Bio-Inspired Optics

Phillips KR, Vogel N, Hu Y, Kolle M, Perry CC, Aizenberg J. Tunable Anisotropy in Inverse Opals and Emerging Optical Properties. Chem. Mater. 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, Loncar M, and Aizenberg J. Structural Colour in Colourimetric Sensors and Indicators. J. Mater. Chem. C. 2013;1 (38) :6075-6086. Publisher's VersionAbstract
Colourimetric sensors and indicators are widely used because of their low cost and simplicity. A significant challenge associated with the design of this type of device is that the sensing mechanism must be simultaneously optimised for the sensitivity of the response and a visually perceptible colour change. Structural colour, derived from coherent scattering rather than molecular absorption, is a promising route to colourimetric sensor design because colour shifts are tied to changes in one of many physical properties of a material, rather than a specific chemical process. This Feature Article presents an overview of the development of low-cost sensors and indicators that exploit structural colour. Building upon recent advances in structurally adaptive materials design, structural colour sensors have been developed for a wide variety of previously inaccessible physical (e.g. temperature, strain, electric fields) and chemical stimuli (e.g. small organic molecules, charged species, biomacromolecules and metabolites). These devices, often exceeding the state of the art in performance, simplicity or both, have bright prospects for market impact in areas such as environmental monitoring, workplace hazard identification, threat detection, and point-of-care diagnostics. Finding the ideal balance between performance (e.g. sensitivity, specificity, reproducibility, etc.) and simplicity (e.g. colourimetric vs. spectroscopic readout) will be one of the most critical elements in the further development of structural colour sensors. This balance should be driven largely by the market demands and competing technologies.
Li L, Kolle S, Weaver JC, Ortiz C, Aizenberg J, Kolle M. A highly conspicuous mineralized composite photonic architecture in the translucent shell of the blue-rayed limpet. Nat. Commun. 2015;6 :6322. Full TextAbstract
Many species rely on diverse selections of entirely organic photonic structures for the manipulation of light and the display of striking colours. Here we report the discovery of a mineralized hierarchical photonic architecture embedded within the translucent shell of the blue-rayed limpet Patella pellucida. The bright colour of the limpet’s stripes originates from light interference in a periodically layered zig-zag architecture of crystallographically co-oriented calcite lamellae. Beneath the photonic multilayer, a disordered array of light-absorbing particles provides contrast for the blue colour. This unique mineralized manifestation of a synergy of two distinct optical elements at specific locations within the continuum of the limpet’s translucent protective shell ensures the vivid shine of the blue stripes, which can be perceived under water from a wide range of viewing angles. The stripes’ reflection band coincides with the spectral range of minimal light absorption in sea water, raising intriguing questions regarding their functional significance.
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. 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.

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. 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.