Bio-Inspired Band-Gap Tunable Elastic Optical Multilayer Fibers

The concentrically-layered photonic structure found in the tropical fruit Margaritaria nobilis serves as inspiration for photonic fibers with mechanically tunable band-gap. The fibers show the spectral filtering capabilities of a planar Bragg stack while the microscopic curvature decreases the strong directional chromaticity associated with flat multilayers. Elongation of the elastic fibers results in a shift of the reflection of over 200 nm.


DOI: 10.1002/adma.201203529
Knowledge of the interplay between the morphology, composition and optical appearance of biological photonic systems can provide broad inspiration for novel artifi cial photonic elements. [1][2][3] On occasion, the study of natural photonics yields specifi c design templates for optical technologies. [4][5][6][7][8][9] To this end, we present the results of the investigation of the hierarchical photonic structure discovered in the seed coat of Margaritaria nobilis fruits, which directly inspired our creation of novel photonic fi bers. The fruit's hue results from the interference of light within a concentrically-layered architecture found inside individual cells in the seed's outer tissue layers. The natural structure presents two codependent, technologically exploitable features for light and color manipulation: regularity on the nanoscale that is superposed with microscale cylindrical symmetry, resulting in wavelength selective scattering of light in a wide range of directions. This is the foundation for novel soft bioinspired photonic fi bers with the spectral fi ltering capabilities and color brilliance of a planar Bragg stack compounded with a large angular scattering range introduced by the microscale curvature, which also decreases the strong directional chromaticity variation usually associated with fl at multilayer refl ectors. Transparent and elastic synthetic materials equip the multilayer interference fi bers with high refl ectance that is dynamically tuned by longitudinal mechanical strain. A two-fold elongation of the elastic fi bers results in a shift of refl ection peak center wavelength of over 200 nm. The bio-inspired design and manufacture of this form of soft photonic fi ber heralds the transition to novel fi ber-based fl exible photonic materials and textiles with colors that are tunable over the entire visible spectrum and optical strain sensors.
Nature's most vivid colors, highest transparencies, strongest whites and deepest blacks rely on ordered, quasi-ordered or disordered structures with lattice constants or scattering element sizes on the order of the wavelength of visible radiation. [10][11][12][13][14][15][16] By inducing interference or diffraction, biological photonic structures of a wide structural diversity strongly alter the spectral composition of refl ected and transmitted light resulting in the stunning structural colors of many organisms. [ 17 , 18 ] Onedimensional multilayer arrangements play an important role in the creation of structural colors in nature and have primarily been studied in the animal kingdom, especially the insect world. [ 10 , 19 , 20 ] Planar layered photonic system have recently also been increasingly frequently reported in various plants. [21][22][23][24] The fruits of the plant Margaritaria nobilis in the rain forests of Middle and South America have a striking blue -green hue ( Figure 1 a). The plant partly relies on seed dispersal by birds which might be attracted by the colorful display. [ 25 , 26 ] The cells in the fruit's blue seed coat are elongated and mostly appear blue or green (Figure 1 a,b). Several layers of cells are stacked on top of each other with varying planar orientation of the individual cell layers (Figure 1 c). A single cell cross-section reveals that the entire interior volume is occupied by a periodic concentricallylayered morphology with an overall periodicity of (180 ± 30) nm (Figure 1 d, e). Light incident on the fruit's surface undergoes interference within the periodic structure in each cell resulting in the refl ection of blue light.
Under directional illumination a planar multilayer interference structure can only display its bright coloration in the specular refl ection direction. The hue of such planar Bragg stacks strongly depends on the angle of incidence. Under diffuse illumination the observed color blue-shifts for increasing observation angle. By contrast, in M. nobilis fruits the superposition of a microscopic curvature on the nanoscale regularity of the layered structure within each individual seed coat tissue cell combined with the fruit's overall macroscopic curvature leads to an increased visibility of the refl ected structural color across a wide angular range. Under directional illumination, a part of the curved multilayer in a majority of the individual cells is oriented to satisfy the specular refl ection condition providing a spatially varying pixelated sparkle of different hues (Figure 1 a) that depend on the locally varying angle of light incidence. In diffuse light this sparkle is suppressed as the light refl ected by each fruit cell towards the observer originates from light incident on the fruit from various directions, producing a more isotropic color that only gradually changes across the fruit due to the macroscopic curvature.

