Terahertz Perturbed, Mid-infrared Driven High-Order Harmonic Generation from Solids*

We have studied high-order harmonic generation from ZnO thin film driven by an 80 fs, 3.6 $\mu $m mid-infrared (MIR) pulse, and dressed by a 2 ps, 600 $\mu $m single-cycle terahertz (THz) pulse. The existence of the weak quasi-DC THz field breaks the symmetry of the MIR driving field and/or the symmetry of the crystal, and we observe the generation of even-order harmonics, up to harmonic order 20. The intensity of each even-order harmonic scales linearly with that of the THz pulse, indicating a perturbation that involves one THz photon. We also find that, when the THz field is applied, the intensities of the odd-order harmonics are reduced (e.g., harmonic order 19, $I (F$THz $=$ 300 kV/cm) \quad $\approx $0.65$ * I (F$THz $=$ 0)), while the intensity of the total harmonics (odd $+$ even) stays constant, indicating that the alternation of the generalized electron-hole recollision by the THz field results in a repopulation of the total harmonics into each harmonic order. Our study paves the way for ultrafast control of the high harmonic generation process in solids by THz fields and suggests a novel method for THz metrology.