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Keywords:

  • Nanofabrication;
  • photoresists;
  • azopolymers;
  • two-photon absorption;
  • near-field fabrication;
  • photoisomerization and photo-orientation;
  • light-activated molecular movement;
  • polymer photomechanics

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Nanofabrication in photoresists
  5. 3. Nanofabrication in azopolymers
  6. 4. Photomechanics and photofluidization in azopolymers
  7. 5. Outlook and conclusions
  8. References
  9. Biographies

Recent progress in the field of single- and two-photon nanofabrication, both 2- and 3-dimensional, in photopolymerizable resins and in films of photoisomerizable azopolymers are reviewed. The basic processes as well as technological advances and applications of nanofabrication by light are discussed. Recent advances and achievements in polymer photomechanics and light-activated molecular movement in azopolymers are also reviewed.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Nanofabrication in photoresists
  5. 3. Nanofabrication in azopolymers
  6. 4. Photomechanics and photofluidization in azopolymers
  7. 5. Outlook and conclusions
  8. References
  9. Biographies

Photolithography has been driving the microelectronics industry for several decades, and nanofabrication by laser light is a natural, yet underdevelopment, extension of photolithography with potential for application in manufacturing structurally complex and functionally advanced devices. Nanofabrication is the process by which functional nanostructures are made. The patterns of the fabricated structures have minimum dimensions smaller than 100 nm, and the commercial implementation of nanofabrication on the large scale have improved microelectronic devices and information technologies. This development is driven by the need to increase the density of components, and increase their performance, and lower their cost. New technologies will certainly evolve to further develop nanofabrication that will have new applications beyond information processing and storage in areas such as optics, biomedicine, and materials science [1-3]. Many “nanomaterials”, e.g. materials with minimum dimensions on the nanoscale, exhibit properties that are different from those observed for the bulk material. Two- and three-dimensional (2D and 3D, respectively) nanoscale structures and nanostructured materials are generated by “top-down” and “bottom-up” approaches. The latter approach uses interactions between molecules or colloidal particles, and the former approach uses various methods of lithographic patterning. This approach includes techniques for patterning features typically in two dimensions. Although bottom-up approaches are feasible to produce 2D and 3D structures at low cost, it may still not accommodate the increasing demand of structural complexity.

We will go on to review recent progress of some unconventional techniques of lithography, e.g. techniques for fabrication of 2D and 3D structurally complexe nanostructures in photoreactive materials by laser light. These techniques are added to the unconventional nanofabrication areas of scanning probe lithography (SPL) [4-10], edge lithography [11-19], molding [20, 21], embossing [22, 23], printing [24-27], and self-assembly [28-31]. Conventional techniques for nanofabrication are commercially available and widely implemented in manufacturing. They are expensive and largely restricted to planar fabrication in semiconductor materials. Such techniques expose substrates to corrosive etchants, high-energy radiation, and relatively high temperatures. The techniques that we will describe in this review, which are based on laser nanofabrication, are unconventional in the sense that none of these conditions is needed, e.g. photolithography in 3D and using photoisomerizable polymers.

Among nanofabrication techniques, nanofabrication by photolithography is the method of choice for manufacturing in the microelectronics industry, and it can pattern 37-nm wide features with 193-nm wavelength light in semiconductor nanofabrication. Photolithographic systems can expose large areas of photoresist coated on a planar substrate, typically a semiconductor wafer in a few seconds. A photoresist is an organic material that crosslinks and becomes insoluble or that changes chemically and becomes more soluble in a basic solution upon exposure to high-energy short-wavelength light, e.g., ultraviolet (UV) light [32, 33]. The exposed photoresist is immersed in solvents that dissolve the exposed (positive photoresist) or unexposed (negative photoresist) regions and provide patterned access to the surface of the substrate. The patterned photoresist masks the substrate during a subsequent step that chemically modifies the exposed regions of the substrate.

Patterning minimum features below 37 nm by photolithography using 193-nm light requires optical proximity correction or phase-shifting mask technology [34, 35]. “Immersion lithography” [36-40], a concept borrowed from immersion microscopy that is often used with biological specimens [41], offers a potential route to high-volume production of devices with sub-50-nm resolution. To pattern smaller features, photolithography will require further advances, such as decreasing the imaging wavelength to 157 nm [42, 43] to soft X-rays (λ = 13.5 nm) known in the microelectronics industry as extreme ultraviolet (EUV) light [44, 45]. There is one photolithographic method that can produce simple patterns (e.g., diffraction gratings) without using a photomask. This process is interferometric lithography [46, 47], which involves the constructive and destructive interference of multiple laser beams at the surface of a photoresist (vide infra). This method does not require most of the expensive projection optics, but the projected patterns are restricted to regularly spaced arrays of lines or dots. Some of the smallest features patterns of 40-nm wide parallel lines separated by 57 nm produced by photolithography have, however, been generated using interferometric lithography [48].

Scanning laser beam lithography is a slow process relative to photolithography. This serial technique can, however, generate arbitrary patterns with 250-nm resolution. Femtosecond-laser nanofabrication can generate patterns with a resolution of tens of nanometers, e.g. a resolution of 65 nm can be reproducibly obtained, e.g. at a level of λ/10, much smaller than the diffraction limit of light [49]. Other key features of nanofabrication by fs-laser technology include maskless photofabrication, and fabrication of complex 3D structures, as well as the ability of nanofabrication in various materials including polymers, and glasses, and ceramics and metals [50-57]. The attraction of this technology is that it can create computer-designed, fully 3D structures with resolution beyond the diffraction limit – no other competing technology offers these advantages. Classic 3D prototyping techniques, such as UV laser stereolithography and 3D inkjet printing, can also reproduce fully 3D structures, however, they provide a maximum resolution of only a few micrometers. Lithographic techniques with superior resolution, such as e-beam lithography, are limited to producing high aspect ratio 2D structures.

This paper is organized as follows: Section 'Nanofabrication in photoresists' discusses nanofabrication in photoresists, including photopolymerization and two-photon materials, and single- versus two-photon absorbtion, as well as nanofabrication beyond the diffraction limit of light, Section 'Nanofabrication in azopolymers' deals with 2D nanstructuring of thin films of photoismerizable azopolymers. Technological advances and applications of nanofabrication by light are discussed for both photoresists and azopolymers. Section 'Photomechanics and photofluidization in azopolymers' deals with recent advances in azopolymer photomechanics, and light-activated polymer movement. An outlook and future prospects are given in Section 'Outlook and conclusions'.

2. Nanofabrication in photoresists

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Nanofabrication in photoresists
  5. 3. Nanofabrication in azopolymers
  6. 4. Photomechanics and photofluidization in azopolymers
  7. 5. Outlook and conclusions
  8. References
  9. Biographies

2.1. Two-photon absorption

Multiphoton absorption (MPA) was predicted as early as 1931 [58] and experimentally observed immediately after the invention of lasers [59]. Two-photon absorption (TPA) can be conceptualized as follows: molecules exposed to high-intensity light can undergo near simultaneous absorption of two photons mediated by a “virtual state”. The combined energy of the two photons accesses an excited state of the molecule. TPA thus involves the interaction of both photons that combine their energies to produce an electronic excitation analogous to that conventionally caused by a single photon of a correspondingly shorter wavelength. Unlike single-photon absorption, whose probability is linearly proportional to the incident intensity, the TPA process depends on both a spatial and temporal overlap of the incident photons and takes on a quadratic (nonlinear) dependence on the incident intensity, resulting in highly localized excitation with a focused light beam. In the early 1990s, some potential applications with this technique were demonstrated, such as two-photon fluorescence (TPF) 3D data storage [60] and TPF microscopy [61]. However, the TPA efficiency of the materials was an issue for these applications, and this has triggered the effort to search for new materials with higher TPA efficiency, see, e.g., the works of the groups of Prasad [62-64] and Marder [65-67], as well as the work of other groups [68-70] for the design of better TPA materials. TPA remains a very active field of research, and recently, with the advent of femtosecond lasers, multiphoton excitation processes found applications in fluorescence imaging of biological molecules [71-75], fluorescence upconversion and two-photon lasing [76, 77], power limiting [78], photodynamic therapy [79-81], optical memory by rewritable photoisomerization and photorefraction effect [60, 82-90], cell surgery [91, 92], synchronization of embryonic heart beating and stimulation of cardiomyocite cells [93, 94], and micro-nanofabrication including micro-optical components, photonic crystals, and complicated 3D patterning by photopolymerization [49, 95-136].

