Influence of Laser Intensity and Location of the Fermi Level on Tunneling Processes for High-Harmonic Generation in Arrayed Semiconducting Carbon Nanotubes

Recent advancement in high-power mid-infrared lasers has facilitated the investigation of extreme nonlinear optical phenomena in solids, which exhibit unconventional behaviors owing to strong light–matter interactions. High-harmonic generation (HHG) is a typical phenomenon that generates harmonics through nonperturbative processes. Herein, the strong laser field tunnels the charged particles from the valence to the conduction band and induces their carrier motion along the band, and then their motion generates high harmonics. In this study, we demonstrate that both the location of the Fermi level and the intensity of the laser field significantly affect the tunneling process in semiconducting carbon nanotubes through experimental measurements and theoretical calculations. Additionally, on- or off-tunneling can determine the nonperturbative or perturbative harmonic generation processes, respectively, and this crossover can be observed owing to the shift in the gate voltage. Tuning the Fermi level and laser intensity is crucial for manipulating the tunneling process, carrier oscillations, and harmonic generation mechanisms. The results provide further insights into the nonlinear optical processes of semiconductors and pave the way for precisely controlling HHG in solids by using a static electric field.