Fig. 1. Experimental setup of HHG in VO2. (a) Schematic diagram of the experimental setup for HHG measurement in reflection geometry. (b) XRD patterns of the sample. The inset is an SEM image of the sample.

Fig. 2. Experimental measurements of HHG in VO2. (a) Typical harmonic spectrum at a laser peak power of 1  TW/cm2. (b) Spectral integral intensity of the harmonics versus peak power of the MIR laser, with a dashed line representing a power law to guide the eye. (c) The high-energy cutoff scales linearly with the MIR laser field.

Fig. 3. Temperature-dependent normalized intensity of the fifth-harmonic in VO2 nanofilm upon heating (purple guide to the eye) and cooling (cyan guide to the eye). The color background marks the state of VO2, where blue represents the insulating phase and red represents the metallic phase.

Fig. 4. Dominant mechanism of harmonics via photo-carrier doping experiments. (a) Schematic of HHG from a photoexcited sample. The vertical arrow denotes the photoexcitation and interband harmonics. (b) Measured harmonic spectrum with and without photo-carrier doping, marked by solid lines and dotted lines, respectively. The black arrows compare the peak intensity of each harmonic with and without the pump. The photon energy of the pump pulse is 0.95 eV (60 fs), while the pump power is set as 9  mJ/cm2 and the time delay between the pump and probe pulses is set as 1 ps. (c) Integrated intensity of the harmonics as a function of the time delay between the pump and probe pulses. The solid line represents a fitting curve applied to the data based on Eq. (1).

Fig. 5. Pump-dependent time-resolved harmonics. (a) Normalized harmonics at several pump powers. The solid line represents a fitting curve applied to the data based on Eq. (1). (b) Modulation depth of harmonics after the photoexcitation versus pump power. Inset: the function between the pump power and the modulation depths of harmonics. The gray dotted line marks the complete modulation (modulation depth = 100%).

Fig. 6. Pump-dependent harmonic manipulation and its mechanism. Lifetime (a), (b) and amplitude (c), (d) of fast and slow processes as a function of pump power. (e) Pump-dependent transmittance of the MIR probe laser while the time delay between the pump and probe pulses is set as 1 ps. The red background marks the metallic phase of VO2. (f) Schematic electronic band structure and multiprocessors of ultrafast relaxation. (g) Schematic electronic band structure and the photo-induced IMT.

Fig. 7. Metallic phase fraction ε as a function of the pump power extracted from the harmonics. Inset: the spatial states of polycrystalline VO2 nanofilm for several pump powers. Segments of insulating (blue filled) and metallic (red filled) phases can exist within the VO2 nanofilm at the same time for moderate pump powers.