Topologically Engineered Strain Redistribution in Elastomeric Substrates for Dually Tunable Anisotropic Plasmomechanical Responses.

The prompt visual response is considered to be a highly intuitive tenet among sensors. Therefore, plasmomechanical strain sensors, which exhibit dynamic structural color changes, have recently been developed by using mechanical stimulus-based elastomeric substrates for wearable sensors. However, the reported plasmomechanical strain sensors either lack directional sensitivity or require complex signal processing and device design strategies to ensure anisotropic optical responses. To the best of our knowledge, there have been no reports on utilizing anisotropic mechanical substrates to obtain directional optical responses. Herein, we propose an anisotropic plasmomechanical sensor to distinguish between the applied force direction and the force magnitude. We employ a simple strain-engineered topological elastomer to mechanically transform closely packed metallic nanoparticles (NPs) into anisotropic directional rearrangements depending on the applied force direction. The proposed structure consists of a heterogeneous-modulus elastomer that exhibits a highly direction-dependent Poisson effect owing to the periodically line-patterned local strain redistribution occurring due to the same magnitude of applied external force. Consequently, the reorientation of the self-assembled gold (Au)-NP array manifests dual anisotropy, i.e., force- and polarization-direction-dependent plasmonic coupling. The cost-effectiveness and simple design of our proposed heterogeneous-modulus platform pave the way for numerous optical applications based on dynamic transformation and topological inhomogeneities.

Spin-Selective Angular Dispersion Control in Dielectric Metasurfaces for Multichannel Meta-Holographic Displays.

Angle-dependent next-generation displays have potential applications in 3D stereoscopic and head-mounted displays, image combiners, and encryption for augmented reality (AR) and security. Metasurfaces enable such exceptional functionalities with groundbreaking achievements in efficient displays over the past decades. However, limitations in angular dispersion control make them unfit for numerous nanophotonic applications. Here, we propose a spin-selective angle-dependent all-dielectric metasurface with a unique design strategy to manifest distinct phase information at different incident angles of light. As a proof of concept, the phase masks of two images are encoded into the metasurface and projected at the desired focal plane under different angles of left circularly polarized (LCP) light. Specifically, the proposed multifunctional metasurface generates two distinct holographic images under LCP illumination at angles of +35 and -35°. The presented holographic displays may provide a feasible route toward multifunctional meta-devices for potential AR displays, encrypted imaging, and information storage applications.

Dielectric chiral metasurfaces for enhanced circular dichroism spectroscopy at near infrared regime.

Numerous applications of chiro-optical effects can be found in nanophotonics, including imaging and spin-selective absorption, particularly in sensing for separating and detecting chiral enantiomers. Flat single-layer metasurfaces composed of chiral or achiral sub-wavelength structures offer unique properties to manipulate the light due to their extraordinary light-matter interaction. However, at optical wavelengths, the generation of strong chirality is found to be challenging via conventional chiral metasurface approaches. This work intends to design and optimize a dielectric chiral meta-nano-surface based on a diatomic design strategy to comprehend giant chiro-optical effects in the near-infrared (NIR) regime for potential application in circular dichroism (CD) spectroscopy. Instead of using a single chiral structure that limits the CD value at optical wavelengths, the proposed metasurface used a diatomic (two meta-atoms with distinct geometric parameters) chiral structure as a building block to significantly enhance the chiro-optical effect. Combining both meta-atoms in a single periodicity of the building block introduces constructive and destructive interferences to attain the maximum circular dichroism value exceeding 75%. Moreover, using multipolar resonance theory, the physics behind the generation of giant chiro-optical effects have also been investigated. The proposed dielectric chiral meta-platform based on the extra degree of freedom can find application in compact integrated optical setups for CD spectroscopy, enantiomer separation and detection, spin-dependent color filters, and beam splitters.

Spin-isolated ultraviolet-visible dynamic meta-holographic displays with liquid crystal modulators.

