Bio-inspired Optics

The most finely tuned, rapidly responsive, and precisely directed optical systems currently known can be found on the surfaces of living organisms. Studies of brittlestars’ tunable microlenses, sea sponges’ optical fibers, butterflies’ and beetles’ intense colors, and squids’ nearly perfect camouflage have revealed 3D architectures so intricately patterned down to the nanoscale that the topography itself controls the wavelengths and direction of reflected light. 

Of particular interest to us, many of these architectures constantly reconfigure and/or control pigment movement to adjust their optical behavior in response to a changing environment. In conjunction with our collaborators’ investigations of the biological mechanisms, our group is developing bottom-up self-assembly techniques that allow us to create comparably elaborate yet tunable hierarchical photonic structures, and integrating these into the design of a new class of dynamic, responsive optical materials, such as self-adapting energy-saving window coatings that adjust their transparency in response to varying temperature, self-reporting sensors, and photonic encryption systems.

England GT, Aizenberg J. Emerging Optical Properties from the Combination of Simple Optical Effects. Rep. Prog. Phys. [Internet]. 2018;81 (1) :016402. Publisher's VersionAbstract

Structural color arises from the patterning of geometric features or refractive indices of the constituent materials on the length-scale of visible light. Many different organisms have developed structurally colored materials as a means of creating multifunctional structures or displaying colors for which pigments are unavailable. By studying such organisms, scientists have developed artificial structurally colored materials that take advantage of the hierarchical geometries, frequently employed for structural coloration in nature. These geometries can be combined with absorbers—a strategy also found in many natural organisms—to reduce the effects of fabrication imperfections. Furthermore, artificial structures can incorporate materials that are not available to nature—in the form of plasmonic nanoparticles or metal layers—leading to a host of novel color effects. Here, we explore recent research involving the combination of different geometries and materials to enhance the structural color effect or to create entirely new effects, which cannot be observed otherwise.

England GT, Russell C, Shirman E, Kay T, Vogel N, Aizenberg J. The Optical Janus Effect: Asymmetric Structural Color Reflection Materials. Adv. Mater. [Internet]. 2017;29 (29) :1606876. Publisher's VersionAbstract
Structurally colored materials are often used for their resistance to photobleaching and their complex viewing-direction-dependent optical properties. Frequently, absorption has been added to these types of materials in order to improve the color saturation by mitigating the effects of nonspecific scattering that is present in most samples due to imperfect manufacturing procedures. The combination of absorbing elements and structural coloration often yields emergent optical properties. Here, a new hybrid architecture is introduced that leads to an interesting, highly directional optical effect. By localizing absorption in a thin layer within a transparent, structurally colored multilayer material, an optical Janus effect is created, wherein the observed reflected color is different on one side of the sample than on the other. A systematic characterization of the optical properties of these structures as a function of their geometry and composition is performed. The experimental studies are coupled with a theoretical analysis that enables a precise, rational design of various optical Janus structures with highly controlled color, pattern, and fabrication approaches. These asymmetrically colored materials will open applications in art, architecture, semitransparent solar cells, and security features in anticounterfeiting materials.
Burgess IB, Nerger BA, Raymond KP, Goulet-Hanssens A, Singleton TA, Kinney MH, Shneidman AV, Koay N, Barrett CJ, Loncar M, et al. Wetting in Color: From photonic fingerprinting of liquids to optical control of liquid percolation. Proc. of SPIE [Internet]. 2013;8632 :863201. Publisher's VersionAbstract

We provide an overview of our recent advances in the manipulation of wetting in inverse-opal photonic crystals. Exploiting photonic crystals with spatially patterned surface chemistry to confine the infiltration of fluids to liquidspecific spatial patterns, we developed a highly selective scheme for colorimetry, where organic liquids are distinguished based on wetting. The high selectivity of wetting, upon-which the sensitivity of the response relies, and the bright iridescent color, which disappears when the pores are filled with liquid, are both a result of the highly symmetric pore structure of our inverse-opal films. The application of horizontally or vertically orientated gradients in the surface chemistry allows a unique response to be tailored to specific liquids. While the generic nature of wetting makes our approach to colorimetry suitable for applications in liquid authentication or identification across a broad range of industries, it also ensures chemical non-specificity. However, we show that chemical specificity can be achieved combinatorially using an array of indicators that each exploits different chemical gradients to cover the same dynamic range of response. Finally, incorporating a photo-responsive polyelectrolyte surface layer into the pores, we are able to dynamically and continuously photo-tune the wetting response, even while the film is immersed in liquid. This in situ optical control of liquid percolation in our photonic-crystal films may also provide an error-free means to tailor indicator response, naturally compensating for batch-to-batch variability in the pore geometry.

