Laser & Photonics Reviews

In the past two decades, semiconductor quantum dots and wires have developed into new, promising classes of materials for next-generation lighting and display systems due to their superior optical properties. In particular, exciton–exciton interactions through nonradiative energy transfer in hybrid systems of these quantum-confined structures have enabled exciting possibilities in light generation. This review focuses on the excitonics of such quantum dot and wire emitters, particularly transfer of the excitons in the complex media of the quantum dots and wires. Mastering excitonic interactions in low-dimensional systems is essential for the development of better light sources, e.g., high-efficiency, high-quality white-light generation; wide-range color tuning; and high-purity color generation. In addition, introducing plasmon coupling provides the ability to amplify emission in specially designed exciton–plasmon nanostructures and also to exceed the Förster limit in excitonic interactions. In this respect, new routes to control excitonic pathways are reviewed in this paper. The review further discusses research opportunities and challenges in the quantum dot and wire excitonics with a future outlook.

A dual-wavelength monolithic Y-branch distributed Bragg reflection (DBR) diode laser at 671 nm is presented. The device is realized with deeply etched surface DBR gratings by one-step epitaxy. A maximum optical output power of 110 mW is obtained in cw-operation for each laser cavity. The emission wavelengths of the device are 670.5 nm and 671.0 nm with a spectral width of 13 pm (0.3 cm⁻¹) and a mean spectral distance of 0.46 nm (10.2 cm⁻¹) over the whole operating range. Together with a free running power stability of ± 1.1% this most compact diode laser is ideally suited as an excitation light source for portable shifted excitation Raman difference spectroscopy (SERDS).

Femtosecond-laser micromachining (also known as inscription or writing) has been developed as one of the most efficient techniques for direct three-dimensional microfabrication of transparent optical materials. In integrated photonics, by using direct writing of femtosecond/ultrafast laser pulses, optical waveguides can be produced in a wide variety of optical materials. With diverse parameters, the formed waveguides may possess different configurations. This paper focuses on crystalline dielectric materials, and is a review of the state-of-the-art in the fabrication, characterization and applications of femtosecond-laser micromachined waveguiding structures in optical crystals and ceramics. A brief outlook is presented by focusing on a few potential spotlights.

The laser is a very powerful and very useful instrument in modern nanoscience and nanotechnology. The knowledge of the interaction mechanism of the laser beam with nanoparticles is needed to control the laser processing of different nano-objects. It was shown that the particle heating–melting–evaporation model can be successfully applied for many phenomena arising when colloidal nanoparticle interact with pulsed laser beams. The general approach of this model is discussed in detail. The two main components of the model, light absorption by particles, and the thermodynamics of phase transitions in particulate material are considered. Special attention is devoted to the correct estimation of the possible heat losses. The way in which the phase diagrams, where the different phase conditions of particle material are presented in laser fluence−particle diameter coordinates, were produced is demonstrated. It is shown how this model can be applied for understanding the mechanism of such complicated processes as particle-size reduction and submicrometer spherical particle growth, as well as other processes that occur when colloidal particles are irradiated by a pulsed laser.

Spatial distribution of superficial blood vessels in human skin in vivo has been observed by using the double correlation Optical Coherence Tomography (OCT). To remove background noise, reduce the artifacts associated with patient motions and to increase the overall quality of the experimental OCT images an adaptive Wiener filtering technique has been employed. Fourier domain correlation has been subsequently applied to enhance spatial resolution of images of vascular network in human skin in vivo. Image processing has been performed on Graphics Processing Units (GPUs) utilizing Compute Unified Device Architecture (CUDA) framework in the frequency-domain. This approach allows carrying out image processing in parallel significantly speeding up the computations. The presented results show that the double correlation method permits obtaining 2D/3D OCT images of subcutaneous microcirculation vascular network and its spatial distribution within the human skin with higher spatial resolution compare to the other OCT correlation-based techniques developed earlier.

