Efficient Nonlinear Modulation for Dynamic Wavefront Shaping with High Quality Factor Phase Gradient Metasurfaces

Dynamic control of wavefront shaping is essential for optical technologies spanning communication, computation, and sensing. While metasurfaces promise high-fidelity light control in a highly reduced footprint, most current designs are static and rely on fixed geometric patterning that once fabricated cannot flexibly tune the device response.<br/>Here we present high quality (high-Q) factor phase gradient metasurfaces as a route to efficient nonlinear modulation of wavefront shaping devices including beamsteerers and lenses. Incorporating high-Q resonances within the metasurface design enables strong electromagnetic field confinement within individual nanoantennas, without compromising the metasurface transfer function. Further, high-Q metasurface resonances make the constituent nanoantennas more sensitive to subtle changes in refractive index, which improves the modulation of tunable devices via optical nonlinearities without sacrificing device efficiency.<br/>We construct our metasurface lens from a series of nanoscale silicon bars. Including subtle, periodic perturbations to individual nanobars allows guided modes in the silicon to couple to free space radiation. In both theoretical and experimental investigations we have demonstrated Q factors well above 10^3 can be achieved by these guided mode resonances. This further corresponds to near field intensity enhancements in the affected nanobars on the order of 10^4x or more. Using full-field simulations, we show how applying a nonlinear perturbation locally modulates the refractive index and shifts the spectral position of the high-Q resonance within individual nanoantennas. Here we construct a cylindrical metalens with integrated high-Q structuring and investigate the optical Kerr effect as a mechanism for modulating the focal characteristics as a function of power. In computational studies, we achieved shifts in the focal position from 4 um to 6.5 um, as the power was increased from only 0.1 mW/um^2 to 0.5 mW/um^2. Additionally, a 4-fold decrease in the normalized focal intensity with increasing incident intensity is observed at the focal spot. Importantly, this power-limiting metasurface can be readily extended for multi-wavelength operation by integrating additional notches into the adjacent Si bars with varying bar widths. Furthermore, utilizing other nonlinearities, such as electro-optic effects, promises further advancements to this modulation scheme. By biasing across an individual nanoantenna, the spectral shifts of each local resonance also allows us to control the phase and amplitude at a set frequency. This effect can demonstrate a full 2pi phase variation to construct phase gradient transfer functions defined by the applied field, rather than varying the size, spacing, or geometry of the constituent nanoantennas. Our presentation will discuss not only the design but also fabrication and characterization en route to a versatile metasurface platform that can reconfigurably achieve a variety of transfer functions.