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We propose an electrically tunable metasurface, which can achieve relatively large phase modulation in both reflection and transmission modes (dual-mode operation). By integration of an ultrathin layer of indium tin oxide (ITO) as an electro-optically tunable material into a semiconductor-insulator-semiconductor (SIS) unit cell, we report an approach for active tuning of all-dielectric metasurfaces. The proposed controllable dual-mode metasurface includes an array of silicon (Si) nanodisks connected together via Si nanobars. These are placed on top of alumina and ITO layers, followed by a Si slab and a silica substrate. The required optical resonances are separately excited by Si nanobars in reflection and Si nanodisks in transmission, enabling highly confined electromagnetic fields at the ITO-alumina interface. Modulation of charge carrier concentration and refractive index in the ITO accumulation layer by varying the applied bias voltage leads to 240° of phase agility at an operating wavelength of 1696 nm for the reflected transverse electric (TE)-polarized beam and 270° of phase shift at 1563 nm for the transmitted transverse magnetic (TM)-polarized light. Independent and isolated control of the reflection and transmission modes enables distinctly different functions to be achieved for each operation mode. A rigorous coupled electrical and optical model is employed to characterize the carrier distributions in ITO and Si under applied bias and to accurately assess the voltage-dependent effects of inhomogeneous carrier profiles on the optical behavior of a unit cell.

Abstract: In this paper, an optically active reflective metasurface with tunable phase performance is designed which operates based on the excitation of both electric and magnetic resonances in a silicon‐made nanoantenna element and the use of layered indium tin oxide (ITO)–alumina under the influence of external multigate biasing. The capability of achieving robust control over the reflection characteristics of two closely spaced electric and magnetic resonances is investigated. Adopting the multigate biasing enables relatively high reflection amplitude of 0.4 over the entire phase‐change coverage (≈180°) in the near‐infrared regime by exploiting the constructive interference between the geometrical resonances. The physical mechanism behind this ITO‐assisted metal–insulator–semiconductor building block can be described as the strong confinement of electromagnetic fields at the vicinity of ITO layers and the possibility of modulating the carrier concentrations of the integrated active materials by electrically varying the bias voltage. The uniformity of reflection magnitude for all reflection phase coverage, relatively high reflectivity, and electro‐optical tunability is leveraged to design three multifunctional devices with step‐by‐step increase in complexity of biasing network including ultrathin reconfigurable linear/circular polarizer (simple identical biasing), dynamical beam steering platform (two‐state biasing), and tunable metalens with controllable on‐ and off‐axes focusing pattern in real‐time (advanced element‐by‐element biasing). [ABSTRACT FROM AUTHOR]