Parikshit Moitra

Dielectric meta-reflectarray for broadband linear polarization conversion and optical vortex generation.

Plasmonic metasurfaces have recently attracted much attention due to their ability to abruptly change the phase of light, allowing subwavelength optical elements for polarization and wavefront control. However, most previously demonstrated metasurface designs suffer from low coupling efficiency and are based on metallic resonators, leading to ohmic loss. Here, we present an alternative approach to plasmonic metasurfaces by replacing the metallic resonators with high-refractive-index silicon cut-wires in combination with a silver ground plane. We experimentally demonstrate that this meta-reflectarray can be used to realize linear polarization conversion with more than 98% conversion efficiency over a 200 nm bandwidth in the short-wavelength infrared band. We also demonstrate optical vortex beam generation using a meta-reflectarray with an azimuthally varied phase profile. The vortex beam generation is shown to have high efficiency over a wavelength range from 1500 to 1600 nm. The use of dielectric resonators in place of their plasmonic counterparts could pave the way for ultraefficient metasurface-based devices at high frequencies.

Realization of an all-dielectric zero-index optical metamaterial

Previously demonstrated zero- or negative-refractive-index metamaterials at optical frequencies suffer from large ohmic losses because of the need to use metals. Metamaterials formed by stacked silicon rod unit cells allow the realization of all-dielectric impedance-matched zero-index metamaterials operating at optical frequencies, potentially benefiting the development of angular-selective optical devices.

Experimental demonstration of a broadband all-dielectric metamaterial perfect reflector

All-dielectric metamaterials utilizing Mie resonances in high-permittivity dielectric resonators offer a low-loss alternative to plasmonic metamaterials. Here we present the demonstration of a single-negative all-dielectric metamaterial, comprised of a single layer of cylindrical silicon resonators on a silicon-on-insulator substrate, that possesses peak reflectance over 99% and an average reflectance over 98% across a 200 nm wide bandwidth in the short-wavelength infrared region. The study is also extended to disordered metamaterials, demonstrating a correlation between the degree of disorder and the reduction in reflectance. It is shown that near-unity reflection is preserved as long as resonator interaction is avoided. Realization of near-unity reflection from disordered metamaterials opens the door to large-area implementations using low-cost self-assembly based fabrication techniques.

Metasurface polarization splitter

Polarization beam splitters, devices that separate the two orthogonal polarizations of light into different propagation directions, are among the most ubiquitous optical elements. However, traditionally polarization splitters rely on bulky optical materials, while emerging optoelectronic and photonic circuits require compact, chip-scale polarization splitters. Here, we show that a rectangular lattice of cylindrical silicon Mie resonators functions as a polarization splitter, efficiently reflecting one polarization while transmitting the other. We show that the polarization splitting arises from the anisotropic permittivity and permeability of the metasurface due to the twofold rotational symmetry of the rectangular unit cell. The high polarization efficiency, low loss and low profile make these metasurface polarization splitters ideally suited for monolithic integration with optoelectronic and photonic circuits. This article is part of the themed issue ‘New horizons for nanophotonics’.

All-dielectric metasurfaces

In this talk, I will discuss our recent efforts to develop purely dielectric metamaterials possessing low absorption loss at optical frequencies. I will discuss implementations ranging from wavefront control to high Q-factor resonances.

Realization of All-dielectric Optical Metamaterials

We present experimental realization of zero-index all-dielectric optical metamaterials. The metamaterials exhibit a nearly isotropic response, leading to angular selectivity of transmission and enhanced directive emission from quantum dots placed within the metamaterial. Article not available.

All dielectric zero-index metamaterials at optical frequencies

Dielectric metamaterials offer a potential low-loss alternative to plasmonic metamaterials at optical frequencies. However, demonstrations of dielectric metamaterials have so far been limited to microwave and mid-infrared frequencies. In this work, we outline the development of purely dielectric zero-index metamaterials operating at optical frequencies. The metamaterial, formed from silicon rods, exhibits impedance matching with air, resulting in unity transmission at the zero-index point. Design and experimental realization of the metamaterials is presented. The metamaterials can potentially be used for a number of applications including compact lens systems, directional emitters, and transformation optics devices.