2.2. Photopolymerization and two-photon materials

Photopolymerization is a photoreaction that converts a liquid or gel monomer into a solid polymer. These reactions require the use of a photosensitive molecule, e.g. photoinitiator, that produces an active species upon light irradiation. Most monomers and oligomers commonly do not produce an initiating species for polymerization, so it is necessary to introduce low molecular weight organic compounds that act as photoinitiators. An additional requirement for effective initiation of a photopolymerization process is a photosensitizer that is capable of absorbing light and transferring the excitation energy to the photoinitiator. An ideal photoinitiator should be easily synthesized, low priced, nontoxic, odorless, highly shelf stable (high thermal stability and stability in darkness), and have a high absorption of incident light and a high quantum yield in the generation of the active moieties, and highly soluble in the polymerization medium. The wavelength of the irradiation light can be in the UV, visible or infrared region. When a photoinitiator absorbs a suitable photon energy, it is transformed into a reactive initiating species like radicals or cation radicals. Cationic initiators are photoacid generators that produce cations upon light irradiation. The latter are used for the polymerization of epoxides or vinyl ethers via a cationic polymerization mechanism. The photolysis of cationic photoinitiators has been shown to result in the generation of free radicals allowing the combination of monomers to be polymerized. Conventional photopolymerization initiators generate free radicals upon light irradiation, which initiate a free-radical polymerization process of acrylates or vinyl ethers. The most commonly used free-radical photoinitiator is benzophenone and its derivatives. Free-radical polymerizations are chain reactions in which the addition of a monomer molecule to an active chain-end regenerates the active site at the chain-end. The free-radical photopolymerization mechanism involves at least three different kinds of reactions [137-139]: The first one is the initiation during which the free-radical initiator is decomposed with light in the presence of a monomer to form an active species. In the next reaction, known as the propagation, the initiator fragment reacts with a monomer molecule to form the first active adduct that is capable of being polymerized. Monomers continue to add in the same manner resulting in the formation of macroradicals that are end-active polymers. The final reaction is the termination, during which the growth center is deactivated and the final polymer molecules are formed. This step normally involves the reaction between two polymers bearing active centers and can proceed by two different mechanisms, combination or disproportionation, leading to the formation of one or two polymer chains, respectively:

  • display math

Here I, I*, M, R• denote a photoinitiator, an intermediate state of I after absorbing a photon, a monomer, and a radical, respectively. refers to the energy of an absorbed photon. Besides the above, other reactions, such as chain transfer and chain inhibition, often take place and complicate the mechanism of free-radical polymerization. Photopolymerizations follow the general scheme for any polymerization, however, the use of light, rather than heat, to drive the reaction has certain advantages, such as the elimination of solvent, the high reaction rates at room temperature and the spatial control of the polymerization. Photopolymerization differs from photocrosslinking, which involves the absorption of a photon in every chain propagation step. Thus, the quantum yield of photopolymerization per absorbed photon can reach thousands.

It is beyond the scope of this paper to present an exhaustive review of the materials and molecules used in two-photon polymerization, a recent comprehensive summary of this topic can be found elsewhere [140]. In this paper, we will briefly, review the main classes of two-photon materials. The first materials employed in two-photon polymerization were acrylic photopolymers and the negative photoresist SU8. Over the last few years, two-photon polymerization research has focused on photosensitive sol-gel hybrid materials [141] such as the commercially available ORMOCER (ORganically-MOdified-CERamic, MicroResist) [142-144]. The sol-gel process is based on the phase transformation of a sol of metallic oxide or alkoxide precursors to form a wet gel. A photosensitive sol-gel process usually involves the catalytic hydrolysis of the sol-gel precursor(s) and the polycondensation of the hydrolyzed products and other sol-gel-active components present in the reaction medium to form a macromolecular hybrid network structure. The gel formed is subsequently reacted through photopolymerization to give a product similar to glass.

Silicate-based photopolymers have proved to be a very popular choice in microfabrication using multiphoton polymerization [142, 145-147]. They combine the properties of silicate glasses such as hardness, chemical and thermal stability, and optical transparency with the laser processing at low temperatures of organic polymers, properties impossible to achieve with just inorganic or polymeric materials. The most widely used hybrid material is ORMOCER. This material comprises an inorganic (–Si–O–Si–) backbone that can be functionalized with a range of organic functionalities. While ORMOCER and other silicate-only based hybrid materials have provided the possibility to fabricate high-resolution 3D structures with good optical properties, they do not allow the optimization and “fine tuning” of the material properties for specific applications. The versatile chemistry of sol-gel composites allows the copolymerization of more than one metal alkoxide, this has been shown to enhance the material's mechanical stability and allows the modification of its optical properties [148-152]. There are several examples of 3D microstructures fabricated using two-photon polymerization, and acrylate photopolymers doped with metal or metal-oxide nanoparticles, e.g. composite materials [153, 154]. However, despite the obvious advantages of preparing true composite photosensitive sol-gels, there are very few such examples in two-photon polymerization. Active hybrid materials are also the subject of intense research because they add functionality to the fabricated structures. Such functionalities may include photochemistry and photoswiching, nonlinearity, conductivity, and so on.

2.3. Two-photon photopolymerization

3D pholithography, e.g. 3D stereolithography by femtosecond lasers, is performed by scanning a tightly focused laser beam into a photopolymerizable resin from the bottom surface to the top surface to fabricate an entire 3D structure (negative photoresist). See, e.g., Fig. 1. In this work, e.g. that corresponding to Fig. 1, a titanium sapphire laser operating in mode-lock at 76 MHz and 780 nm with a 150-fs pulse width was used as an exposure source and it was focused by an objective lens of high numerical aperture (∼1.4) yielding an intensity on the sample in the order of 1013 W/μm2. This is a typical intensity for fs-laser fabrication. Two- or multiphoton absorption occurs at the high photon density of a laser beam. The wavelength of the fs-laser light is in the near-infrared (NIR) region, a range in which most of the photoresists are transparent, a feature that allows a deep penetration in the medium with negligible absorptive power loss. Nonlinear absorption, e.g. TPA or MPA and photoreaction occur in a volume, e.g. voxel, much smaller than the cube of the wavelength, λ3, of the laser, a feature that is a key to high special resolution [105, 106]. The precision of the focal spot sample positioning is obtained by a galvanomirror set in the plane of the sample and a piezostage in the out-of-plane direction. A charge-coupled device (CCD) camera is used for monitoring the focal spot on the sample and in situ fabrication [49].

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Figure 1. (Top) Scanning electron microscopic (SEM) image of a microbull sculpture produced by raster scanning (left) and surface profile scanning (right). For the latter, only the crust was defined by the two-photon process, the inside was solidified by illumination under a mercury lamp. In this particular structure, the two-photon scanning time was reduced by 90% due to the use of surface profile scanning and subsequent solidification under a mercury lamp. Nanofabrication took 13 min. (Bottom) schematics of (a) raster, and (b) surface profile scanning. Reproduced with permission from [49].

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Stereo photolithography dates back to the work of Kodama in 1981 [155] and it was further developed by several other researchers to improve the speed, and the resolution [156-163]. However, the dimensions of the fabricated structures are several orders of magnitudes larger than those that can be fabricated by TPA or MPA processes. Kawata's group developed the technology of fs 3D nanofabrication as reported in a series of works (see for example, references 49 and 95). The absorption of UV-curable resin is due to valence electron transition and it is located in the UV or the blue side of the visible spectrum, and when exposed to NIR, TPA or MPA occurs. Evidence of the occurrence of nonlinear absorption can be obtained when using a mode-locked femtosecond laser. Linear absorption is governed by photon flux, (photon/cm2), expressed by the product of irradiation duration and photon flux intensity (photon/s cm2), which is proportional to light intensity (W/cm2). The adequate energy dose needed for absorption is reachable either by intense irradiation for short time or weak irradiation for long times. To date there is no experimental report on polymerization observed when NIR lasers work in CW mode, where the power is uniformly delivered over time. On the other hand, polymerization occurs with the same irradiation dose under pulsed mode. When switched to pulsed mode, 82 MHz, 100 fs and 1 mW mean power, a diffraction-limited focal spot has a photon flux density of 1029 photons/s cm2 in the pulse duration, which is 1.2×105 times larger than when the laser is operating in CW mode. This shows that polymerization is due to a nonlinear optical absorption effect.

The first reported real 3D microstructure [95] is a 7-μm diameter and 50-μm long spiral coil with a line cross section of 1.3–2.2 μm (Fig. 2). Due to the use of a relatively low NA (0.85) objective, the feature size is larger than the potential limit provided by the technology. This work confirmed the feasibility of two-photon photopolymerization in three dimensions. In this experiment, a 790-nm, 200-fs laser was focused into SCR 500 [Japan Synthesis Rubber Company, JSR], a commercial urethane acrylate resin. Both nonlinear optical absorption and thresholding, e.g. chemical and material photoreactive nonlinearity (vide infra), contribute to high spatial resolution of nanofabrication. Nonlinear absorption results from a light-intensity distribution that is spatially narrower than that of linear absorption used for UV lithography [106]. For example, the squared light intensity distribution related to TPA reduces the volume of interaction of light with matter, thereby improving the resolution of nanofabrication. Thresholding is a nonlinear photochemical effect. For example, for two-photon induced polymerization of resins, the photogenerated radicals are spatially distributed, obeying the square law of the light intensity distribution. The rate of photopolymerization of monomers is therefore expected to follow the same distribution. However, due to the existence of the radical quenchers, like dissolved oxygen molecules, radicals survive and initiate polymerization only at the region with highest light intensity, and thus a threshold is formed. Since the light intensity relative to the threshold is continuously adjustable by exposure duration or laser pulse intensity, the size of a voxel, which defines the spatial resolution, can be tuned to be much smaller than that defined by the diffraction limit [110-114]. This effect is called material nonlinearity [49].

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Figure 2. Scanning electron microscopic (SEM) image of a spiral coil structure made by two-photon photopolymerization after removal of unsolidified resin. (a) A view of the entire structure and (b) the magnified top end. Reproduced with permission from [95].

