Ultrafast All-Optical Light Manipulation with a Local High-Q Metasurface

Local modulations in refractive index due to ultrafast nonlinear effects present a promising route for realizing space-time-varying optical materials. However, the typically small changes in refractive index prohibit their application for spatial wavefront patterning. Here, we report on space-time light modulation using local ultrafast all-optical modulation of the refractive index in a dielectric, higher-order Mie resonant metasurface. This is enabled by extreme local field enhancement stimulating increased Kerr nonlinearity and free carrier effects in the nanostructure. The metasurface consists of amorphous silicon nanoparticles exhibiting a spectrally overlapped electric dipole and electric octupole mode, and allows for local wavefront manipulation. First, we investigate the time-dependent transmission modulation of the metasurface by resonantly pumping the metasurface at a wavelength of 1305 nm with an intense pulsed illumination at normal incidence (100 fs pulse length, 1 kHz, fluence up to<i> F </i>= 2.3 mJ/cm²) and analyzing the spectrum of the transmitted light. We observe strong pump self-modulation evidenced by a red shift of the resonance at low power (<i>F</i> &lt; 24 μJ/cm²) due to the optical Kerr effect, and a pronounced blue shift of up to 3.1 nm at higher power originating from free carrier effects. Next, we experimentally demonstrate space-time diffraction of a wavefront by spatially patterning a transient ultrafast grating using two pump beams incident on the metasurface at angles θ = ±6° and measure the diffraction of the pump on the grating. Finally, we present results from time-dependent degenerate pump-probe experiments of the space-time light modulation.