Fig. 1
Self-induced blue-shift of harmonics from a nonlinear photon accelerating semiconductor infrared metasurface (PASIM). a The concept of blue-shifted harmonics generation. Mid-infrared (MIR) photons are trapped by the metasurface (MS) cavity, blue-shifted by the rapid refractive index variation due to free carrier generation inside the MS, and then nonlinearly upconverted to near-infrared (NIR) photons via the standard THG process. The blue-shifted MIR and NIR photons then leave the MS, and their spectra are detected in transmission. Red shaded areas in the plots illustrate the radiation spectra, the gray area represents the temporal profile of the incoming pulse. Solid and dashed curves illustrate the time-dependent nature of the MS resonance. b Schematic of the sample and the MIR beam setup. In experiments, the following dimensions of the sample were used: wx = 0.87 μm, wy = 1.54 μm, g = 400 nm, and h = 600 nm. Inset: a close-up scanning electron micrograph of the metasurface elements; scale bar: 1 μm. c Transmittance spectra of the metasurface measured using Fourier-transform infrared spectroscopy. Dashed line: central wavelength λL of the MIR fs pulses. Inset: simulated electric field E (arrows) and intensity |E|2 (color) inside a metasurface unit cell for the incident light at λ = λL. The red region shows the Gaussian fit to the spectrum of the pump pulses. d Experimentally measured third harmonic generation (THG) in the metasurface (blue curve) and in an unstructured Si film (black curve) of the same thickness h. Both spectra are measured at the same MIR fluence of F = 2.3 mJ cm−2 (peak intensity I = 11 GW cm−2). A notable blue-shift of the upconversion peak from the metasurface with respect to the expected THG is a result of the plasma-induced acceleration of MIR photons. Inset: the same data plotted on the logarithmic scale. Gray region: the noise floor. The THG from the metasurface extends to λ = 1.06 μm, with intensities eight orders of magnitude higher than projected from the unstructured film (Gaussian fit given in dashed line): a clear indication of the emergence of the photons that are not present in the incident spectrum. e Spectra of MIR pulses transmitted through the metasurface for different input fluences revealing significant pulse self-modulation