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Nanofabricated structures may shrink in two-photon lithography, an effect that can be precompensated in the computer design of the structure [164], or used to advantage to reduce the size of the fabricated structure [165]. There are also resins that do not shrink after nanofabrication. One of which is SU-8, which is an ultrathick negative photoresist that has much more strength than liquid-based resins [166, 167]. Figure 3 shows fabricated 3D spiral structures without shrinkage employing two-photon-induced crosslinking in SU-8.

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Figure 3. (a) Schematic principle of the fabrication of a spiral structure in SU-8 by direct laser writing. SEM images of spiral-type photonic crystals fabricated with a pulse energy of 0.6 nJ, (b) full shape that has a period of 1.8 μm, length of spiral arm (L) of 2.7 μm, and a vertical pitch (P) of 3.04 μm as illustrated in (c). (d) and (e), Fabricated circular-spiral structure with a L-shaped waveguide and its magnified image, respectively. Reprinted with permission from [166].

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Besides laser scanning, laser interference has been used in the fabrication of optical gratings, 2-dimensional rod arrays, 3-dimensional diamond-like photonic crystals and 3-dimensional networks [168-172]. An interesting example of nanofabrication of functional nanostructures is shown in Fig. 4. Indeed, this figure shows 2D and 3D patterns of CdS-polymer nanocomposites fabricated by a four-beam interference technique in films with various thicknesses [173]. Micro-nanofabrication by laser-beam interference and single-beam irradiation is discussed in Section 'Nanofabrication in azopolymers' for azopolymers.

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Figure 4. SEM images of microstructures of CdS–polymer nanocomposites fabricated by four-beam laser interference with various film thicknesses and exposure times, (a) thickness, 2 μm, exposure time, 20 s, (b) thickness, 2 μm, exposure time, 10 s, (c) thickness, 2 μm, exposure time, 8 s, (d) thickness, 10 μm, exposure time, 30 s, (e) thickness, 10 μm, exposure time, 20 s, (f) thickness, 10 μm, exposure time, 10 s. The experimental details can be found in [173]. The scale bar is 2 μm in all images. Reproduced with permission from [173].

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2.4. Photopolymerization beyond the diffraction limit of light

We will go on to discuss more applications of two-photon photopolymerization. Innovative techniques of multiphoton polymerization that are based on stimulated-emission-depletion (STED) fluorescence microscopy, developed by Hell and Wichmann [174] and Hell's group [175], have been recently introduced. They include single-photon rather than multiphoton excitation [176], a one-color scheme [177], a multiphoton two-color scheme [178, 179], and diffusion-assisted STED [180]. In STED, fluorescent molecules are excited by a short laser pulse. A second laser pulse, which is tuned to a substantially longer wavelength than the first pulse, is used to de-excite the molecules through stimulated emission. This depletion pulse must arrive after vibrational relaxation is complete in the excited electronic state but before significant fluorescence has occurred. Spatial phase shaping of the depletion beam causes de-excitation to occur everywhere except in a region at the center of the original focal volume. The size of this region depends on the intensity of the depletion beam and the corresponding degree of saturation of stimulated emission. Thus, fluorescence can be localized in a zone much smaller than the excitation wavelength. World-record lateral resolutions down to 5.6 nm using visible light and nitrogen-vacancy centers in diamond have been reported by Hell's group [181]. It is interesting to translate this success of optical microscopy to optical lithography. This possibility was already mentioned by Hell's group in 2000 [182].

In principle, STED should work equally well to deactivate polymerization in multiphoton polymerization or conventional photopolymerization. Typical radical photoinitiators undergo intersystem crossing to a triplet state on a time scale on the order of 100 ps [183]. The radicals that lead to polymerization are formed in the triplet state, so de-excitation of molecules before intersystem crossing occurs will turn off photopolymerization. Furthermore, radical photopolymerization only occurs above a threshold concentration of radicals, so de-excitation of a small fraction of excited photoinitiator molecules could be sufficient to halt polymerization if the concentration of excited molecules is just above this threshold. Li et al. [177] achieved λ/20 resolution by one-color initiation and deactivation of photopolymerization. They used a technique inspired by STED that they call resolution augmentation through photoinduced deactivation (RAPID) lithography. In RAPID lithography, one laser beam is used to initiate polymerization in a negative-tone photoresist. A second laser beam is used to deactivate the photoinitiator, preventing photopolymerization from occurring. Multiphoton absorption of pulsed 800 nm light is used to initiate crosslinking in a polymer photoresist and one-photon absorption of continuous-wave (CW) 800-nm light is used simultaneously to deactivate the photopolymerization. By spatial shaping of the phase of the deactivation beam, features far smaller than the excitation wavelength can be fabricated. They demonstrated the fabrication of features with scalable resolution along the beam axis, down to a 40-nm minimum feature size.

Scott et al. [176] demonstrated a technique of photopolymerization, which is also inspired by STED, and that uses a two-color irradiation scheme, whereby initiating species are generated by single-photon absorption at one wavelength (473 nm from a CW laser) while inhibiting species are generated by single-photon absorption at a second, independent wavelength (364 nm from a CW Argon ion laser). Coirradiation at the second wavelength thus reduces the polymerization rate, delaying gelation of the material and facilitating enhanced spatial control over the polymerization. Appropriate overlapping of the two beams produces structures with both feature sizes and monomer conversions otherwise unobtainable with the use of single- or two-photon absorption photopolymerization. Additionally, the generated inhibiting species rapidly recombine when irradiation with the second wavelength ceases, allowing for fast sequential exposures not limited by memory effects in the material and thus enabling fabrication of complex two- or three-dimensional structures (Fig. 5) [176]. This figure demonstrates 65-nm dots with a single-photon fabrication process. The optical approach demonstrated in [176] produces confinement of the polymerized region along only two axes, and manipulation of the photoinhibiting wavelength into a bottle beam profile [184] would induce confinement along the third axis, thus allowing fabrication of 3D structures with sub-100-nm isotropic resolution. Because single-photon absorption cross sections are often orders of magnitude larger than two-photon cross sections, this photoinitiation–photoinhibition system facilitates the use of inexpensive CW diode lasers and very high write velocities. Thus, this single-photon approach to nanolithography uses dramatically cheaper hardware and scales to much higher throughput.

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Figure 5. Scanning electron micrographs of polymerized features. (a) Voxels polymerized on a microscope slide using a 0.45-NA singlet lens and a coincident Gaussian blue / UV irradiation scheme. The blue power was held constant at 10 μW while the UV was progressively increased. The UV power, from left to right, was 0, 1, 2.5, 10, and 100 μW. The exposure time was 8 s for each dot. Scale bars, 10 μm. (b) Profile of a voxel similarly fabricated but with 10 μW of blue power and 110 μW of UV focused at 1.3 NA, then imaged via SEM at normal incidence. The SEM intensity on the white line (inset) is plotted as squares against the expected polymerization profile (green) obtained by a double-parameter fit of the initiation (blue) and inhibition (violet) rate profiles, as shown. (c) Polymer column fabricated by using the same conditions as Fig. B. The focus was translated normal to the glass slide at a velocity of 0.125 μm/s for 3 μm. The high aspect ratio caused the column to fold during the solvent wash, leaving it lying on the glass surface. Scale bar, 200 nm. Reproduced with permission from [176].

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Along the same lines, e.g. a two-color photopolymerization scheme based on STED, Gu's group fabricated 40 nm features [179]. They used a direct laser writing technique based on single-photon photoinhibited polymerization. They used a CW-mode 375-nm inhibiting laser for inhibiting polymerization and a CW mode 488-nm initiating laser. The super-resolution feature is realized by overlapping two laser beams of different wavelengths to enable the wavelength-controlled activation of photoinitiating and photoinhibiting processes in the polymerization.

Fisher and Wegener [178] explored stimulated emission as a superior depletion mechanism and developed a multiphoton two-color photopolymerization scheme based on STED. The strategy of this scheme is briefly described as follows. They use a pulsed 810-nm Ti-Sapphire fs laser for photoinitiation by two-photon absorption and a cw green (532 nm) laser as a depletion beam. The relevant states and transitions of a photoinitiator molecule are schematically shown in Fig. 6a. The excitation (or writing) laser excites photoinitiator molecules from their ground states to an electronically and vibrationally excited level S1* via two-photon absorption. Some molecules may directly decay back to the ground-state nonradiatively. However, the majority of molecules will relax to an intermediate state S1 from where they can either fluoresce or undergo intersystem crossing (ISC) to the triplet state T1. From the T1, a chemical reaction may be initiated in the photoresist. The idea of STED is to bring the molecules from the intermediate state S1 back to the ground state S0 via stimulated emission (SE) induced by a second laser of a different color that is called the depletion laser.

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Figure 6. (a) Energy-level scheme with transitions in a photoinitiator molecule for stimulated emission- depletion (STED) direct-laser-writing optical lithography. SE, stimulated emission, ESA, excited-state absorption, TTA, triplet–triplet absorption, ISC, intersystem crossing. (b) Ingredients of the photoresist used by the authors, pentaerythritol tetraacrylate (monomer) and 7-diethylamino-3-thenoylcoumarin (photoinitiator). Reproduced with permission from [178].

