Optical metasurfaces are regular quasi-planar nanopatterns that can apply diverse spatial and spectral transformations to light waves. A critical challenge is to realize a technique of tuning their optical properties that is both fast and efficient. We will overview our recent results on controlling the properties of semiconductor-based metasurfaces with femtosecond laser pulses. Starting from silicon metasurfaces, where the instantaneous effect of two-photon absorption can be utilized to show all-optical switching as fast as 65 fs, we will show how direct-gap semiconductors can enable extremely nonlinear metasurfaces. In particular, it is experimentally shown that magnetic dipole resonances of GaAs nanoresonators can be tuned by almost full width at half maximum by ultrafast injection and relaxation of free carriers. We will also show that dynamic epsilon-near-zero and plasmonic regimes can be realized at longer wavelengths, making a smooth transition between plasmonic and all-dielectric metasurfaces.
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