This paper reviews recent progress in the nascent field of semiconductor optical fibres, from the fundamentals through to device demonstration. The incorporation of semiconductor materials into both the step-index and microstructured fibre geometries provides a route to introducing new optoelectronic functionality into existing glass fibre technologies. Herein, the various fabrication methods that have been developed as of to date are described, and their compatibility with the different semiconductor materials and fibre designs discussed. Results will be presented on the optical transmission properties of several fibre types, with particular attention being paid to the observation of nonlinear propagation in silicon core fibres. Finally, some speculation regarding the future prospects and applications of this new class of fibre will be provided.

Low-level light therapy (LLLT) using red to near-infrared (NIR) (630–1000 nm) light has gained attention in recent years as a therapy in ophthalmology, neurology, dermatology, dentology, and regenerative medicine. Advancement in the basic science fields of photobiology has propelled LLLT into the therapeutic revolution. The potential mechanisms on LLLT-induced biological effects have been investigated by numerous researchers throughout the world. This article reviews the current intracellular signaling cascades in photobiology and photomedicine under the influence of red to NIR light on mammalian cells. Specifically, mitochondrial retrograde signaling initiated by cytochrome c oxidase photomodulation is discussed in detail in the treatment of indications using LLLT, such as vitiligo management, retinal protection, and tumor therapy. The pathways through activating receptor tyrosine kinases are also highlighted in LLLT-induced neuroprotection, wound healing, and skeletal muscle regeneration. The understanding of the LLLT-induced biological reactions in cellular and subcellular levels is crucial for the advancement of LLLT in treatment of diseases.

A novel fiber technology is presented that enables the transmission of 200 nm wide spectra over meter-long distances with minimal temporal reshaping and acceptable losses down to about 3 dB/m. Delivery of a 10 fs pulse over nearly meter distance is experimentally demonstrated, which sets a new standard for the fiber-based delivery of few-cycle pulses. Numerical simulations provide insight into the unique guiding mechanism in the novel hollow-core fiber technology, enabling dispersion parameters that are within an order of magnitude of those available in free space propagation.

Symmetric anisotropy in wurtzite semiconductors, e.g., AlGaN, has led to the significant optical anisotropy that is rather difficult to resolve. Here, a novel scheme for achieving optical isotropization in Al-rich AlGaN through the introduction of additional asymmetric elements is demonstrated to compensate the native asymmetry. Asymmetric modulation of alloy composition and periodicity of (GaN)_m/(AlN)_n superlatices was proposed with first-principles simulations. Results showed that the compensation for the c-axial symmetry with the asymmetric ultrathin (GaN)_m/(AlN)_n superlatices (m ≤ 2) could well achieve the equivalence of the ordinary and extraordinary imaginary dielectric functions ε₂ at the band edge. Measurement with spectroscopic ellipsometry for this (GaN)_m/(AlN)_n superlatice insertion in AlGaN host confirmed the theoretical predictions of the optical isotropization. This method can be transferred to other semiconductors in anisotropic structure and with troubles of optical anisotropy.

The defect chalcopyrite crystal HgGa₂S₄ has been employed in a 1064-nm pumped optical parametric oscillator operating at 100 Hz, to generate ∼5 ns long idler pulses near 4 µm with energies as high as 6.1 mJ and average power of 610 mW. At crystal dimensions comparable to those available for the commercial AgGaS₂ crystal, operation of the 1064-nm pumped HgGa₂S₄ OPO is characterized by much lower pump threshold and higher conversion efficiency, with the most important consequence that such a device might become practical at pump levels sufficiently lower than the optical damage threshold.

The spontaneous fluorescence background in optical parametric amplifiers is generally attributed to the zero-point fluctuations of the electromagnetic field. These are amplified in parallel to the seed light and lead to an uncompressible superfluorescence background that deteriorates the contrast in optical parametric chirped pulse amplifiers (OPCPA). The absolute level of the underlying parametric fluorescence has not been reported so far. Comparing the fluorescence to low level cw seed light and quantitatively monitoring the output of a noncollinear optical parametric amplifier for both sources, the level is now determined. In a situation of 50 nm visible output bandwidth and low Gaussian spatial modes about 58 photons are found in the signal direction within the femtosecond time window of the amplifier. The superfluorescence level is observed to be proportional to the pump area for constant signal amplification. The implications for the background in high power OPCPA are discussed.

