Universal active metasurfaces for ultimate wavefront molding

Optical metasurfaces are becoming ubiquitous optical components to mold the amplitude, phase, and polarization of light. So far, most of these devices are passive in essence, that is, they cannot be arbitrarily reconfigured or optimized according to the user's interest and/or change in the surrounding environment. In this study, we propose an innovative design strategy relying on the position of topological singularities, namely zeros and poles of the reflection coefficient, to address full phase modulation of light reflected off an active metasurface with almost unity efficiency. The active metasurface unit cells, consisting of asymmetric Gires-Tournois resonators filled with either silicon or hetero-structured materials to leverage on the thermo-optical or electro-optical effects, respectively. In both cases, a full phase modulation associated with 100 % reflection amplitude is observed even when dealing with extremely low refractive index change, on the order of 1 %. Improving the deflection efficiencies for each deflection angle and accounting for the near-field coupling between strongly resonant pixels is performed by calculating the refractive index modulation profile in the extended unit cell using an advanced optimization methodology relying on statistical learning. Consequently, active beam steering designs for active thermo-optical effect with ultimate performance exceeding have been optimized. Furthermore, active wavefront splitting using electro-optics materials was optimized to reach ultimate modulation performances with nearly efficiency. The realization of highly efficient active beam-forming operating at high frequencies would open important applications in imaging microscopy, high-resolution image projection, optical communication, and 3D light detection and ranging (LiDAR).

Schematic representation of an ideal active metasurface unit-cell based on thermo-optical effect to actively tune the reflected phase. (a-c) refer to a Si unit-cell resonator surrounded by SiO, that is without a reflecting substrate and excited by a normally incident x-polarized light impinging from the top, whereas (d-g) represents the same resonator disposed on a thin gold layer of thickness 200 nm. The excitation also consists of an x-polarized beam coming at normal incidence. (a) A homogeneous Sio encapsulates a Si nano-ridge with dimensions of nm and width nm. The period of the unit cell is fixed at 600 nm, and the width and the thickness have been optimized to observe a sharp feature in the reflection spectrum around the desired wavelength of nm, as shown in (b). The corresponding cross-section of the electric field distribution (along the plane) is shown in (c). (d) To take advantage of the G-T phase modulation mechanism discussed earlier, the resonator is placed on a thin reflective gold layer (thickness 200 nm) separated from the Si nano-ridge by a distance.