The defect-state-assisted enhancement of high harmonic generation in bulk ZnO

Optical modulation of high harmonic generation (HHG) at ultrashort timescales is of fundamental interest and central importance for emerging photonic applications. Traditionally, this modulation is realized by injecting incoherent electrons into the conduction band, which can only result in the suppression of HHG intensity. In this work, we have proposed and demonstrated an all-optical route to amplify a specific order of high harmonic generation in (11-20)-cut wurtzite zinc oxide (ZnO) based on the pump-probe configuration. Specifically, intensity enhancement is demonstrated by tuning the wavelength of the generation middle-infrared pulse when the wavelength of HHG matches the energy of a specific defect state. The maximum enhancement factor is observed to be 1.8, while the modulation speed varies with different defect states, which are 0.1 ps for the 5th HHG and 1.5 ps for the 4th HHG. This work might enlighten a new path for ultrafast modulation of HHG in solids for the future development of all-optical devices.