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From this scheme, it can be seen that the search of photoinitiators is a key to the success of STED direct laser writing. The photoinitiator selected by these authors is shown in Fig. 6b. These authors succeeded in fabricating a photonic crystal woodpile structure with a clear stopgap by using STED direct laser writing with 52 layers (corresponding to a total height of 6.4 μm) and a rod spacing a = 350 nm, which is the intralayer periodicity (Fig. 7a). The quality of the sample interior was excellent over its entire thickness. Figure 7b shows the corresponding optical spectra with a pronounced stop band and a minimum transmittance of 3.5%.

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Figure 7. (a) Oblique-view electron micrograph of a woodpile photonic crystal with 52 layers and a rod spacing of a = 350 nm made by STED direct laser writing. The sample has been milled with a focused-ion beam to reveal its interior. (b) Corresponding reflectance and transmitted spectra (normalized to substrate transmittance and the reflectance of an 80-nm silver film, respectively). Reproduced with permission from [178].

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The implementation of direct laser writing with STED is complicated, requiring very fine beam control and photoinitiators that have not only a high two-photon cross section but also high fluorescence quantum efficiency [176-179]. When absorbing the light, the photoinitiator becomes excited and produces radicals that attack monomer molecules to form macroradicals of the polymer. Finally, the radicals are terminated to give the polymer. Radical termination can be also induced by oxygen and other molecules in the system, known as scavengers. Quenching competes with photopolymerization and is usually considered detrimental to the process. In multiphoton photopolymerization, however, it can be used to circumvent the diffraction limit and produce structures of very high resolution. Farsari's group [180] showed that it is possible to increase the resolution of multiphoton polymerization by employing a scheme based on quencher diffusion, in a chemical equivalent of STED. This is based on the combination of a mobile quenching molecule and a slow laser scanning speed, allowing the diffusion of the quencher in the scanned area, the depletion of the generated radicals, and the regeneration of the consumed quencher. They fabricated woodpile structures with an interlayer period of 400 nm, comparable to what has been achieved by STED direct laser writing, as discussed in the previous paragraph.

2.5. Applications of two-photon photopolymerization

Two-photon photopolymerization has found applications in photonic crystals and metamaterials fabrications [185-193] as well as in biomedical applications, including biodegradable materials and protein microstructuring [194-198]. We do not intend to review all the literature in these kinds of applications, rather we will highlight some important achievements in application of two-photon fabrication. For example, Wegener group [188] fabricated 3D woodpile photonic crystals with defects, e.g. they created a simple resonator by altering just one elementary cell of the periodic structure (Fig. 8). As the woodpile photonic crystal is constructed from a two-rod motif, they actually leave out exactly one of these building blocks to create a defect. Farsari's group [189] reported three-dimensional metallic photonic crystals with optical bandgaps in the near-IR to optical region. They prepared woodpile structures and they metalized them with silver. The fabricated structures had 900-nm and 600-nm periodicity and resolution below 100 nm. Using sol-gel chemistry, direct laser writing and electroless plating metallization techniques, they were able to create the appropriate polymer network, fabricate the structures and metalize them selectively. Farsari's group [192] also reported 3D structuring and metallization of a zirconium-based organic–inorganic photosensitive material doped with metal-binding tertiary amine moieties.

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Figure 8. (a) Schematic representation of the resonator design after silicon double-inversion. (b) Focused ion beam cross section of a silicon photonic crystal with defect. (c) Numerical calculation of the mode profile. The gray structure represents the distribution of the silicon and acts as a guide for the eyes. (d) Perpendicular incidence transmittance spectra into the forward direction (entire half-space) of a structure containing one resonator, green (blue) solid curve. The inset is a close-up of the transmittance in the forward direction. The gray area displays the width of the complete photonic bandgap. Reproduced with permission from [188].

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Metal nanostructures have been fabricated by Prasad's group [187] by two-photon lithography. They fabricated highly conductive gold nanostructures within a polymeric matrix with subwavelength resolution. The nanostructures are directly written in a gold-precursor-doped photoresist using a femtosecond pulsed laser. The laser energy is absorbed by a two-photon dye, which induces simultaneous reduction of gold in the precursor and polymerization of the negative photoresist. This results in gold-nanoparticle-doped polymeric lines that exhibit both plasmonic effects, due to the constituent gold nanoparticles, and relatively high conductivity due to the high density of particles within these lines. Line widths from 150 to 1000 nm have been achieved with this method, and several functional structures have been written including optically active planar chiral structures and plasmonic nanostructures, which are used in the development of composite metamaterials. Gu's group [185] fabricated and characterized three-dimensional photonic structures that represent a novel class of biomimetic photonic chiral composites inspired by butterfly wings. They fabricated different types of chiral composites that can be engineered to form novel photonic devices. They experimentally showed strong circular dichroism and compare with numerical simulations to illustrate the high quality of their 3D photonic crystals.

Metamaterials are artificial materials that, unlike naturally occurring substances, exhibit magnetism at optical frequencies, it is therefore essential that the structures are conducting. While most studies on fs-laser fabrication have focused on the realization of polymeric structures [49, 95-116], this technique was recently used for the fabrication of metallic 3D patterns over large areas using a microlens array [199-202], a feature which is a step forward from standard, single-beam laser writing into a polymer matrix, that is time consuming and not suitable for large-scale fabrication. The proposed method, however, enabled the simultaneously writing of more than 700 polymer structures that were uniform in size. The metallization of the structures was then achieved through the deposition of thin films composed of small silver particles by means of electroless plating (Fig. 9). A hydrophobic coating on the substrate prevented silver deposition in unwanted areas and allowed the formation of a large number of isolated and highly conducting objects. This method of metallization is flexible in that it can produce either polymer structures covered with metal or numerous isolated insulating polymer objects spread over a metallic film, depending on the resin properties and treatment procedures [201, 202]. Femtosecond-laser writing is considered to be one of the most promising methods for future manufacturing of large-area, true 3D metamaterials, offering intrinsic 3D parallel processing capability with good resolution (100 nm), and it can be successfully combined with selective metal deposition by electroless plating [201, 202].

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Figure 9. Scanning electron microscope images of (a) self-standing empty cubic structures (height∼4.6 μm) connected in pairs and (b) a silver-coated polymer structure composed of a cube (2 μm in size) holding up a spring (inner diameter 700 nm). The structures are made by a two-photon-induced photopolymerization technique combined with electroless plating. Reprinted with permission from [201].

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Recently, Pendry [203] pointed out an alternative route to negative refraction by exploiting chirality in combination with a resonant system with a photonic bandgap. Wegener's group [204] fabricated, by direct laser writing, chiral photonic crystals consisting of 3D dielectric (photoresist) helices arranged vertically on a two-dimensional (2D) square lattice. Transmittance measurements for circularly polarized light propagating along the helix axis found stop bands when the pitch of the dielectric helices matched the wavelength of light in the dielectric. Since this structure was uniaxial, its optical properties depended strongly on the angle of incidence. In a subsequent publication, Wegener's group [205] presented bichiral dielectric photonic crystals with a 3D simple cubic lattice. The chosen architecture was inspired by blue-phase cholesteric liquid crystals [206-208]. The photonic crystal consisted of three sets of 3D dielectric helices, one set oriented along each of the three orthogonal axes of the simple cubic lattice. Giessen's group [190] fabricated and characterized a metallic counterpart of the 3D bichiral dielectric crystals starting from the same point as Wegener's group. Giessen's group first fabricated a 3D bichiral crystal by two-photon femtosecond-laser writing in a negative-tone photoresist. Next, this dielectric template was coated with a conformal metal film via electroless silver plating. Electroless plating is a wet-chemical metallization technique based on an autocatalytic redox reaction that does not require an external current source, and thus coated all exposed surfaces. Finally, since electroless plating also coated the substrate with silver, the silvered crystal was detached with a glass capillary and transferred to a clean substrate in order to facilitate transmission spectroscopy. Note that standard deposition methods such as sputter coating or vacuum evaporation are unsuitable for metal coating due to self-shadowing of the complex 3D geometry.

Along the same lines of the work of Kawata's group [201, 202], Wegener's group [193] fabricated a uniaxial gold helix metamaterial shown in Fig. 10 (Left). Such structures can only be fabricated by direct laser writing. The polymer structures produced through direct laser writing lithography can be filled with gold using electroplating [209] (Fig. 10 (Right). Electroplating setups can be extremely simple and inexpensive, often requiring only a bias voltage between a transparent electrode on the substrate and a macroscopic counterelectrode within a beaker. The metamaterial sample footprint and height is therefore limited only by the direct laser-writing lithography process itself. Past achievements and future challenges in the development of three-dimensional photonic metamaterials have reviewed by Soukoulis and Wegener [186].

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Figure 10. (Left) Chiral metamaterial made using direct laser writing and electroplating. (Right) Connected cubic-symmetry negative-index metamaterial structure amenable to direct laser writing [209]. Reproduced with permission from [193]. In the schematics the yellow and blue colors represent gold and dielectric materials, respectively.

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Farsari's group [191] fabricated a functional photonic crystal, e.g. a photonic crystal made of a nonlinear optical material that consists of photosensitive sol-gel material that contains the nonlinear optical chromophore Disperse Red 1, an azodye chromophore, and that can be structured accurately using the two-photon polymerization technique. Farsari's group used this material to fabricate 3D photonic crystals with band stops in the near-infrared. The optical properties of such a functional photonic crystal can be controlled by the material's optical nonlinearity and photochemical switching capabilities of the azodye chromophore [210].