Pouya C, Overvelde JTB, Kolle M, Aizenberg J, Bertoldi K, Weaver JC, Vukusic P. Characterization of a Mechanically Tunable Gyroid Photonic Crystal Inspired by the Butterfly Parides Sesostris. Adv. Optical Mater. 2016;4 (1) :99-105.Abstract
A mechanically tunable macroscale replica of the gyroid photonic crystal found in the Parides sesostris butterfly's wing scales is systematically characterized. By monitoring both photonic frequency changes and the distribution of stress fields within the compressed structure, electromagnetic transmission features are found and can be frequency-tuned and the structure only contains localized high stress fields when highly compressed.
England G, Kolle M, Kim P, Khan M, Munoz P, Mazur E, Aizenberg J. Bioinspired micrograting arrays mimicking the reverse color diffraction elements evolved by the butterfly Pierella luna. Proc. Nat. Acad. Sci. [Internet]. 2014;111 (44) :15630–15634. Full TextAbstract

Recently, diffraction elements that reverse the color sequence normally observed in planar diffraction gratings have been found in the wing scales of the butterfly Pierella luna. Here, we describe the creation of an artificial photonic material mimicking this re- verse color-order diffraction effect. The bioinspired system con- sists of ordered arrays of vertically oriented microdiffraction gratings. We present a detailed analysis and modeling of the cou- pling of diffraction resulting from individual structural compo- nents and demonstrate its strong dependence on the orientation of the individual miniature gratings. This photonic material could provide a basis for novel developments in biosensing, anticoun- terfeiting, and efficient light management in photovoltaic systems and light-emitting diodes.

Kim P, Hu Y, Alvarenga J, Kolle M, Suo Z, Aizenberg J. Rational Design of Mechano-Responsive Optical Materials by Fine Tuning the Evolution of Strain-Dependent Wrinkling Patterns. Adv. Optical Mater. [Internet]. 2013;1 (5) :381-388. Publisher's VersionAbstract
Rational design strategies for mechano‐responsive optical material systems are created by introducing a simple experimental system that can continuously vary the state of bi‐axial stress to induce various wrinkling patterns, including stripes, labyrinths, herringbones, and rarely observed checkerboards, that can dynamically tune the optical properties. In particular, a switching of two orthogonally oriented stripe wrinkle patterns from oxidized polydimethylsiloxane around the critical strain value is reported, as well as the coexistence of these wrinkles forming elusive checkerboard patterns, which are predicted only in previous simulations. These strain‐induced wrinkle patterns give rise to dynamic changes in optical transmittance and diffraction patterns. A theoretical description of the observed pattern formation is presented which accounts for the residual stress in the membrane and allows for the fine‐tuning of the window of switching of the orthogonal wrinkles. Applications of wrinkle‐induced changes in optical properties are demonstrated, including a mechanically responsive instantaneous privacy screen and a transparent sheet that reversibly reveals a message or graphic and dynamically switches the transmittance when stretched and released.
Phillips KR, Vogel N, Hu Y, Kolle M, Perry CC, Aizenberg J. Tunable Anisotropy in Inverse Opals and Emerging Optical Properties. Chem. Mater. [Internet]. 2014;26 (4) :1622-1628. Publisher's VersionAbstract

Using self-assembly, nanoscale materials can be fabricated from the bottom up. Opals and inverse opals are examples of self-assembled nanomaterials made from crystallizing colloidal particles. As self-assembly requires a high level of control, it is challenging to use building blocks with anisotropic geometry to form complex opals, which limits the possible structures. Typically, spherical colloids are employed as building blocks, leading to symmetric, isotropic superstructures. However, a significantly richer palette of directionally dependent properties are expected if less symmetric, anisotropic structures can be created, especially originating from the assembly of regular, spherical particles. Here we show a simple method for introducing anisotropy into inverse opals by subjecting them to a post-assembly thermal treatment that results in directional shrinkage of the silica matrix caused by condensation of partially hydrated sol−gel silica structures. In this way, we can tailor the shape of the pores, and the anisotropy of the final inverse opal preserves the order and uniformity of the self-assembled structure. Further, we prevent the need to synthesize complex oval-shaped particles and crystallize them into such target geometries. Detailed X-ray photoelectron spectroscopy and infrared spectroscopy studies clearly identify increasing degrees of sol−gel condensation in confinement as a mechanism for the structure change. A computer simulation of structure changes resulting from the condensation-induced shrinkage further confirmed this mechanism. As an example of property changes induced by the introduction of anisotropy, we characterized the optical spectra of the anisotropic inverse opals and found that the optical properties can be controlled in a precise way using calcination temperature.

Burgess IB, Abedzadeh N, Kay TM, Shneidman AV, Cranshaw DJ, Loncar M, Aizenberg J. Tuning and Freezing Disorder in Photonic Crystals using Percolation Lithography. Scientific Reports [Internet]. 2016;6 (1) :19542. Full TextAbstract
Although common in biological systems, synthetic self-assembly routes to complex 3D photonic structures with tailored degrees of disorder remain elusive. Here we show how liquids can be used to finely control disorder in porous 3D photonic crystals, leading to complex and hierarchical geometries. In these optofluidic crystals, dynamically tunable disorder is superimposed onto the periodic optical structure through partial wetting or evaporation. In both cases, macroscopic symmetry breaking is driven by subtle sub-wavelength variations in the pore geometry. These variations direct site-selective infiltration of liquids through capillary interactions. Incorporating cross-linkable resins into our liquids, we developed methods to freeze in place the filling patterns at arbitrary degrees of partial wetting and intermediate stages of drying. These percolation lithography techniques produced permanent photonic structures with adjustable disorder. By coupling strong changes in optical properties to subtle differences in fluid behavior, optofluidic crystals may also prove useful in rapid analysis of liquids.
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Spotlight summary: The Brighter The Better!, Virtual Journal for Biomedical Optics, December 23, 2014. 

Butterfly's colorful trick of the light recreated in the lab, Physics World, October 10, 2014. 

Lifelike cooling for sunbaked windows, Wyss Institute press release, July 30, 2013. 

"Watermark Ink" device wins R&D 100 Award, Harvard press release, July 8, 2013. 

Bioinspired fibers change color when stretched, Harvard press release, January 28, 2013.