Wearable displays or head-mounted displays (HMDs) have the ability to create a virtual image in the field of view of one or both eyes. Such displays constitute the main platform for numerous virtual reality (VR)- and augmented reality (AR)-based applications. Meta-holographic displays integrated with AR technology have potential applications in the advertising, media, and healthcare sectors. In the previous decade, dielectric metasurfaces emerged as a suitable choice for designing compact devices for highly efficient displays. However, the small conversion efficiency, narrow bandwidth, and costly fabrication procedures limit the device's functionalities. Here, we proposed a spin-isolated dielectric multi-functional metasurface operating at broadband optical wavelengths with high transmission efficiency in the ultraviolet (UV) and visible (Vis) regimes. The proposed metasurface comprised silicon nitride (Si3N4)-based meta-atoms with high bandgap, i.e., ∼ 5.9 eV, and encoded two holographic phase profiles. Previously, the multiple pieces of holographic information incorporated in the metasurfaces using interleaved and layer stacking techniques resulted in noisy and low-efficiency outputs. A single planar metasurface integrated with a liquid crystal was demonstrated numerically and experimentally in the current work to validate the spin-isolated dynamic UV-Vis holographic information at broadband wavelengths. In our opinion, the proposed metasurface can have promising applications in healthcare, optical security encryption, anti-counterfeiting, and UV-Vis nanophotonics.

Single-layered meta-reflectarray for polarization retention and spin-encrypted phase-encoding.

Broadband communication with high data rates is a dire need for state-of-the-art wireless technologies. For achieving efficient wireless communication (particularly in an indoor environment), the electromagnetic (EM) waves should maintain their state of polarization despite encountering multiple reflections. Metasurfaces provide a unique platform to design subwavelength-featured meta-reflectarrays which enable the desired retention of the polarization state of an EM wave upon reflection. We present a single-layered broadband meta-reflectarray, simultaneously breaking n-fold (n > 2) rotational and mirror symmetry, which exhibits an unprecedented control over the phase, amplitude, and polarization of a reflected EM wave. This unique control enables the retention of polarization state and recording of spin-encrypted information for the reflected EM waves. Such novel multifunctional meta-reflectarray can be crucial to building an indoor setup for high data rate wireless communications. Meanwhile, the meta-array's ability to encode phase information provides an extra degree of freedom to structure and control (via incident spin) the reflected EM beam in the desired way. For the proof of concept, we have experimentally demonstrated a spin-encrypted holographic display which reconstructs the recorded holographic image at an image plane for the left circularly polarized (LCP) illumination and exhibits circular dichroism for the right circularly polarized (RCP) incident waves. The proposed meta-array can find applications in 5G indoor wireless communication, chiral sensing, spin-selective imaging, holography, and encryption.

Engineering tunability through electro-optic effects to manifest a multifunctional metadevice.

Ultrafast modulation of the refractive index exhibits either linear or nonlinear electro-optic (EO) effect, which is extensively utilized in tunable photonic circuits. Silicon, a mature material for on-chip devices, lacks a strong electro-optic (EO) Pockels effect. Utilization of the Pockels effect alters the intrinsic property (refractive index) of the material that manifests tunability and offers expanded functionalities. Driven by the limited space constraints in data storage, sensing, and imaging applications, we propose an electrically tunable meta-device whose resonance wavelength and focal length can be tuned by varying applied electric fields in the visible range. The fundamental unit of a metalens is the barium titanate (BTO) diffractive optical element placed on indium titanium oxide (which serves as an electrode) with SiO2 as the substrate. The metalens' tunability is characterized by point spread function (PSF), full-width half-maximum (FWHM), and imaging bandwidth that demonstrates the tuning of resonance wavelength and focal length. Moreover, polarization-insensitive meta-holograms are realized at a wavelength of 633 nm without utilizing propagation and Pancharatnam-Berry (PB) phase. The proposed study can find exciting applications in machine vision, broadband microscopy, and spectroscopy.