Another area where the potential of two-photon laser technology is being realized is in biomedical applications. Several photopolymers are biocompatible and able to be structured in complicated 3D shapes, a feature that makes them attractive for applications such as implantable microelectromachining systems (MEMS), and drug-delivery devices, such as microneedle arrays, devices that enable transdermal delivery of different pharmacological substances, and scaffolds for tissue engineering [97, 121, 124]. Figure 11 (left) shows an example of implantable MEMS, e.g. a microvalve designed to prohibit the reversal of blood flow in human veins, which may be caused by standing for too long. The valve is designed to open under forward fluid flow and close firmly in the case of backward flow. Figure 11 (right) shows an example of a high-porosity tissue engineering scaffold. The material used for two-photon fabrication of the valve and scaffold was a zirconium/silicon hybrid sol-gel SZ2080 [97]. The scaffolds are required in order to produce living tissue that can integrate with host tissue inside the body. Micrometer-sized topography has an essential role in determining cell adhesion and surface-bound characteristics, influencing important cellular functions such as survival, proliferation, differentiation, and migration or mediator release. 3D cell cultures offer a more realistic local environment where the properties of cells can be observed and manipulated. An important factor in the production of tissue-engineering scaffolds is the ability to reproducibly manufacture 3D nanostructures, a feature that is a direct application of fs-laser fabrication scanning technology.

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Figure 11. , Biomechanical parts made by two-photon fabrication. (left) Microvalve designed to prohibit the reversal of blood flow in human veins. Only part of the valve cover was built, to enable visualization of the interior. (Right) Scanning electron microscope image of high-porosity tissue engineering scaffold. The material used for two-photon fabrication was a zirconium/silicon hybrid sol-gel. Reproduced with permission from [97].

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Engelhardt et al. [194] showed that two-photon polymerization offers the possibility of creating artificial cell scaffolds composed of micro- and nanostructures with spatial resolutions of less than 1 μm. For use in tissue engineering, the identification of a two-photon processable polymer that provides biocompatibility, biofunctionality and appropriate mechanical properties is a difficult task. Extracellular matrix proteins such as collagen or fibronectin, which could mimic native tissues best, often lack the mechanical stability. Hence, by generating polymer–protein hybrid structures, the beneficial properties of proteins can be combined with the advantageous characteristics of polymers, such as sufficient mechanical stability. They demonstrated the fabrication of 3D microstructures of polymer–protein hybrid structures and crosslinked gelatin microstructures, which clearly forced porcine chondrocytes to adapt their cell morphology, e.g. chondrocyte grows on crosslinked gelatin microlines, thereby showing the tissue engineering capabilities of two-photon fabrication. Along the same lines, Farsari's group [195] reported laser-induced photocrosslinking of protein microstructures. They fabricated 2D and 3D microstructures of avidin, bovine serum albumin and biotinylated bovine serum albumin. The multiphoton absorption-induced photocrosslinking of proteins was demonstrated with a nontoxic biomolecule flavin mononucleotide as the photosensitizer. Submicrometer- and micrometer-scale structures were fabricated from several different compositions of protein and photosensitizer. In addition, the retention of ligand-binding ability of the crosslinked protein structures was shown by fluorescence imaging of immobilized biotin or streptavidin conjugated fluorescence labels.

Seidlits et al. [196] reported high-resolution patterning of hydrogels in 3D using direct-write photofabrication for cell guidance. The development of 3D, spatially defined neuronal cultures that mimic chemical and physical attributes of native tissue is of considerable interest for various applications, including the development of tailored neuronal networks and clinical repair of damaged nerves. Seidlits et al. used multiphoton excitation to photocrosslink protein microstructures within 3D, optically transparent hydrogel materials, such as those based on hyaluronic acid. These methods can be used to create complex 3D architectures that provide both chemical and topographical cues for cell culture and guidance, providing a means to direct cell adhesion and migration on size scales relevant to in vivo environments. Using this approach, Seidlits et al. demonstrated guidance of both dorsal root ganglion cells and hippocampal neural progenitor cells along arbitrary, three-dimensional paths.

Two-photon fabrication of scaffolds for neural tissue engineering applications were reported by Farsari's group [198] . These authors reported the production of high-resolution 3D structures of polylactide-based materials via multiphoton polymerization and explored their use as neural-tissue engineering scaffolds. To achieve this, they synthesized a liquid polylactide resin and rendered it photocurable via attaching methacrylate groups to the hydroxyl end groups of the small molecular weight prepolymer. This resin cures easily under UV irradiation, using a mercury lamp, and under femtosecond IR irradiation. The results showed that the photocurable polylactide resin can be readily structured via direct laser writing with a femtosecond Ti:sapphire laser and submicrometer structures can be produced. The maximum resolution achieved is 800 nm. Neuroblastoma cells were grown on thin films of the cured polylactide material, and cell viability and proliferation assays revealed good biocompatibility of the material.

Sun's group [197] used two-photon fabrication to prepare protein microarchitectures. The protein molecules are crosslinked by a two-photon polymerization approach to form a hydrogel. They fabricated protein microlenses from bovine serum albumin by maskless femtosecond-laser direct writing. They used a commercial bovine serum albumin and a photosensitizer (methylene blue) to fabricate micro/nanoarchitectures. The photocrosslinked protein microstructures exhibited rapid and reversible swelling-to-shrinking behavior when stimulated by chemical signals. As a result, the focal distance of the as-formed microlenses can be tuned regularly and reversibly by changing the pH value. Because of their dynamically adjustable properties and full biocompatibility, microlenses show great promise for optical, electronic, and biomedical applications.

Both photoresists and azopolymers can be structured by light, but each with distinctive features and response to light absorption. Furthermore, two-photon direct laser writing is starting to be used for the combination of these two-kind of materials for added functionality [191]. In contrast to photoresists, azopolymers have their own way of being structured by light, the reaction is not photopolymerization, it is rather induced mass movement of the polymer that fades away from light. This is what we discuss next.

3. Nanofabrication in azopolymers

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Nanofabrication in photoresists
  5. 3. Nanofabrication in azopolymers
  6. 4. Photomechanics and photofluidization in azopolymers
  7. 5. Outlook and conclusions
  8. References
  9. Biographies

Photoisomerizable polymers, namely azopolymers, have attracted much attention because of their tremendous importance in a large spectrum of adjacent research fields, including photochemistry, polymer science and engineering, chemical engineering, optics and nonlinear optics, and so on. The basic phenomena discovered in the field, including photoinduced mass movement of polymers, are summarized in a book that we edited in 2002 [210]. Photoinduced patterns of surface deformations in azobenzene-containing polymer films have attracted much attention because of possible applications in optical data storage and in micro-/nanofabrication, and it is well known that such patterns reflect the state of the incident light polarization and the light-intensity distribution [210-215]. The photoinduced patterns are due to light-induced mass movement of the polymer chains which in turn is triggered by the photoisomerization of the azochromophores. An example of an azopolymer is shown in Fig. 12.

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Figure 12. Chemical structure (inset) and absorption spectrum of the trans-DR1-PMA thin film. PMA-DR1, Poly(Disperse Red 1 methacrylate), (Product No. 579009, Aldrich, Tg = 82 °C). The arrow labelled pump indicates an irradiation wavelength for photoisomerization. Reproduced with permission from [215].

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The molecular machine, e.g. the azobenzene derivative, that fuels the polymer motion has two geometric isomers, the trans and the cis forms (see Fig. 13a for the trans and cis azobenzenes), and the isomerization reaction is a light- or heat-induced interconversion of the two isomers. The trans isomer is thermodynamically more stable than the cis isomer – the energy barrier at room temperature is about 50 kJ/mol for the azobenzene – and generally, the thermal isomerization is in the cis[RIGHTWARDS ARROW]trans direction. Light induces isomerization in both directions. Photoisomerization begins by elevating the isomers to electronically exited states, after which nonradiative decay brings them to the ground state either in the “cis” form or in the “trans” form, the ratio depending upon the quantum yields (QYs) of the isomerization reaction. From the cis form, molecules come back to the trans form by two mechanisms: the spontaneous thermal reaction and the reverse cis[RIGHTWARDS ARROW]trans photoisomerization [216]. Figure 13b shows a simplified model of the excited states. Only two excited states are represented, but each may represent a set of actual levels. The lifetimes of all these levels are assumed to be very short in comparison to those of these two excited states. σt and σc are the cross sections of absorption for the absorption of one photon by the trans and the cis isomers, respectively. The cross sections are proportional to the isomer's molar absorptivities. γ0 is the thermal relaxation rate, and it is equal to the reciprocal of the lifetime of the cis isomer (τc). Φtc and Φct are the QYs of photoisomerization, and they represent the efficiency of the trans [RIGHTWARDS ARROW] cis and cis [RIGHTWARDS ARROW] trans photochemical conversion per absorbed photon, respectively. When the irradiation light is polarized, the azomolecules are redistributed perpendicular to the irradiation light polarization after light absorption [217-219]. Briefly, photo-orientation results from photoselective isomerization, e.g. molecules that are oriented along the irradiation light polarization have the highest probability of isomerization and those that are oriented perpendicular to the excitation light polarization have the lowest probability of isomerization. Photoselection, burns a hole into the molecular orientational distribution, and the long molecular axis of the azochromophore fades from the exciting light polarization due to rotation during photoisomerization, thereby inducing molecular photo-orientation.

