Time-Modulated Conducting Oxide-Based Nanoantennas for Shared-Aperture Multi-Frequency Metasurfaces

Actively tunable optical nanoantennas are of great importance for dynamic control over the wavefront of light, where real-time and reprogrammable functions are required. Electro-optical tuning mechanisms based on electro-refraction effects induced by the free carriers in doped semiconductors are of growing interest for the realization of active metasurfaces. In particular, indium tin oxide (ITO) is a degenerately doped semiconductor, whose incorporation into the subwavelength unit cells of the geometrically-fixed metasurfaces enables post-fabrication electrical tuning of the optical response at near-infrared frequencies. The epsilon-near-zero transition in the ITO as well as novel design paradigms leveraging high-Q resonant modes allow for extreme light-matter interactions for enhancing the tunability of optical response. Despite the fruitful progress, the operating principle of ITO-integrated quasi-static metasurfaces relies on the modulation of resonant modes between the over- and under-coupled regimes. This limits the performance of such quasi-static metasurfaces to negligibly narrowband regime due to the strong resonant dispersion. Introducing time-modulation into the nanoantennas as an additional degree of freedom, renders a four-dimensional design space and surmounts the limitations of the quasi-static metasurfaces by converting the incident signal at the fundamental frequency to the higher-order sidebands. The phase of higher-order sidebands generated by a time-modulated metasurface can be tuned with uniform amplitude via a non-resonant dispersionless geometric phase shift induced by the modulation phase delay. This dispersionless phase elevates time-modulated metasurfaces beyond their quasi-static counterparts in that it increases the functionality bandwidth, expands the angle-of-view, and minimizes the power coupled into undesired sidelobes by providing access to the full phase span (2π) with uniform amplitude. The current architecture of deep space and local area networks calls for high capacity and high-speed platforms that can simultaneously address multiple users with minimal crosstalk. Time-modulated nanoantennas are promising candidates for multi-frequency functioning as they feature all-angle and all-wavelength optical response due to the access to dispersionless phase span at the sidebands. To demonstrate the adaptive multi-frequency multi-beam steering by the time-modulated metasurface, an array of plasmonic nanostrips integrated with ITO in metal-insulator-metal configuration is considered, wherein two sets of time-varying biasing signals are independently applied to the dual-gated metasurface for modulating the permittivity of ITO in space and time. The aperture of the metasurface is then divided into several interleaved orthogonally modulated sub-array nanoantennas with distinct modulation frequencies to render a shared-aperture metasurface in space and time. The spatially interleaved sub-arrays are programmed to exploit multi-beam scanning via a pixelated control over the modulation phase delays assigned to their constituent elements. The number of sub-arrays and distinct channels can be scaled easily without suffering from crosstalk due to the orthogonality of the channels or the efficient metasurface design. The results point toward high-capacity platforms with low size, weight, and power for next-generation free-space optical (FSO) communication systems.