Structured Optical Materials Controlled by Light

Abstract

light of wavelengths commensurate to their periodicity and, depending on their arrangement and composition, they form photonic structures of different complexity, ranging from 1D gratings (Bragg grating) up to 3D photonic crystals. In all these cases, light-matter interaction leads to characteristic optical features, such as reflection and transmission profiles of diffractive gratings and, even more interestingly, the propagation inhibition of certain frequencies (photonic band gap) in photonic crystals. Surprisingly, nature presents numerous examples of how optical properties can be affected by nanoscale structuration. In fact, many living organisms adopt structural coloration, [1] a nanometric arrangement of dielectric geometric units on their skin or shell, to create vivid and bright structural colors. This way they adapt to the surrounding environment, send warning messages, or mislead their natural enemies through camouflage. [2] Mimic nature, and in particular actual reconfigurable mechanisms, [3] is fundamental also in man-made photonic applications to control the optical response of devices or materials. Current strategies include the use of the electrooptic effect, [4] temperature sensitivity, [5] and carrier injection. [6] Another method relies on the birefringent behavior of liquid crystals (LCs), [7] which can be infiltrated in photonic structures allowing to control the refractive index through temperature, [8] electric, [9] or magnetic [10] fields. Materials with structural color can be effectively-but invasively and slowly-tuned by deforming them, for instance by applying mechanical pressure or stress. [11] This is the only proposed tuning method over the optical response that acts on the unit cell variation instead of relying on refractive index control. Here we wish to explore a different route to achieve tuning and switching of photonic materials, using the light itself as a means to control the mechanical state of a periodic material. By creating a structured material out of photoresponsive polymer, we show that it is possible to induce mechanical deformation-and hence a huge change in the optical response of the material-by simply shining light on it. De facto, this constitutes a nonlinear optical effect, with sub-millisecond time response, using mechanical deformation as an intermediate step. The deformation, and hence the nonlinear response, takes place only in the illuminated region and therefore can be precisely local. The photoresponsive polymers that we use are light sensitive liquid crystal networks (LCN), [12] or elastomers, which have the ability to strongly deform in a reproducible way. [13] It has been demonstrated that LCN can be Materials whose optical response is determined by their structure are of much interest both for their fundamental properties and applications. Examples range from simple gratings to photonic crystals. Obtaining control over the optical properties is of crucial importance in this context, and it is often attempted by electro-optical effect or by using m...

3D Printed Photoresponsive Materials for Photonics

“…This creates an optically tunable micro beam steerer. Depending on the grating shape and on the molecular alignment of the liquid crystalline matrix, the diffracted orders exhibited a center‐symmetric deviation or an anisotropic one in a sub‐millisecond time scale . The operation has been demonstrated both in air and in water, with different dynamics, confirming the thermal nature of the process.…”

“…Most of the examples of light‐responsive DLW microstructures have been reported by using Liquid Crystalline Networks (LCNs) or hydrogels. In the first case, big anisotropic deformations (up to 20% of the structure length) have been reported in different environments (from air to solvents) and demonstrated in tunable photonic platforms . LCNs have an anisotropic molecular structure created by mesogenic units bonded to a polymeric backbone (both as side chain or directly inserted in the main chain) .…”

“…b) SEM image of the LCN micro grating operating as a beam steerer (top panel), optical image of the diffracted beams (in the center), and temporal dynamics of the normalized deflection of the first diffraction order for an actuation power of 2.8 mW (bottom panel). Adapted with permission . Copyright 2018, Wiley‐VCH.…”

Photoreversible Soft Azo Dye Materials: Toward Optical Control of Bio‐Interfaces

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Dynamic Control of Light Direction Enabled by Stimuli‐Responsive Liquid Crystal Gratings

“…The polymer stabilization process allows for electric-field controllable, 2D gratings. [158] Figure 11C-E shows a close look of the polymerized area under three different modes of a POM: panel (C) shows the structure under plane polarized light, panel (D) shows the same area under crossed polarizers and E with a full-wave plate.…”