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Figure 13. (a) Trans–cis isomerization of azobenzenes. (b) Simplified model of the molecular states. Reproduced with permission from [216].

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A surface relief is fabricated by an interference pattern of light (Fig. 14). The polymer moves from the high- to the low-intensity regions in the direction of light polarization, and the trans [LEFT RIGHT ARROW] cis photoselective isomerization and molecular reorientation play important roles in the deformation process. Although there are many attempts to theoretically describe photoisomerization-induced mass movement in polymers [220-234], till now there is no theoretical model that fully describes the phenomenon taking into account the combined aspects of photochemistry, and photo-orientation and polymer science and polymer mechanics. This is an area of research that still needs investigation. From an experimental point of view, photoisomerization was shown to enhance molecular mobility far below the glass-transition temperature (Tg) of azopolymers in the beginning of the past decade [235-238], and considerable exploration of sub-Tg photoinduced molecular movement was performed especially targeting polymer structural effects, including Tg, the free volume and free-volume distribution, the mode of the attachment of the chromophore to a rigid or flexible chain, the molecular weight, and so on [217-219]. Light-induced mass movement in azopolymers, i.e. surface-relief gratings (SRGs) [237, 238] triggered many studies to understand the mechanism of polymer migration, and most of the studies have focused on SRGs that are fabricated by the interference pattern of two coherent laser beams [225, 226, 239]. Both photo-orientation and surface reliefs can also be induced by two-photon isomerisation [240-256].

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Figure 14. AFM image of a typical surface relief grating inscribed on an azopolymer film displaying a grating amplitude of 300 nm. The original film thickness was 300 nm. Reproduced with permission from [233].

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Recently, azopolymer-based patterns have been used as templates for fabricating periodic arrays of, e.g., titanium dioxide [257-259], indium tin oxide [260], and metallic [261, 262] nanostructures, a technology named directional photofluidization lithography (DLP) developed by Lee and coworkers [263]. This technology is based on irradiating, with an interference pattern, stripes (line array) of an azopolymer, prefabricated with a polymer mold – PDMS mold for example – rather than a film (Fig. 15). The irradiation conditions are arranged in such a way that the photoinduced movement of the azopolymer extends to areas that are not covered with it, thereby controlling the space, between the azopolymer features, available for evaporating a metal for example. Such technology is pretty much parenting to colloidal nanosphere lithography, with the exception that the space between the colloidal particles can be controlled by light in the case of DLP using azopolymers.

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Figure 15. Monolithic evolution of hierarchical SRGs by holographic photofluidization of azopolymer line arrays, STEP 1) fabrication of original azopolymer line arrays by micromolding in capillaries (MIMIC) using a solvent. SEM image of the obtained original azopolymer line array. 2) monolithic evolution of hierarchical SRGs by interference pattern irradiation. Reproduced with permission from [263].

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Figure 16 shows free-standing gold ellipsoid arrays fabricated by DPL on an azopolymer after development. The SEM image of the gold ellipsoid arrays in Fig. 16b shows that the individual gold ellipsoids are highly ordered over a large area with few defects and structural variation (the structural variation was less than 25 nm). DPL enables the production of large-area metallic nanostructures with controlled shapes and structural homogeneity. The structural features of the obtained gold ellipsoids including the length (x, the long axis of the gold ellipsoid), width (y, the short axis of the gold ellipsoid), period, and gaps between the sharp edges were commensurate with those of the patterns of the azopolymer. A magnified image of single gold ellipsoid (inset in Fig. 16a) reveals a smooth surface morphology and low line-edge roughness (below 5 nm). The radii of curvature of the edge at the end of the ellipsoid ranged from 12 to 25 nm (see Figs. 16b and c). Such values of radii of curvatures are comparable to that prepared by e-beam lithography or focused ion-beam lithography. Such structures can be used as plasmonic nanoantenna. DPL allows nanotemplate shape/size to be precisely controlled as a nanolithographic technique.

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Figure 16. SEM images of gold ellipsoids fabricated by DPL on an azopolymer after development, (a) Gold ellipsoids arranged over a large area with structural uniformity. (b) High-magnification SEM image of a single gold ellipsoid. (c) Tips of a single gold ellipsoid. The radii of curvature are less than 25 nm (from 12 to 25 nm). Scale bars are (a) 20 μm (inset, 1 μm), (b) 1 μm, and (c) 250 nm. Reproduced with permission from [262].

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Kravchenko et al. [264], extended the concept of azopolymer-based nanofabrication to patterning of silicon by using photoinduced SRGs as lithographic masks. They fabricated large-area of silicon nanostructures with feature sizes of the order of 100 nm (Fig. 17). The proposed fabrication technique is fast and straightforward, providing an attractive alternative to optical interference lithography based on conventional photoresists. Lithographic masks are made from azopolymers rather than conventional photoresists.

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Figure 17. (a) Schematic illustration of the fabrication process using a soft azopolymer mask, I) spin coating of an azopolymer thin film, II) SRG inscription, III) partial etching of the polymer in O2, IV) dry etching of Si, and V) stripping the mask. (b) An alternative fabrication process using an intermediate hard mask, I) deposition of Al2O3 (5 nm), amorphous silicon (20 nm), azopolymer (100 nm), and SRG inscription, II) partial etching of the polymer in O2, III) etching of amorphous silicon and Al2O3, IV) dry etching of Si, and V) stripping the mask. Reproduced with permission from [264].

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The fabrication procedure (Fig. 18) involves spin coating a thin film of azopolymer onto the top of a silicon substrate. Illuminating the surface with an optical interference pattern generates a surface-relief grating in the film. The next step is to partially etch the polymer by reactive-ion etching and then dry etch the silicon substrate in regions that do not contain the polymer mask. The final step is to strip away the remaining mask, leaving only the desired silicon pattern. Because the azopolymer mask is soft, etches in the silicon are slightly angled rather than perfectly rectangular, which limits the achievable etching depth of the technique. To overcome this problem, an advanced fabrication approach that adds a 20-nm layer of amorphous silicon and a 5-nm thick alumina layer beneath the azopolymer film was adopted. Alumina is more resistant than azopolymer to reactive-ion etching and thus functions as a hard mask with steep side walls. The amorphous silicon provides good adhesion to the azopolymer film. The amorphous silicon layer is etched through the soft mask using reactive-ion etching followed by wet etching of the alumina layer, after which the silicon is dry etched and the mask is stripped off. The use of azopolymers relaxes the environmental requirements of optical lithography, allowing it to function at longer wavelengths and under room lighting. The proposed technique is fast and cost effective in comparison with electron-beam lithography or optical nanolithography, as the azopolymer-based surface patterning does not require UV light sources and mask aligners. The phenomenon of induced photofluidization in azopolymers can be best understood by single-beam-induced surface deformations in the polymer, a feature that is discussed next together with the related nanopatterning capabilities.

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Figure 18. (a) An atomic force microscopy (AFM) image (top) and surface profile (bottom) of an azopolymer soft mask with a period of 320 nm and surface-modulation depth of 100 nm. (b) A scanning electron microscopy (SEM) image of an etched silicon wafer through a soft azopolymer mask. (c) a silicon wafer etched using different processing parameters compared to those in (b). Reproduced with permission from [264].

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4. Photomechanics and photofluidization in azopolymers

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Nanofabrication in photoresists
  5. 3. Nanofabrication in azopolymers
  6. 4. Photomechanics and photofluidization in azopolymers
  7. 5. Outlook and conclusions
  8. References
  9. Biographies

4.1. Far-field patterning

Most of the SRGs on azopolymers are fabricated by the interference pattern of two coherent laser beams [225, 226, 239]. However, there are few reports on surface deformations that are induced by a single focused laser beam [265-267]. To fabricate deformation structures with high spatial resolution, a small irradiation spot is required, a feature that can be achieved by focusing the laser beam by using a high numerical aperture (NA) objective lens. Single-beam-induced surface deformations give a clear insight into the mechanism of photoinduced mass movement of the polymer. Figure 19 shows AFM images of the surface deformation induced by (a, b) linear and (c) circular polarizations on an azopolymer film PMA-DR1 (Fig. 19). The laser beam was focused on the film surface. Irradiation with linearly polarized light induced the deformation pattern shown in Figs. 19a and b. It is clearly shown in this figure that the polymer moved along the polarization direction from the center to the outside of the focused spot, thus producing two side lobes along the polarization direction and a pit at the center. Indeed this polarization-dependent deformation was confirmed by an experiment in which the polarization direction of the irradiation light was rotated through an angle of 90 degrees and the induced pattern followed the polarization of the light (see Fig. 19a versus b). In contrast to irradiation with linear polarization, irradiation with circularly polarized light induced a deformation pattern in which the polymer moved from the center to the outside of the focused laser spot, thus forming a doughnut shape pattern (Fig. 19c).

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Figure 19. AFM images of the deformation induced by a tightly focused laser beam polarized (a) horizontally, (b) vertically, and (c) circularly, respectively. The irradiation intensity and the exposure time were 12.5 mw/cm2 and 30 s, respectively, and the laser beam was focused on the film surface. The film was prepared by spin casting from a chloroform solution. The remaining solvent was removed by heating the film for an hour at 100 °C. Reproduced with permission from [215].

