Polarization Analysis of High Harmonic Generation in Solids Using Intense Mid-IR Sources

Carrier-envelope phase-stable, femtosecond intense light sources in the mid-IR region are developed to produce high harmonics in solids. Using these light sources, high harmonics are produced in GaSe. Polarization analysis along with a first-principle simulation suggests that the intraband current is the primary source for lower-order odd harmonics, which show unusual 30-degree periodicity. High harmonic generation with circularly polarized electric field is also investigated. The selection rule for circularly polarized high harmonics is experimentally confirmed for the first time.