An integrated intra-laser-cavity microparticle sensor based on a dual-wavelength distributed-feedback channel waveguide laser in ytterbium-doped amorphous aluminum oxide on a silicon substrate is demonstrated. Real-time detection and accurate size measurement of single micro-particles with diameters ranging between 1 µm and 20 µm are achieved, which represent the typical sizes of many fungal and bacterial pathogens as well as a large variety of human cells. A limit of detection of ∼500 nm is deduced. The sensing principle relies on measuring changes in the frequency difference between the two longitudinal laser modes as the evanescent field of the dual-wavelength laser interacts with micro-sized particles on the surface of the waveguide. Improvement in sensitivity far down to the nanometer range can be expected upon stabilizing the pump power, minimizing back reflections, and optimizing the grating geometry to increase the evanescent fraction of the guided modes.

The traditional optical frequency comb (OFC) based microwave photonic filters (MPFs) are rigidly restricted to be operated in a single “Nyquist zone”, as varieties of spurious frequencies signals coexist in the output. Here, a method for spurious frequencies suppression in the OFC-based MPF is proposed and experimentally demonstrated. The method is achieved by applying group velocity dispersion on the carrier combs to separate the filter transfer functions of the spurious frequencies from that of the input radio frequency signal. It is fairly simple and effective, and has no effect on the filter characteristic. With this method, the filter pass band can be freely tuned without the limitation of the “Nyquist zone”. It can be considered as a step forward for the practical application of the OFC-based MPF.

Multi-shot pulse-shape measurements of trains of ultrashort pulses with unstable pulse shapes are studied. Measurement techniques considered include spectral-phase interferometry for direct electric-field reconstruction (SPIDER), second harmonic generation frequency-resolved optical gating (FROG), polarization gate FROG, and cross-correlation FROG. An analytical calculation and simulations show that SPIDER cannot see unstable pulse-shape components and only measures the coherent artifact. Further, the presence of this instability cannot be distinguished from benign misalignment effects in SPIDER. FROG methods yield a better, although necessarily rough, estimate of the pulse shape and also indicate instability by exhibiting disagreement between measured and retrieved traces. Only good agreement between measured and retrieved FROG traces or 100% SPIDER fringe visibility guarantees a stable pulse train.

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.

Optical resonators are important devices that control the properties of light and manipulate light–matter interaction. Various optical resonators are designed and fabricated using different techniques. For example, in coupled resonator optical waveguides, light energy is transported to other resonators through near-field coupling. In recent years, magnetic optical resonators based on LC resonance have been realized in several metallic microstructures. Such devices possess stronger local resonance and lower radiation loss compared with electric optical resonators. This study provides an overall introduction on the latest progress in coupled magnetic resonator optical waveguide (CMROW). Various waveguides composed of different magnetic resonators are presented and Lagrangian formalism is used to describe the CMROW. Moreover, several interesting properties of CMROWs, such as abnormal dispersions and slow-light effects, are discussed and CMROW applications in nonlinear and quantum optics are shown. Future novel nanophotonic devices can be developed using CMROWs.

This paper reviews the state-of-the-art of grating fabrication in silica and polymer microstructured optical fibres. It focuses on the difficulties and challenges encountered during photo-inscription of such gratings and more specifically on the effect of the air hole lattice microstructure in the cladding of the fibre on the transverse coupling of the coherent writing light to the core region of the fibre. Experimental and computational quantities introduced thus far to assess the influence of the photonic crystal lattice on grating writing efficiency are reviewed as well, together with techniques that have been proposed to mitigate this influence. Finally, early proposals to adapt the microstructure in view of possibly enhancing multi-photon grating fabrication efficiency are discussed.

Surface plasmon propagating modes supported by metal/dielectric interfaces in various configurations can be used for radiation guiding similarly to conventional dielectric waveguides. Plasmonic waveguides offer two attractive features: subdiffraction mode confinement and the presence of conducting elements at the mode-field maximum. The first feature can be exploited to realize ultrahigh density of nanophotonics components, whereas the second feature enables the development of dynamic components controlling the plasmon propagation with ultralow signals, minimizing heat dissipation in switching elements. While the first feature is yet to be brought close to the domain of practical applications because of high propagation losses, the second one is already being investigated for bringing down power requirements in optical communication systems. In this review, the latest application-oriented research on radiation modulation and routing using thermo-optic dielectric-loaded plasmonic waveguide components integrated with silicon-based photonic waveguides is overviewed. Their employment under conditions of real telecommunications is addressed, highlighting challenges and perspectives.