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The observed polarization dependence is consistent with the one obtained after irradiation with a low-NA lens [265]. For both linear and circular polarizations, the polymer migrates in the direction of the light gradient from high to low light intensity regions, and the polarization dependence demonstrates that the light-induced polymer movement is anisotropically photofluidic [214]. When a laser beam is tightly focused by a high NA (∼1.4) objective lens, the electric-field component of the light along the polarization direction is predominant, e.g. 200 times larger than the inplane component perpendicular to the polarization direction and 7 times larger than the component in the optical axis, e.g. the z component.

Besides the anisotropic photofluidity, experiments performed by Ishitobi et al. [215] proof the existence of an optical gradient force in polymer mass migration. In these experiments, the z-position of the focus was varied with respect to the film surface. When the z-position was just on the film surface (z = 0 nm), the deformation pattern was the same as the one shown in Fig. 19a or b. It is interesting to note that at distances larger than 200 nm above the film surface in air, the polymer formed a protrusion coming out towards the center of the laser focus and suggesting the existence of a gradient force that pulls the polymer towards the region of maximum intensity (see Fig. 20). This is optical trapping of a viscoelastic polymer showing nanoelasticity over 20 nm, i.e. the maximum height of the protrusion obtained at z = + 500 nm. For z distances between 200 and 0 nm, the overlap of the laser intensity and the film are large enough to produce dips at the center as explained above. When the laser is focused into the glass substrate, there is no protrusion formed, because the polymer movement is blocked by the substrate.

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Figure 20. (Left column) Line plots of the surface deformation at (a) Z = +500 nm, (b) Z = 0 nm, and (c) Z = –500 nm. The positions of the each plot correspond to the line scan of the surface structure. (Right column) Schematics describing the relationship between the z-position of the focus and the film surface. The azopolymer film is red and the cover glass is blue. The center of the light beam, in green, is indicated by a white spot. The arrows oriented laterally and longitudinally in these schematics indicate the direction of the anisotropic photofluidity and the optical gradient force, respectively. Reproduced with permission from [215].

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The extent to which the polymer moves upon photoinduced mass movement is discussed next. For example, Ishitobi et al. [215] found that the size of the photoinduced deformation depends on the intensity of the irradiation light and the exposure time. The deformation patterns obtained at all intensities at all times of irradiation were the same for the same lens, but the size of the deformation was different. The height and the full width at half-maximum (FWHM) of the deformation pattern along the direction parallel to the light polarization depend on the irradiation intensity and the exposure time. The rate of the deformation of the height and FWHM decreased with increasing irradiation dose, and the higher the irradiation intensity, the faster the increase of both the height and FWHM. The height increases more rapidly than FWHM, which needs more time to reach saturation. The height increases rapidly at small irradiation doses, and saturates at larger irradiation doses near 90 nm, a value that corresponds to the film thickness. The azopolymer kept moving with larger irradiation doses exceeding the μm range for a diffraction-limited laser spot of 200 nm, a size that corresponds to the minimum FWHM of the fabricated pattern.

4.2. Near-field patterning

The contribution of an optical gradient force to the photoinduced mass movement of the polymer can also be seen in nanopatterning by a nanosource of light generated at the apex of a silver-coated AFM tip (Fig. 21). Nanopatterning using near-field optical probes can be found in recent reviews [268, 269]. Briefly, SPL provides a versatile set of tools for both manipulating and imaging the topography of a surface with atomic-scale resolution [270-273]. At present, these tools seem well suited for applications in research but will require substantial development before they can be used for patterning large areas in manufacturing. The most important SPL techniques include scanning tunneling microscopy (STM), AFM, and near-field scanning optical microscopy (NSOM). A striking example of the potential of these techniques for nanoscale fabrication is the precise positioning of individual Fe atoms with an STM tip [274, 275]. This atomic-scale manipulation is interesting scientifically but is not yet a practical technology. SPL can precisely position atoms on a surface and selectively deposit or remove regions of etch resist to pattern surfaces. These techniques may find applications in mask or device repair and information storage. Parallel approaches in SPL are being developed to overcome the serial limitations of standard SPL technologies. Nanofabrication by SPL is extensively reviewed elsewhere [271, 273, 268, 269, 276], and in this review we only emphasize the use of SPL in nanopatterning azopolymers.

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Figure 21. (top) (Left) SEM image of the silver-coated AFM tip. (Right) Schematic of the near-field surface deformation configuration. (bottom) AFM image of the near-field surface deformation. The polarization direction and the scale bar are indicated. (a) and (b) show the line profiles in the directions near parallel and near perpendicular to the incident light polarization, respectively. The FWHMs of the surface deformation in the parallel and perpendicular directions are also indicated. Reproduced with permission from [283].

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Nanopatterning of azopolymers by SPL has been studied by several authors [277-283]. Figure 21 shows an AFM image of a surface deformation photoinduced on an azopolymer film (PMA-DR1) with a silver-coated AFM tip in a near-field configuration as schematically depicted in Fig. 21. The irradiation (3 mW/cm2) was on the sample for 10 s, an irradiation dose which permitted fabrication by only the near-field component of the light. Larger irradiation doses lead to lateral polymer migration due to the far-field component of the light, as discussed previously. With such a low irradiation dose, e.g. that corresponding to 3 mW/cm2 and 10 s of irradiation, the protrusion was induced only with the tip present inside the focused spot, and no deformation was induced when the tip was away from it. The height of the protrusion was found to be 7 nm, and the lateral size was less than the diffraction-limited spot size, indicating that the surface deformation was induced by only the near-field component of the light. It is well known that the component of the electric field parallel to the tip axis (Ez) is effectively enhanced near the tip end due to the local surface plasmons [284]. The polymer under the tip is pulled by the optical gradient force that is generated by Ez [285, 286] and formed a nanoprotrusion. The nanolight spot was placed just onto the film surface, and it was kept on it while the protrusion was being formed during irradiation. Here too, the film was pulled by the optical gradient force towards the nanosource of light, e.g. the tip end, much like the case of a tightly focused beam on top of the film (vide infra). The field-enhancement effect due to the plasmonic metal tip allowed for the fabrication of near-field surface features with a light intensity that is as low as 3 mW/cm2.

Figure 21 also shows that the long axis of the deformation pattern was along a direction that is, not quite, but nearly parallel to the incident light polarization. This uniaxial anisotropic polymer movement suggests that the polymer moved along the polarization direction. The full width of half-maximum of the protrusion was found to be 65 and 47 nm in the direction nearly parallel (//) and perpendicular (⊥) to the incident light polarization, respectively. Here too, the anisotropic photofluidity of the polymer tends to induce a polymer mass movement in the direction parallel to the incident light polarization [225, 226]. This anisotropic polymer movement implies the presence under the tip end of a component of the electric field that is nearly parallel to the polarization direction [283]. The azopolymer helps map the electric field just at the tip apex. In fact, the fabricated feature that is due to Ex is much smaller than the one due to Ez and it confirms the well-known phenomenon that only the longitudinal component Ez drives efficient oscillations of the surface plasmons polaritons at the metal tip and leads to a strong field enhancement, i.e. enhancement of Ez, at the tip apex [285].

4.3. Photogenerating work from polymers

The previous subsections show that azopolymers can be moved by light irradiation over large distances, a mechanical work that is fuelled by the trans[LEFT RIGHT ARROW]cis isomerisation cycling of azobenzene derivatives. Such molecules are in fact machines doing work by reversibly changing shape and size between an elongated trans form and a more globular cis form [287]. This shows that the properties of a host material, such as mechanical properties, can be controlled by the change in shape of a guest photoactive unit. Such a chromophore for reversible shape change is the rhodopsin-retinal protein photoswitch system that enables vision, can be found in nature. Perhaps the best artificial mimic of this strong photoswitching effect is that of azobenzene derivatives. Besides light-induced mass movement, photoinduced contraction and bending can also generate work from azopolymers. Such light-sensitive systems may be referred to as light-energy transducers, and a nice review perspective about light-to-work transduction in photosensitive systems was recently published by White [288]. Light-to-work transduction, observed as uniaxial contraction/expansion cycles upon irradiation, was first observed by Angolini and Gay in the synthesis and photomechanical characterization of azobenzene-functionalized aromatic polyimides, subsequently, a variety of novel synthetic efforts reported a number of materials capable of transducing light into mechanical work mostly based on azobenzene, although spiropyran and other photochromic moieties have been studied, either covalently attached or included as a guest in both glassy and elastomeric amorphous, semicrystalline, and liquid-crystalline polymeric materials [289-296]. It is beyond the scope of this paper to exhaustively review these materials and their subsequent photomechanical responses. Rather, we will focus on contraction and bending effects leading to work in some liquid-crystalline elastomers.

