Fig. 1. Schematic diagram of the experimental setup.

Fig. 2. (a) Laser beam intensity profiles simulation for the energies 87 μJ, 93 μJ, 100 μJ and 107 μJ. (b) Simulation of the significant role avalanche plays after photoionization (PI) introduces "seed" electrons in sapphire when irradiated with different laser energies. (c) Full and PI electron densities as a function of laser energy.

Fig. 3. (a) Total photoionization, MPI and tunneling ionization rate as a function of laser intensity in sapphire. (b) Enlarge image where MPI and tunneling ionization rate overlap at a Keldysh parameter of about 1.5. (c) Keldysh parameter as a function of laser intensity in sapphire.

Fig. 4. (a) Effective bandgap and (b) number of photons required for MPI process as a function of laser intensity in sapphire.

Fig. 5. [in Chinese]