COMMUNICATION
The emergence of unique structural and optical properties from combinations of structures on different length scales within hierarchical synergistic assemblies is a principle often encountered when studying natural systems. [ 19 , 27 ] Increasingly frequently this concept is also applied in novel optical technologies. [ 6 , 7 , 9 ] The hierarchical photonic architecture in the seed coat of the M. nobilis fruit is the key element involved in the creation of intense blue and green hues. It provides inspiration for the manufacture of artifi cial photonic fi bers with the optical functionality being defi ned by the interplay of nanoscopic regularity and superimposed microscopic curvature. The artifi cial bio-inspired system presented in this article consists of concentric multilayer-wrapped fi bers with a radially periodic refractive index profi le built from only two alternating phases. While similar to its natural model in dimensions and underlying optical interactions, the artifi cial system avoids many of the complexities in the natural structure including the ellipticity of the fruit cells' cross-section and any existing fi ne structure within the periodic layers.
Optical fi ber systems with multilayer claddings have theoretically been discussed since the late 70's and have more recently been manufactured using standard fi ber drawing processes from macroscopic preforms. [28][29][30][31][32] The choice of materials that can be drawn into multilayer fi bers is constrained to a limited, albeit continuously expanding set of components. [ 33 ] In particular, the preform material combinations need to provide an appropriate refractive index contrast and have matching thermal expansion coeffi cients in order to prevent fracturing at the material interfaces during the processing at elevated temperature. [ 33 ] Here, we present an alternative approach that allows fabrication of fi bers at room temperature from a wide range of soft organic and also inorganic materials with varying optical and mechanical properties that are not restricted to a translational symmetry along the fi ber axis as in thermally drawn fi bers. The fi bers reported in this article consist of two elastomeric dielectrics, polydimethylsiloxane (PDMS) and polyisoprene-polystyrene triblock copolymer (PSPI), two inexpensive materials that are commercially available in industrial quantities and provide a suffi ciently high refractive index contrast ( n PDMS = 1.41 ± 0.02, n PSPI = 1.54 ± 0.02, determined by ellipsometry). Multilayer fi bers are produced by initially forming a bilayer of the two constituent materials, which is subsequently rolled up onto a thin glass fi ber ( ≈ 10-20 μ m diameter) to form the multilayer cladding ( Figure 1 f and Figure S1 in the Supporting Information). [ 34 , 35 ] This material system, which was reported earlier in the context of planar fl exible multilayer systems, [ 34 ] was chosen here for its advantageous set of properties for the controlled tuning of the fi ber's optical performance, which is demonstrated below. The fi ber rolling technique used to produce the fi bers has previously been employed for the creation of multilayer claddings on rods with macroscopic diameter to facilitate the manufacture of microscopically planar multilayer stacks. Here, we show for the fi rst time that this technique can be employed to form concentric multilayers on core fi bers of only ∼ 15 μ m diameter using elastic materials. Such multilayer fi bers with curvature on the microscale display optical properties that are distinctly different from the macroscopic rolls previously produced. [ 35 ] Other dielectric materials that have been used in the fi ber manufacture include thermoplastics, for instance polystyrene and poly(methyl methacrylate). Thin metal fi lms have been incorporated in non-stretchable fi bers with macroscale diameter employing the rolling technique. [ 35 ] Spray coating or blade coating, two industrially well-established techniques that are compatible with roll-to-roll processing, could be explored as viable alternatives for the bilayer production on a larger scale. the fi ber's near-fi eld after excitation with a broadband pulse. An appropriate far-fi eld transformation yields the refl ected intensities as a function of frequency or photon energy and wave vector component k x -i.e. the dispersion relation of the multilayer fi bres. [ 38 , 39 ] The band structures obtained in these simulations (Figure 3 f, g) match the experimentally acquired dispersion diagrams of the fi bers (Figure 3 c,d). The main Bragg refl ection peak at a photon energy of 2.5 eV (corresponding to a The individual thicknesses of the two fi lms in the initial bilayer can be tuned during the fi lm deposition. Consequently, the spectral position of the refl ection band of the fi bers can be freely adjusted. Three fi bers with high refl ectivity in different color ranges and the corresponding complementary color in transmission are shown in Figure 1 g. Scanning electron micrograph images of the cross-section of a green fi ber visualize the concentric multilayer cladding with 80 periods wrapped around the core glass fi ber (Figure 1 h, i).
