All-Dielectric High-Q Thermo-Optically Tunable Transmissive Metasurfaces

We theoretically demonstrate all-dielectric low-loss active metasurfaces, which can dynamically manipulate the wavefront of the transmitted light in the near-infrared wavelength range. Our active metasurfaces feature an array of amorphous silicon (a-Si) pillars on a silica (SiO₂) substate, which support supercavity resonances with quality factors (Q-factors) as high as 9800, as well as lower-Q resonances. We show that the considered supercavity mode is also supported by an isolated pillar with the highest achievable quality factor of 676. We observe that the quality factor of the supercavity mode increases by a factor of 14 when the metasurface elements are arranged in a metasurface array.<br/>Next, we propose a metasurface geometry and a realistic interconnect architecture, which enable thermo-optic dynamic beam steering and dynamic beam switching with switching times as low as 7.3 μs. In the proposed metasurface implementation, the top and bottom 50 nm of Si are n-doped with a carrier density of 6 × 10¹⁸ while the carrier density of the lightly doped Si core is 3.2 × 10¹⁷cm^-3. By applying voltage between the top and bottom doped Si layers, we flow a current between the doped layers through the lightly doped a-Si layer. The induced current raises the temperature a-Si pillars by means of Joule heating, resulting in a modified refractive index of a-Si. To realize dynamic beam switching or beam steering in transmission, we utilize modest refractive index modulation of amorphous Si ranging between Δn = 0.0026 to Δn = 0.006, depending on the specific implementation. In our proposed metasurface structures, we benefit from both relatively low-Q modes as well as high-Q supercavity mode supported by an individual a-Si pillar. We use full wave optimization to obtain high directivity steered beam using the particle swarm optimization algorithm. We show that that that our full wave optimization approach increases the directivity of the steered beam as compared with the case when the intuitive forward design approach is used.<br/>We observe that both the high-Q and lower-Q modes can steer the beam for two orthogonal incoming polarization directions at two different wavelengths, indicating that with an appropriate interconnect design, the proposed metasurface can potentially be used for two-dimensional beam steering.<br/>It is also worth mentioning that, so far, most of the active metasurfaces enabling dynamic wavefront shaping operate in reflection. On the other hand, active metasurfaces operating in transmission can readily be integrated with chip-scale light sources, yielding ultra-compact wavefront shaping devices.