Fig. 2
Spectral characteristics of blue-shifted harmonics in photon accelerating semiconductor infrared metasurfaces (PASIMs). a Third harmonic generation (THG) spectra measured for various input fluences for the unpatterned Si film and for the metasurface. Shaded gray area: THG from the unstructured film at the highest fluence Fmax = 5.5 mJ cm−2. Shaded blue areas indicate the integration range of λTH < 1.17 μm. b Spectrally integrated third harmonic generation as a function of the mid-infrared fluence the film (open circles) and metasurface (closed circles), log–log scales. Lines denote the guide-to-the-eye ITH ∝ F3 dependence. Inset: THG spectrum integrated over 1.1 μm < λTH < 1.17 μm as a function of F for the unstructured film (open circles) and the metasurface (filled circles). The THG spectrum from the PASIM reveals self-induced blue-shifting by ≈30 nm (c) and broadening by ≈50% (d) as a function of F; both effects are negligible for the unpatterned film. Error bars on the plots represent standard errors of the mean