Similar to the optical signature of the tropical fruit ( Figure 2 a,b), these fi bers show a pronounced refl ection in a fi nite wavelength range imposed by the multilayer periodicity (Figure 2 d,e) and a corresponding drop in transmission (Figure 2 f,g). Fibers rolled with multilayer claddings of up to 150 periods provide a refl ectivity of more than 90% in their refl ection band and a bandwidth varying from 70 nm to 30 nm, decreasing with increasing number of layers in the cladding.
Collection of spectra along a line perpendicular to the fruit cell axis (white dashed line in Figure 2 a) allows for the reconstruction of the axially symmetric spatial refl ection intensity distribution (Figure 2 c). This measurement was also performed on the fi bers to determine their refl ection and transmission intensity distribution. Note that due to its curvature, the concentric multilayer refl ector translates the lateral distance from the fi ber's symmetry axis at which each spectrum was taken into a corresponding incident angle ( Figure 3 a, b). This allows for the direct measurement of the dispersion relation of the multilayer fi ber system in refl ection and transmission (Figure 3 c,d, for details see Figure S2 and discussion in the Supporting Information).
Under directional illumination the difference in radial variation of the peak wavelength from fi ber and individual fruit cells results from a subtle difference in geometry. Cross-sections of the fruit cells show that the multilayer is arranged in concentric ellipses aligned with the major axis in the plane of the fruit's surface which leads to a less pronounced color variation across a single cell. In contrast, the multilayer cladding of the artifi cial fi bers has a circular concentric cross-section inducing a radial color variation with larger gradient.
Finite difference time domain (FDTD) simulations of the fi bers' optical performance were carried out on a high performance computing cluster using the MIT electromagnetic equation propagation software package (MEEP) [ 36 ] to explain the complexity of the measured dispersion diagrams and to predict the change in optical properties due to a variation in the fi ber design. This approach was motivated by the recent investigation of a photonic structure in a fi sh retina, where FDTD simulations of the interaction of light with the complex hierarchical biological photonic system could provide valuable insight into the interplay of all components on the different length scales. [ 37 ] The simulations presented here result from interactions of light with a fi ber of 60 layers with 340 nm periodicity in the outer multilayer cladding zone matching the geometry of the blue fi ber from which the experimental dispersion diagrams were obtained. The inner cladding zone adjacent to the fi ber core was occupied by a small number of bilayers of ≈ 710 nm periodicity also found in the real fi ber. In the simulations the spatiotemporal electromagnetic fi eld amplitude distributions of light refl ected and transmitted by the fi ber are collected in in the fi ber's axial direction (see Figure S3 in the Supporting Information). A detailed investigation of this effect is beyond the scope of this paper and will be addressed elsewhere.
The glass fi ber that acts as the substrate for the multilayers in the rolling process can be removed from the fi ber by dissolution in hydrofl uoric acid or by simply pulling it out of the multilayer cladding. Once the glass core is removed, the fi ber, being composed of two elastomers, can be elastically deformed by stretching it along its axis. An elongation along the fi ber axis leads to a compression perpendicular to it, causing a decrease of its overall diameter and a reduction of the thickness of each individual layer. Due to the comparable Poisson's ratio of the constituent elastic materials the thickness ratio and the refl ection intensity remain constant while the refl ection band blueshifts. This way, the refl ected and transmitted color can be reversibly tuned by axial extension of the fi bers ( Figure 4 a,d). A reversible peak wavelength shift of over 200 nm has been recorded for axial elongations of a fi ber to over 200% of its original length (Figure 4 b,c).
From Poisson's ratio and the proportionality between the thickness of individual layers in the cladding and the spectral band-gap position, the refl ection peak center wavelength λ peak is predicted to follow the relation λ peak = λ 0 peak (1 + ε ) − v , where λ 0 peak represents the refl ection peak centre wavelength at zero axial strain, ε the applied axial strain and v the fi bre's Poisson's ratio. Fitting the experimental data with this relation yields a Poisson's ratio of v = 0.46 ± 0.02 for the stretchable multilayer fi bers matching the Poisson's ratios of the constituent rubber wavelength of 496 nm) and its spectral variation with propagation direction is captured in both sets of data for transmission and refl ection. A blue-shift of the refl ection band is observed with increasing propagation angle in experiment and simulation. However, this spectral variation amounts to less than 40% of the blue-shift expected from a comparable fl at multilayer structure, showing that the curvature of the multilayer cladding more than halves the angular variation in chromaticity usually associated with multilayer refl ectors. The same effect arises for refl ection of light from the multilayered cells in the seed coat of the M. nobilis fruit (Figure 3 c) explaining its isotropic coloration in diffuse illumination.
