Ultrafast Diffraction Shaping by Periodic Arrays of GaAs Nanoantennas

Efficient ultrafast control of the intensity and wavefront of light is essential for many modern optical devices, from LiDAR scanning systems to advanced laser beam shapers. A CMOS-compatible approach to spatial light control is based on tailoring the directional scattering from metasurfaces made of arrays of Mie-resonant semiconductor nanoparticles─meta-atoms. All-optical ultrafast tuning is possible by generating photoinduced carriers in semiconductor meta-atoms and is usually traced by overall transient reflection or transmission. However, the high intensities of light pulses required for effective modulation impose a limit for applications. Here, we introduce a diffractive metasurface based on gallium arsenide and demonstrate that ultrafast tuning of intensity is an order of magnitude stronger for the diffracted beam than for the transmitted one. Experimentally, the intensity modulation of the first diffraction order reaches 16% at a pump fluence level of only a few μJ/cm2, with a switching time of 200 fs and a relaxation time of 1.2 ps. The results constitute an important milestone in ultrafast optical switching device development for efficient low-power spatial light control.