  High harmonic generation in solids is becoming an important method for strong
field solid state physics research. The power scale relationship between high
harmonics and the driving laser is investigated both experimentally and
theoretically. Results of the power scale dependence clearly divided the
interaction into two regimes. The modification of the bandgap by intense laser
proved to be very important for theoretically reproducing the experimental
result. Combining with the Keldysh theory analysis, the harmonic generation
process is found to be transitioned from multiphoton excitations to the
diabatic tunneling.