The recently introduced Gas in Scattering Media Absorption Spectroscopy (GASMAS) technique provides novel possibilities for analysis in biophotonics. Free gas in pores or cavities is monitored with narrow-band laser radiation, which can discern the gas absorptive imprints which are typically several orders of magnitude more narrow than the features of the surrounding tissue through which the diffusely scattered light emerges to the detector. Important gases monitored are oxygen and water vapour. Applications include diagnosis of human sinus cavities and surveillance of neonatal children, but also characterization of food-stuffs, food packages and pharmaceutical preparations. Non-biological applications include the study of construction materials such as wood, polystyrene foams and ceramics. For nano-porous materials, information on the pore sizes can be obtained from observed line broadening. Apart from concentration measurements, the GASMAS technique also allows the study of gas transport and diffusion, and pressure and temperature information can also be obtained.

Fluorescence spectroscopy and imaging have been widely used for in vivo cancer diagnosis and therapy monitoring in preclinical models, as well as clinical translation. Great attempts have been made to develop novel fluorescence techniques and improve on existing ones, which can now be used in conjunction with newly developed fluorescent probes for specific cancer imaging. In this review, a broad overview of fluorescence techniques is provided, including photodynamic diagnosis, laser confocal endomicroscopy and fluorescence lifetime imaging, coupled with endogenous and exogenous fluorophores. In particular, endogenous fluorophores, such as nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD), are highlighted as they are linked to cellular metabolism in precancer growth. The use of near-infrared dyes, such as indocynanine green (ICG), for imaging deep-tissue regions is also reviewed. In addition, diagnostic algorithms used for tissue classification and cancer detection will be discussed. Lastly, emerging technologies in fluorescence diagnosis will also be included.

Orthogonal frequency division multiplexing (OFDM) can provide spectrally efficient communication channels because it can utilize carrier orthogonality and various impairment mitigation methods. An optical OFDM signal can be generated electronically to multiplex lower-rate carriers. In recent advancements, OFDM signals are also shown to be generated and demultiplexed by all-optical discrete Fourier transform (DFT), overcoming the speed limit of electronics for >Tbps capacity. High-performance DFT devices, such as arrayed waveguide grating (AWG) or planar lightwave circuit (PLC), are critically required to obtain strong orthogonality for scalable all-optical OFDM (AO-OFDM) system implementations. Advanced techniques such as coherent modulation and detection with digital impairment mitigation are also important for long-reach AO-OFDM transmissions. More recently, optical superchannel schemes have been introduced utilizing coherent detection for multi-Tbps AO-OFDM transmissions. This paper reviews the device and system aspects for the AO-OFDM technology, including a generalized theoretical model to provide an indepth understanding.

Tissue optical clearing technique provides a prospective solution for the application of advanced optical methods in life sciences. This paper gives a review of recent developments in tissue optical clearing techniques. The physical, molecular and physiological mechanisms of tissue optical clearing are overviewed and discussed. Various methods for enhancing penetration of optical-clearing agents into tissue, such as physical methods, chemical-penetration enhancers and combination of physical and chemical methods are introduced. Combining the tissue optical clearing technique with advanced microscopy image or labeling technique, applications for 3D microstructure of whole tissues such as brain and central nervous system with unprecedented resolution are demonstrated. Moreover, the difference in diffusion and/or clearing ability of selected agents in healthy versus pathological tissues can provide a highly sensitive indicator of the tissue health/pathology condition. Finally, recent advances in optical clearing of soft or hard tissue for in vivo imaging and phototherapy are introduced.

