Excitonic Beam Steering in an Active van der Waals Metasurface

Two-dimensional transition metal dichalcogenides (2D TMDCs) are promising candidates for ultrathin active nanophotonic elements due to the strong tunable excitonic resonances that dominate their optical response. Here, we demonstrate dynamic beam steering by an active van der Waals metasurface that leverages large complex refractive index tunability near excitonic resonances in monolayer molybdenum diselenide (MoSe₂). Through varying the radiative and nonradiative rates of the excitons, we can dynamically control both the reflection amplitude and phase profiles, resulting in an excitonic phased array metasurface. Our experiments show reflected light steering to angles between -30° and 30° at different resonant wavelengths corresponding to the A exciton and B exciton. This active van der Waals metasurface relies solely on the excitonic resonances of the monolayer MoSe₂ material rather than geometric resonances of patterned nanostructures, suggesting the potential to harness the tunability of excitonic resonances for wavefront shaping in emerging photonic applications.

Kerr Metasurface Enabled by Metallic Quantum Wells

Optical metasurfaces have emerged as promising candidates for multifunctional devices. Dynamically reconfigurable metasurfaces have been introduced by employing phase-change materials or by applying voltage, heat, or strain. While existing metasurfaces exhibit appealing properties, they do not express any significant nonlinear effects due to the negligible nonlinear responses from the typical materials used to build the metasurface. In this work, we propose and experimentally demonstrate one kind of Kerr metasurface that shows strong intensity-dependent responses. The Kerr metasurface is composed of a top layer of gold antennas, a dielectric spacer, and a ground layer of metallic quantum wells (MQWs). Because of the large Kerr nonlinearity supported by the MQWs, the effective optical properties of the MQWs can change from metallic to dielectric with increasing of the input intensity, leading to dramatic modifications of the metasurface responses. This opens up new routes for potential applications in the field of nonlinear optics.

Dual-Gated Active Metasurface at 1550 nm with Wide (>300°) Phase Tunability

Active metasurfaces composed of electrically reconfigurable nanoscale subwavelength antenna arrays can enable real-time control of scattered light amplitude and phase. Achievement of widely tunable phase and amplitude in chip-based active metasurfaces operating at or near 1550 nm wavelength has considerable potential for active beam steering, dynamic hologram rendition, and realization of flat optics with reconfigurable focal lengths. Previously, electrically tunable conducting oxide-based reflectarray metasurfaces have demonstrated dynamic phase control of reflected light with a maximum phase shift of 184° ( Nano Lett. 2016 , 16 , 5319 ). Here, we introduce a dual-gated reflectarray metasurface architecture that enables much wider (>300°) phase tunability. We explore light-matter interactions with dual-gated metasurface elements that incorporate two independent voltage-controlled MOS field effect channels connected in series to form a single metasurface element that enables wider phase tunability. Using indium tin oxide (ITO) as the active metasurface material and a composite hafnia/alumina gate dielectric, we demonstrate a prototype dual-gated metasurface with a continuous phase shift from 0 to 303° and a relative reflectance modulation of 89% under applied voltage bias of 6.5 V.

Out-of-Plane Designed Soft Metasurface for Tunable Surface Plasmon Polariton

Reliable and repeatable tunability gives functional diversity for reconfigurable plasmonics devices, while reversible and large mechanical deformation enabled by soft materials provides a new way for the global or partial regulation of metadevices. Here, we demonstrate a soft metasurface with an out-of-plane design for tuning the energy of surface plasmon polaritons (SPPs) bloch wave using theory, simulation, and experiments. Our metasurface is composed of two-layered gold nanoribbon arrays (2GNRs) on a soft substrate. The out-of-plane coupling mechanism is systematically analyzed in terms of separation height effect. Moreover, by harnessing mechanical deformation, continuously tunable plasmonic resonance has been achieved in the visible and near-infrared ranges. We further studied the angle-dependent reflection spectra of our metastructure. Compared with its planar counterpart, our soft and two-layered metastructure exhibits diverse tunability and significant field enhancement by out-of-plane interactions. Our approach in designing soft metasurface with out-of-plane structures can be extended to more-complex photonic devices and finds prominent applications such as biosensing, high-density plasmonic circuits, surface-enhanced luminescence, and surface-enhanced Raman scattering.

Multi-Direction-Tunable Three-Dimensional Meta-Atoms for Reversible Switching between Midwave and Long-Wave Infrared Regimes

We introduce an electromechanically tunable metasurface composed of an array of three-dimensional nanosplit-rings for reversible and large-range changes of optical characteristics in infrared (IR) regime. When a current is induced or withdrawn, each nanosplit ring in the surface can deform in multi directions and consequently become a closed (OFF) or an open (ON) state. Theoretical and experimental results manifest that, as the metasurface is dynamically manipulated between the ON and OFF states, the corresponding resonance absorption will reversibly switch between the long wave (around 10.4 μm) and midwave (around 6.3 μm) IR regimes, two key IR spectral windows, and the reversible relative reflection changes can reach up to 95%.