In this particular fi ber sample, the splitting in the refl ection band observed in experiment and simulation is caused by interference of light in the inner zone of the multilayer cladding with ≈ 710 nm periodicity surrounding the fi ber core. High frequency intensity fl uctuations as a function of photon energy in the simulations are caused by the fi ber core, which acts as a larger resonant cavity introducing an abundance of defect modes. This is not captured in the experimental data due to a limited spectral resolution. In fi ber designs with smaller cores these cavity modes would be detectable and potentially even be manifested in angle-dependent spectrally varying intensity fl uctuations visible to the naked eye. Some of the light propagating transversely through the fi ber experiences guiding within the individual layers resulting in additional weak modes seen at photon energies between 1.7-2.4 eV captured in simulations and experiments. These modes only occur for light polarized materials (Figure 4 c). This is in good agreement with results obtained earlier for the deformation of planar elastic multilayers. [ 34 ] In this paper, we have given an example of the versatility of bio-inspired approaches for the manufacture of novel photonic elements. The hierarchical photonic architecture in individual cells of the fruits of the tropical plant Margaritaria nobilis served as a model for the creation of novel tunable band-gap multilayer fi bers with a large tuning range in the visible spectrum. Our room temperature fi ber rolling technique allows fabrication of multilayer fi bers with hundreds of layers from a wide range of polymeric material combinations that would not be realizable by conventional thermal fi ber drawing. The fi bers' band-gap center frequency can initially be tuned by adjusting the individual fi lm thicknesses of the two constituent layers prior to the rolling process, which also allows shifting of the fi bers' tuning range into the near UV or near IR. Chirped multilayer fi bers can be realized by applying an appropriate force on the elastic bilayer during rolling of the multilayer cladding (see Figure S1 in the supp. inf.).
The incorporation of gold or silver layers into the concentric multilayer offers potential for the development of novel micronscale fi ber-based meta-materials. Asymmetric structures or chirality could be incorporated into the fi bers by patterning of the initial bilayer prior to the rolling, promising additional interesting optical properties. [ 40 ] Removal of the fi ber core from inside the multilayer cladding permits mechanical deformation of fi bers to more than twice their original length, which causes a tuning of the band-gap and a spectral blue-shift of over 200 nm. In the future, the incorporation of fl exible core fi bers will render the removal step obsolete. The fl exibility in the choice of constituent materials for the multilayer fi bers and their unique combination of mechanical and optical properties holds great potential for applications in mechanically tunable light guides or optical strain sensing. The fi bers' mechanical fl exibility and elasticity, in addition to the demonstrated color brilliance and tunability, can make them a versatile novel material for smart, color-dynamic textiles. The reported multilayer fi ber manufacturing process can in principle be applied to a wide range of synthetic materials with varying optical and mechanical properties. Large area deposition of the initial bilayer can be achieved by spraying or blade coating in a roll-to-roll process before fi nal rolling of the multilayer with the layer thicknesses and the core fi ber diameter acquired from the scanning electron microscope images of fi ber cross-sections (d PDMS = 240 nm, d PSPI = 100 nm, d core = 14 μ m, Figure 1 h,i) and refractive indices of the constituent materials measured by ellipsometry (n PDMS = 1.41 n PSPI = 1.54, n core = 1.5). The modeling cell was bordered by absorbing boundaries simulating the effect of a fi nite numerical aperture of the microscope objective. A light pulse containing light of wavelengths 300-900 nm was propagated through the simulation cell. Detectors positioned above and below the fi ber captured the electromagnetic fi elds as a function of time and space. The distance of the detectors to the fi ber was chosen in order to detect waves propagating in an angular range that is commensurate with the numerical aperture of the microscope objective in the experiments. The detected fi elds served to deduce the refl ection and transmission dispersion diagrams of the fi bers using a suitable Fourier transformation algorithm to perform the far-fi eld transformation and the spectral analysis. The simulations were repeated with periodic boundary conditions to ensure that no artifact spatial frequencies result from the absorbing boundaries and to ensure that conservation of energy was observed in the simulations (i.e refl ection and transmission add up to unity).

