Dynamically Tunable Optical Cavities with Embedded Nematic Liquid Crystalline Networks

Abstract

prompted a renewed interest in asymmetric Fabry-Pérot (FP) cavities, also termed Gires-Tournois resonators. They are comprised of one optically thick and one optically thin metallic mirror through which light can enter the structure. These optical elements are known for their ease of fabrication and effectiveness in resonating and enhancing light-matter interaction at selected wavelengths. [4,6,7] The general strategy to achieve dynamic tuning in FP resonators is to replace the passive insulator that commonly sits between the mirrors by dynamically tunable materials such as graphene, [11][12][13] phase-change magnesium, [14] electro-optical polymers, [15] liquid crystals (LCs) [16][17][18] and conducting oxides [19][20][21] via the field-effect. [22] Several works showed that electrical gating of indium-tin-oxide incorporated in the cavity facilitates control over light absorption [12,19] and its reflection phase in mid-infrared [20] and near-infrared. [21] Other research exploited electrochromic oxide [23] and polymers [24][25][26] with nanostructures to tune the resulting reflected color. Researchers showed that optical pumping of gallium-doped zinc oxide [27] and alumina [28] allows for ultrafast modulation of cavity resonances in a sub-picosecond regime. Tuning can also be achieved in non-conventional ways by light pressure, [29] directed self-assembly of nanoparticles in a liquid electrolyte [30] and by phase-tunable meta-mirrors. [31] To reduce fabrication complexity, a large variety of responsive materials Tunable metal-insulator-metal (MIM) Fabry-Pérot (FP) cavities that can dynamically control light enable novel sensing, imaging and display applications. However, the realization of dynamic cavities incorporating stimuliresponsive materials poses a significant engineering challenge. Current approaches rely on refractive index modulation and suffer from low dynamic tunability, high losses, and limited spectral ranges, and require liquid and hazardous materials for operation. To overcome these challenges, a new tuning mechanism employing reversible mechanical adaptations of a polymer network is proposed, and dynamic tuning of optical resonances is demonstrated. Solid-state temperature-responsive optical coatings are developed by preparing a monodomain nematic liquid crystalline network (LCN) and are incorporated between metallic mirrors to form active optical microcavities. LCN microcavities offer large, reversible and highly linear spectral tuning of FP resonances reaching wavelength-shifts up to 40 nm via thermomechanical actuation while featuring outstanding repeatability and precision over more than 100 heating-cooling cycles. This degree of tunability allows for reversible switching between the reflective and the absorbing states of the device over the entire visible and near-infrared spectral regions, reaching large changes in reflectance with modulation efficiency ΔR = 79%.

Unlocking Optical Coupling Tunability in Epsilon‐Near‐Zero Metamaterials Through Liquid Crystal Nanocavities

“…For instance, elastomeric LCs have been used to tune photonic crystals [ 31 ] and Fabry‐Perot cavities. [ 32 ] LC‐based metasurfaces have also been recently implemented, confirming the extraordinary capabilities of LCs in the active control of visible light, [ 33–36 ] while extensions to the microwave [ 37,38 ] and terahertz regimes [ 39,40 ] are under way. The primary challenge to developing an active, LC‐based ENZ metamaterial is to reduce the LC thickness to a few hundred nanometers.…”