In azobenzene-functionalized liquid-crystal polymer networks (azo-LCNs), photoinduced order–disorder transitions have been widely used to trigger uniaxial contraction of thin films or bending of cantilevers [297-317]. For example, White's group [318] reported the photomechanical response of cantilevers composed of acrylate-based glassy, polydomain azo-LCNs, observed in the bending of cantilevers, to irradiation from both UV and blue-green laser exposure as a function of linear polarization. Figure 22 shows the comparative photomechanical response of a representative azo-LCN material to irradiation with 375-nm and 442-nm light. Figure 22a shows the response to 442-nm irradiation (100 mW/cm2), and Fig. 22b the response to 375-nm irradiation (50 mW/cm2). The azo-NLC cantilever was identical for both experiments (5 mm x 1 mm x 15 μm : length, width, and thickness), and it was entirely covered by the irradiation light, in such a way that there is no gradient of light at the film sample, and consequently no surface relief created. The cantilever is originally vertical (Fig. 22a-i). Upon irradiation with 442-nm light polarized parallel to the long axis (E // x) of the cantilever, the cantilever bends approximately 21° in the direction of the incident exposure (Fig. 22a-ii). When the polarization direction of the incident 442-nm irradiation is rotated such that it is orthogonal to the long axis of the cantilever (Ex), the direction of the cantilever reverses to approximately –22° (Fig. 22a-iv). Irradiation of glassy azo-LCN materials with 442-nm irradiation fixes the material in temporary shapes. This optical fixing is evident in Figs. 22a-iii and a-v upon removal of the 442-nm irradiation and can retain this temporary state for many months.

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Figure 22. (a) Polarization-controlled forward and reverse bending in a PD20CL-80CL cantilever. i) Before exposure, ii) after exposure to 100 mWcm2 442 nm light polarized parallel to the long axis of the cantilever (E//x), iii) shape fixing observed one hour after removal of irradiation, iv) after exposure of i) or ii) to 100 mWcm2 442-nm light polarized orthogonal to the long axis of the cantilever (Ex), and v) shape fixing observed one hour after removal of irradiation. (b) Bending observed in PD20CL-80CL cantilever upon exposure to 50 mWcm2 375-nm light polarized parallel to the long axis of the cantilever. i) Before exposure, ii–vi) during UV exposure, vii) 16 h after exposure. (c) The temporal response for bending of the cantilevers observed upon exposure to 375 nm (blue disk) and 442 nm (red square) with E//x is summarized. Reproduced with permission from [318].

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Figure 22b shows the response to 375-nm light polarized parallel to the long axis of the cantilever (E//x). As is apparent in Fig. 22b-ii, the cantilever initially bends to a large angle. The directionality and magnitude of the bending is initially similar to that of 442-nm irradiation evident in Fig. 22a-ii. However, upon continued irradiation to 375 nm the magnitude of the bending angle of the cantilever lessens and eventually reverses before eventually returning to the original vertical position (images in Figs. 22b-ii–vi). In contrast to the optical fixing observed to 442-nm irradiation, if the UV light is shuttered after the cantilever is bent to Fig. 22b-ii the magnitude of the bending gradually lessens over approximately 16 h, eventually restoring the film to the original vertical position (Fig. 22b-vii). Figure 22c directly compares the temporal response of the cantilever to 375-nm and 442-nm irradiation. Clearly, the responses observed at both irradiation wavelengths mirror the photochemical reaction, e.g. while 375-irradiation depletes the trans isomer leaving the surface of cantilever facing the irradiation light with cis content mostly, and 442-nm reaction in both directions occur [216, 319]. The bending and contraction occur in such films because the actinic light is absorbed only in the top fraction of the film cantilever (15 mm thick), thereby creating a gradient of mechanical properties of the film in the direction perpendicular to the film thickness, e.g. the mechanical properties of one side of the film become different from those of the other side under light irradiation.

Taking advantage of photoinduced changes of the mechanical properties of azopolymers, Ikeda's group [320] have developed a motor, based on a plastic liquid-crystalline elastomer (LCE) containing an azobenzene derivative, that can convert light energy into mechanical work (Fig. 23). A motor device converts input energy directly into a continuous rotation. A plastic belt of the LCE laminated film was prepared by connecting both ends of the film, and then the belt was placed on a custom-made pulley system as illustrated in Fig. 18a. By irradiating the belt with UV light from top right and visible light from top left simultaneously, the belt rotated to drive the two pulleys in a counterclockwise direction at room temperature (Fig. 23b). The machine, e.g. a motor, was driven by contraction and expansion forces due to UV and visible light irradiation, respectively. Upon exposure to UV light, a local contraction force is generated at the irradiated part of the belt near the right pulley along the alignment direction of the azobenzene mesogens, which is parallel to the long axis of the belt. This contraction force acts on the right pulley, leading it to rotate in the counterclockwise direction. At the same time, the irradiation with visible light produces a local expansion force at the irradiated part of the belt near the left pulley, causing a counterclockwise rotation of the left pulley. These contraction and expansion forces produced simultaneously at the different parts along the long axis of the belt give rise to the rotation of the pulleys and the belt with the same direction. The rotation then brings new parts of the belt to be exposed to UV and to visible light, which enables the motor system to rotate continuously. Reverse rotation of this belt could also be induced just by changing the irradiation positions of the UV and visible light.

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Figure 23. A light-driven plastic motor with the LCE laminated film. (a) Schematic illustration of a light-driven plastic motor system used in this study, showing the relationship between light-irradiation positions and a rotation direction. (b) Series of photographs showing time profiles of the rotation of the light-driven plastic motor with the LCE laminated film induced by simultaneous irradiation with UV (366 nm, 240 mW/cm2) and visible light (>500 nm, 120 mW/cm2) at room temperature. Diameter of pulleys, 10 mm (left), 3 mm (right). Size of the belt, length, 36 mm, width, 5.5 mm. Thickness of the layers of the belt, PE, 50 μm, LCE, 18 μm. Reproduced with permission from [320].

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Azopolymers parent to photoresists in their ability to be structured by light, however, the fundamental photochemical mechanisms that govern structuring in both kinds of material lead to quite different, albeit complementary, effects in azopolymers versus photoresists. Although several reviews have appeared on fs-laser polymerization in photoresists, such as for example references [321-323], our review (i) summarizes the latest results in the field, and (ii) connects photoresists to azopolymers. For example, azopolymers can be used instead of photoresists as lithographic masks to pattern silicon, and the fabrication process is fast and straightforward, providing an interesting alternative to photoresists [264]. Furthermore, the photochemical mechanism of patterning azopolymers is fundamentally different from that of patterning photoresists, and it adds more options to patternable materials. Photoinduced mass movement in azopolymers and photoinduced change in their mechanical properties, also discussed in this review, bring about quite exciting applications such as generating work from soft materials, a property that could be exploited in possible future generations of smart materials that bring together the patternability of resists and the photofunctionalities of azopolymers. If the intrinsic properties of both materials are combined one might expect the emergence of the next-generation highly functional nanophotonic devices such as photofunctional photonics crystals.

5. Outlook and conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Nanofabrication in photoresists
  5. 3. Nanofabrication in azopolymers
  6. 4. Photomechanics and photofluidization in azopolymers
  7. 5. Outlook and conclusions
  8. References
  9. Biographies

The unconventional techniques of nanofabrication discussed in this review cover technologies of laser nanofabrication in photosensitive materials, namely resins and azopolymers, and show that photolithography continues to overcome obstacles to achieve new resolution requirements, especially with the advent of STED nanofabrication, as well as increasing complexity in fabricated 2- and 3-dimensional nanostructures. The applications of laser nanofabrication encompass photonics crystals and metamaterials and biocompatible scaffolds for tissue engineering. The expansion of two-photon fabrication as a production tool looks very promising. Two-photon technology machines are commercially available, and the technology has started to expand and industrial interest is growing, and the number of specially designed materials available has increased, and there is also the potential of increasing the processing speed using multibeam parallel processing. The azopolymer option as a photosensitive material for nanopatterning is starting to emerge and there are lots of intriguing, albeit, highly important issues to be studied related to creating work by light from such polymers. It is worth noting that the impressive resolution, beyond the diffraction limit of light, obtained by STED in laser writing in photoresists could open new avenues in diffraction-unlimited photostructuring in azopolymers.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Nanofabrication in photoresists
  5. 3. Nanofabrication in azopolymers
  6. 4. Photomechanics and photofluidization in azopolymers
  7. 5. Outlook and conclusions
  8. References
  9. Biographies

Biographies

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Nanofabrication in photoresists
  5. 3. Nanofabrication in azopolymers
  6. 4. Photomechanics and photofluidization in azopolymers
  7. 5. Outlook and conclusions
  8. References
  9. Biographies
  • Image of creator

    Zouheir Sekkat is Professor at University Mohamed V, Department of Chemistry, Rabat, and the director of the Optics and Photonics Centre (OPC) of the Moroccan Foundation for Science, Innovation and Research (MAScIR). Sekkat completed his Master and PhD and Habilitation degrees, all from Paris-Sud University, Orsay. Sekkat did post-doctoral research stays Max-Planck Institute for Polymer Research (Prof. W. Knoll), and at CPIMA jointly between the University of California-Davis and IBM-Almaden at San Jose, and Stanford. In 1999 Zouheir Sekkat joined Osaka Univeristy as an Associate Professor of Applied Physics.

  • Image of creator

    Satoshi Kawata received his BSc and PhD in Applied Physics from Osaka University in 1974 and 1979 respectively. After working in University of California, Irvine as a postdoctoral scientist, he joined Osaka University as a faculty member, where he is now the Professor of Applied Physics and Executive Director of Photonics Center. In 2002 he joined RIKEN as a Chief Scientist as a head of Nanophotonics Laboratory. He is a Fellow of OSA, IOP, SPIE, and JSPS. The “8-micron bull” fabricated with his invented two-photon technology has been awarded in Guinness World Record Book 2004 Edition.