Ultrafast intersubband relaxation and nonlinear susceptibility at 1.55 μm in GaN/AlN multiple-quantum wells

Using a degenerate pump–probe technique on the Brewster configuration at room temperature, we investigate the dynamics of the intersubband transition in GaN/AlN multiple-quantum wells. The relaxation dynamics is found to consist of ultrafast (∼140 fs) and slower (∼1.3 ps) components. We estimate the third-order susceptibility for the Brewster configuration to be ∼5.5×10−18 m2/V2, which indicates that its value becomes ∼2.2×10−16 m2/V2 when a light beam is parallel to the well.