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author. fi bers. Hollow photonic fi bers providing good thermal insulation can be produced by employing hollow polymeric microtubing as the inner fi ber core. Exposure of the fi bers to different solvents in the vapor or liquid phase would result in varying degrees of swelling and a corresponding refl ection peak red-shift endowing the fi bers or textiles made thereof with optical solvent sensing capacities. The fi nal formation of the multilayer rolls could in principle also be achieved by self-induced rolling of the bilayer caused by directional stresses, which can be induced by gradually swelling one of the bilayer phases selectively. [ 41 ]

Experimental Section
Fiber Manufacture : A thin polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning) fi lm was spun from a 4%wt solution in heptane onto a sacrifi cial water-soluble polystyrene-sulfonic acid layer on a silicon wafer. The PDMS fi lm was cross-linked by curing it on a hotplate for 2h at 70 ºC. Subsequently, a bilayer was formed by spin-coating a polystyrenepolyisoprene-polystyrene triblock copolymer (PSPI, Sigma Aldrich, 14%wt content polystyrene) fi lm on top of the cross-linked PDMS layer from a 4%wt solution in toluene (Figure 1 f left). Stripes of 12 cm by 1-2 cm of the bilayer were then released from the wafer onto the surface of a water bath. This was achieved by immersing the sample slowly into the water at an angle varying between 30-45º allowing the water to dissolve the sacrifi cial water-soluble fi lm between the elastomer bilayer and the substrate, thereby detaching the bilayer from the substrate. A thin glass fi ber (10-20 μ m diameter) was then lowered onto the end of the fl oating bilayer, where it adhered to the PSPI fi lm, the top layer in the bilayer. Once the core glass fi ber had attached it was rotated at a speed of 10-20 turns per minute rolling up the bilayer to form the multilayer cladding (Figure 1 f right). [ 34 , 35 ] Structural and Optical Analysis : Images of the cross-sections of the fruit's seed coat tissue cells were obtained by scanning electron microscopy (Hitachi S-3200N SEM). For imaging, the samples were coated with a 3 nm thick fi lm of a gold/palladium alloy. The cells internal periodic structure was visualized by transmission electron microscopy (JEOL 100S TEM). Images were acquired after fi xing and staining samples according to the protocol described elsewhere. [ 42 ] Crosssections of the fi bers were obtained by cryo-fracture. Fractured fi bers were coated with a 3-5nm thick platinum fi lm to avoid charging artifacts during imaging and visualized using a fi eld emission scanning electron microscope (Zeiss Supra55VP).
Simultaneous imaging and micro-spectroscopic spatial refl ection/ transmission intensity mapping of the fruit's surface and the fi bers was performed in a modifi ed optical microscope (Leica DMRX). The samples were illuminated in the area of interest with a halogen lamp in refl ection or transmission. Via an additional microscope port, a fraction of the refl ected light was collected confocally and guided by a fi ber to a spectrometer (Maya 2000 Pro, Ocean Optics). The detection spot size depends on the diameter of the fi ber and the magnifi cation of the objective lens. Measurements with a 50x objective (NA = 0.55) and a fi ber with 50 μ m core diameter resulted in a spatial resolution of 1 μ m. All spectra are referenced against a fl at silver mirror of ≥ 95% refl ectance in the wavelength range of 400-800 nm. In order to acquire spatially and spectrally resolved intensity distributions of a specifi c area on a sample, the sample was translated step-wise in the focal plane of the microscope with a minimum step size of 1 μ m using an automated, remote-controlled stage (Prior ES110). Individual spectra were acquired after each scanning step resulting in a complete map of the spectrally-resolved intensity distribution of the samples in refl ection or transmission.
Optical Modeling : The transverse refl ection and transmission of cylindrical multilayer fi bers was modeled using the MIT Electromagnetic Equation Propagation package (MEEP) [ 36 ] interfaced with custommade C + + code to feed MEEP with the refractive index distributions in the simulation cell and to deduce the dispersion relations from the simulated fi eld distributions. The fi ber was modeled in cross-section