We have investigated the ultrafast coherent dynamics of the intersubband transition in GaN/AlN multiple quantum wells, using a spectral resolved two-color pump-probe technique. We have found a significant spectrum change as a function of the delaytime τ. For negative delaytimes corresponding to τ < 0, coherent spectrum oscillations have been observed. At τ ~ 0, asymmetric dispersion has been observed. For positive delaytimes, τ > 0, no measurable spectrum change has been observed. From the analysis of these results, we have estimated that intersubband transitions dephasing time is more than 100 fs measured at room temperature.
We have investigated the ultrafast relaxation dynamics of intersubband transition (ISBT) in GaN/AlN, using a two-color pump-probe technique, in a wide energy range around the optical communication wavelength. We suggest that the origin of the signal depends on the relation between the pump and probe pulse energies. We have observed an ultrafast induced absorption signal and a slow negative component which are due to the absorption of electrons during intra-subband scattering and a carrier cooling process with a hot-phonon effect, respectively. Moreover, we clarify the origin of the inhomogeneous broadening width of the ISBT and of the intrinsic absorption width of ISBT from the detailed analyses of the result. We have reproduced the relaxation dynamics by a numerical calculation to confirm this interpretation of ISBT relaxation dynamics.