Nanofabricated metallic tips are at the core of important research in single-molecule imaging, near-field scanning optical microscopy, tip-enhanced Raman spectroscopy, as well as potential commercial applications such as heat-assisted magnetic recording. While challenging to fabricate, much progress has been made towards the reliable production of extremely sharp (10 nm) metallic probes. In this review, we discuss the various factors that go into the design of metallic tips, their fabrication, packaging and system integration, characterization, passivation, and eventual use. Fabrication challenges, implementation issues, optical excitation schemes, and various current and emerging applications are also discussed. For the rapidly emerging fields of plasmonics and nanophotonics, the use of sharp metallic tips has generated significant scientific progress and will play an integral role well into the future.

Photoacoustic microscopy (PAM) is a hybrid in vivo imaging technique that acoustically detects optical contrast via the photoacoustic effect. Unlike pure optical microscopic techniques, PAM takes advantage of the weak acoustic scattering in tissue and thus breaks through the optical diffusion limit (∼1 mm in soft tissue). With its excellent scalability, PAM can provide high-resolution images at desired maximum imaging depths up to a few millimeters. Compared with backscattering-based confocal microscopy and optical coherence tomography, PAM provides absorption contrast instead of scattering contrast. Furthermore, PAM can image more molecules, endogenous or exogenous, at their absorbing wavelengths than fluorescence-based methods, such as wide-field, confocal, and multi-photon microscopy. Most importantly, PAM can simultaneously image anatomical, functional, molecular, flow dynamic and metabolic contrasts in vivo. Focusing on state-of-the-art developments in PAM, this Review discusses the key features of PAM implementations and their applications in biomedical studies.

This article is mainly devoted to the modeling and measurement of the absorption and fluorescence angular distributions in polarized light of monoclinic crystals. Up to now theoretical crystal optics were mostly devoted to crystals having a high crystallographic symmetry. In these crystals belonging to the cubic, hexagonal, tetragonal, trigonal or orthorhombic lattice classes, the tensor properties related to the real part of the dielectric permittivity and to its imaginary part can be described in the same frame which orientation does not vary as a function of wavelength. The situation is much more complicated in the case of monoclinic crystals because it is necessary to define a specific frame for each property and each wavelength that are considered. The main features of monoclinic crystal optics are described in detail, followed by a review of monoclinic materials and the consequence of these features on their related optical properties.

Microwave photonics (MWP) is an emerging field in which radio frequency (RF) signals are generated, distributed, processed and analyzed using the strength of photonic techniques. It is a technology that enables various functionalities which are not feasible to achieve only in the microwave domain. A particular aspect that recently gains significant interests is the use of photonic integrated circuit (PIC) technology in the MWP field for enhanced functionalities and robustness as well as the reduction of size, weight, cost and power consumption. This article reviews the recent advances in this emerging field which is dubbed as integrated microwave photonics. Key integrated MWP technologies are reviewed and the prospective of the field is discussed.

Upconverting nanoparticles (UCNPs) are a class of recently developed luminescent biomarkers that – in several aspects – are superior to organic dyes and quantum dots. UCNPs can emit spectrally narrow anti-Stokes shifted light with quantum yields which greatly exceed those of two-photon dyes for fluence rates relevant for deep tissue imaging. Compared with conventionally used Stokes-shifting fluorophores, UCNP-based imaging systems can acquire completely autofluorescence-free data with superb contrast. For diffuse optical imaging, the multi-photon process involved in the upconversion process can be used to obtain images with unprecedented resolution. These unique properties make UCNPs extremely attractive in the field of biophotonics. UCNPs have already been applied in microscopy, small-animal imaging, multi-modal imaging, highly sensitive bioassays, temperature sensing and photodynamic therapy. In this review, the current state-of-the-art UCNPs and their applications for diffuse imaging, microscopy and sensing targeted towards solving essential biological issues are discussed.

Although silver nanowires as plasmonic components have been investigated extensively in both theoretical and experimental studies, a systematic study is still lacking. In this work, a review is given to explain some basic features of experimentally prepared nanowires and their optical properties in different situations, such as waveguides, resonators, and antennas. The review also lists several possible applications of nanowires for enhanced light-emitting, photonic device fabrication, sensors, lasers, and nonlinear optics. Combined with the merits of both nanowires and surface plasmon polaritons, silver nanowires are certain to show their potential in photonics in the near future.

