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.

Burgess IB, Aizenberg J, Loncar M. Creating bio-inspired hierarchical 3D–2D photonic stacks via planar lithography on self-assembled inverse opals. Bioinspiration & Biomimetics [Internet]. 2013;8 :045004. Publisher's VersionAbstract
Structural hierarchy and complex 3D architecture are characteristics of biological photonic designs that are challenging to reproduce in synthetic materials. Top–down lithography allows for designer patterning of arbitrary shapes, but is largely restricted to planar 2D structures. Self-assembly techniques facilitate easy fabrication of 3D photonic crystals, but controllable defect-integration is difficult. In this paper we combine the advantages of top–down and bottom–up fabrication, developing two techniques to deposit 2D-lithographically-patterned planar layers on top of or in between inverse-opal 3D photonic crystals and creating hierarchical structures that resemble the architecture of the bright green wing scales of the butterfly, Parides sesostris. These fabrication procedures, combining advantages of both top–down and bottom–up fabrication, may prove useful in the development of omnidirectional coloration elements and 3D–2D photonic crystal devices.
Phillips KR, England GT, Sunny S, Shirman E, Shirman T, Vogel N, Aizenberg J. A colloidoscope of colloid-based porous materials and their uses. Chem. Soc. Rev. [Internet]. 2016;45 (2) :281-322. Full TextAbstract
Nature evolved a variety of hierarchical structures that produce sophisticated functions. Inspired by these natural materials, colloidal self-assembly provides a convenient way to produce structures from simple building blocks with a variety of complex functions beyond those found in nature. In particular, colloid-based porous materials (CBPM) can be made from a wide variety of materials. The internal structure of CBPM also has several key attributes, namely porosity on a sub-micrometer length scale, interconnectivity of these pores, and a controllable degree of order. The combination of structure and composition allow CBPM to attain properties important for modern applications such as photonic inks, colorimetric sensors, self-cleaning surfaces, water purification systems, or batteries. This review summarizes recent developments in the field of CBPM, including principles for their design, fabrication, and applications, with a particular focus on structural features and materials' properties that enable these applications. We begin with a short introduction to the wide variety of patterns that can be generated by colloidal self-assembly and templating processes. We then discuss different applications of such structures, focusing on optics, wetting, sensing, catalysis, and electrodes. Different fields of applications require different properties, yet the modularity of the assembly process of CBPM provides a high degree of tunability and tailorability in composition and structure. We examine the significance of properties such as structure, composition, and degree of order on the materials' functions and use, as well as trends in and future directions for the development of CBPM.
Singleton TA, Burgess IB, Nerger BA, Goulet-Hanssens A, Koay N, Barrett CJ, Aizenberg J. Photo-tuning of Highly Selective Wetting in Inverse Opals. Soft Matter [Internet]. 2014;10 (9) :1325-1328. Publisher's VersionAbstract

Crack-free inverse opals exhibit a sharply defined threshold wettability for infiltration that has enabled their use as colourimetric indicators for liquid identification. Here we demonstrate direct and continuous photo-tuning of this wetting threshold in inverse opals whose surfaces are function- alized with a polymer doped with azobenzene chromophores.

Vogel N, Utech S, England GT, Shirman T, Phillips KR, Koay N, Burgess IB, Kolle M, Weitz DA, Aizenberg J. Color from hierarchy: Diverse optical properties of micron-sized spherical colloidal assemblies. Proc. Nat. Acad. Sci. [Internet]. 2015;112 (35) :10845-10850. Publisher's VersionAbstract
Materials in nature are characterized by structural order over multiple length scales have evolved for maximum performance and multifunctionality, and are often produced by self-assembly processes. A striking example of this design principle is structural coloration, where interference, diffraction, and absorption effects result in vivid colors. Mimicking this emergence of complex effects from simple building blocks is a key challenge for man-made materials. Here, we show that a simple confined self-assembly process leads to a complex hierarchical geometry that displays a variety of optical effects. Colloidal crystallization in an emulsion droplet creates micron-sized superstructures, termed photonic balls. The curvature imposed by the emulsion droplet leads to frustrated crystallization. We observe spherical colloidal crystals with ordered, crystalline layers and a disordered core. This geometry produces multiple optical effects. The ordered layers give rise to structural color from Bragg diffraction with limited angular dependence and unusual transmission due to the curved nature of the individual crystals. The disordered core contributes nonresonant scattering that induces a macroscopically whitish appearance, which we mitigate by incorporating absorbing gold nanoparticles that suppress scattering and macroscopically purify the color. With increasing size of the constituent colloidal particles, grating diffraction effects dominate, which result from order along the crystal’s curved surface and induce a vivid polychromatic appearance. The control of multiple optical effects induced by the hierarchical morphology in photonic balls paves the way to use them as building blocks for complex optical assemblies—potentially as more efficient mimics of structural color as it occurs in nature.
Vasquez Y, Kolle M, Mishchenko L, Hatton BD, Aizenberg J. Three-Phase Co-Assembly: In-situ Incorporation of Nanoparticles into Tunable, Highly-Ordered, Porous Silica FIlms. ACS Photonics [Internet]. 2014;1 (1) :53-60. Full TextAbstract

We present a reproducible, one-pot colloidal co-assembly approach that results in large-scale, highly ordered porous silica films with embedded, uniformly distributed, accessible gold nanoparticles. The unique coloration of these inverse opal films combines iridescence with plasmonic effects. The coupled optical properties are easily tunable either by changing the concentration of added nanoparticles to the solution before assembly or by localized growth of the embedded Au nanoparticles upon exposure to tetrachloroauric acid solution, after colloidal template removal. The presence of the selectively absorbing particles furthermore enhances the hue and saturation of the inverse opals’ color by suppressing incoherent diffuse scattering. The composition and optical properties of these films are demonstrated to be locally tunable using selective functionalization of the doped opals.

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.