Fig. 3.

Transient optical properties of GaAs metasurfaces. a Transient reflectance measurement results for the sample (red curve) and for the substrate (black curve; ordinate values are multiplied by 20 for comparison) at a pump fluence of 380 μJ cm⁻². An enhancement factor of about 50 is attained for the modulation depth at the metasurface sample. b Measured differential reflectance as a function of probe wavelength and time delay between the pump and probe pulses. White dashed lines denote the spectral positions of the ED and MD modes, while the black one indicates the area of low reflectance situated in between the resonances. c Power-dependent transient reflectance traces. The low-perturbation regime (black curve) reveals exponential decay with a time constant of τ _relax ≈ 2.5 ps, whereas higher fluences (red and green curves) provide more complicated traces owing to the blue shift of the MD resonance