Observing chirality changes as they occur is an important topic of research. It provides information that deepens the understanding of biomolecular configuration and conformation under environmental changes. Also, knowing the specific steps in chiral synthesis would simplify the production of specific chiral enantiomers that have a specific function. To gain better insight to the initial steps of conformational and configurational changes, the time-resolution of chiral spectroscopy is continually pushed toward a shorter time-scale. Recent advances have produced measurements of chirality changes with a femtosecond time-resolution. These measurements are hindered by the inherently weak chirality signal, which can be overshadowed by different optical artefacts. This minireview will look at the so far successful techniques which measure chirality changes with femtosecond time-resolution and discuss the advantages and disadvantages of these techniques. A short outlook will also look at new techniques that could improve the ability to measure chirality changes on an ultrafast time-scale.

This review focuses on fiber optic probes for linear and nonlinear Raman spectroscopy, especially for medical applications. It aims at providing an overview over contemporary technology, recent first clinical trials, and helps identifying future developments necessary to bring the emerging technology to clinical end users. After a short introduction to linear and nonlinear Raman spectroscopic modalities, general design considerations will be discussed and compared to common fiber probe setups. Subsequently, examples for medical applications of fiber optic Raman probes will be given concentrating on probes for linear Raman spectroscopy as these devices are technologically more mature compared to their counterparts based on nonlinear Raman spectroscopy. The review also includes a brief summary of first multimodal fiber optic probes and highlights their benefits for clinical applications. Finally, probes are introduced which employ either nonlinear Raman spectroscopy or surface enhanced spectroscopy.

As typical one-dimensional nanostructures for waveguiding tightly confined optical fields beyond the diffraction limit, metal nanowires have been using as versatile nanoscale building blocks for functional plasmonic and photonic structures and devices. Metal nanowires, especially those fabricated by bottom-up synthesis such as Ag and Au nanowires, usually exhibit excellent diameter uniformity and surface smoothness with diameters down to tens of nanometers, which offers great opportunities for plasmonic waveguiding of optical fields with deep-subwavelength confinement, coherence maintenance and low scattering losses. Based on nanowire plasmonic waveguides, a variety of applications ranging from plasmonic couplers, interferometers, resonators to photon emitters have been reported in recent years. In this article, significant progresses in these nanowire plasmonic waveguides, circuits and devices are reviewed. Future outlook and challenges are also discussed.

Optics is usually integrated into robotics as part of intelligent vision systems. At the microscale, however, optical forces can cause significant acceleration and so optical trapping and optical manipulation can enable the noncontact actuation of microcomponents. Microbeads are ubiquitous optically actuated structures, from Ashkin's pioneering experiments with polystyrene beads to contemporary functionalized beads for biophotonics. However, micro- and nanofabrication technologies are yielding a host of novel synthetic structures that promise alternative functionalities and new exciting applications. Recent works on the actuation of synthetic microstructures using optical trapping and optical manipulation are examined in this review. Extending the optical actuation down to the nanoscale is also presented, which can involve either direct manipulation of nanostructures or structure-mediated approaches where the nanostructures form part of larger structures that are suitable for interfacing with diffraction-limited optical fields.

Optical tweezers, a simple and robust implementation of optical micromanipulation technologies, have become a standard tool in biological, medical and physics research laboratories. Recently, with the utilization of holographic beam shaping techniques, more sophisticated trapping configurations have been realized to overcome current challenges in applications. Holographically generated higher-order light modes, for example, can induce highly structured and ordered three-dimensional optical potential landscapes with promising applications in optically guided assembly, transfer of orbital angular momentum, or acceleration of particles along defined trajectories. The non-diffracting property of particular light modes enables the optical manipulation in multiple planes or the creation of axially extended particle structures. Alongside with these concepts which rely on direct interaction of the light field with particles, two promising adjacent approaches tackle fundamental limitations by utilizing non-optical forces which are, however, induced by optical light fields. Optoelectronic tweezers take advantage of dielectrophoretic forces for adaptive and flexible, massively parallel trapping. Photophoretic trapping makes use of thermal forces and by this means is perfectly suited for trapping absorbing particles. Hence the possibility to tailor light fields holographically, combined with the complementary dielectrophoretic and photophoretic trapping provides a holistic approach to the majority of optical micromanipulation scenarios.