AlGaN/GaN intersubband transitions for Tb/s 1.55-μm optical switches

The feasibility of the inter-subband transition (ISBT) in AI(Ga)N/GaN quantum wells (QWs) as a device mechanism for ultrafast optical switches is theoretically investigated, The 1.55-mu m ISBT is shown to be feasible because of its large conduction band discontinuity. The inter-subband relaxation time at 1.55 mu m is estimated to be about 100 fs, which is 20-30 times shorter than that in InGaAs QWs. A large electron-electron scattering rate causes a short dephasing time (about 10 fs), which reduces the peak value of the third-order nonlinear susceptibility. At a high carrier density, however, the dephasing time is increased because of the screening and the exclusion, which enhances the nonlinear susceptibility. Ultrafast relaxation of the inter-subband optical nonlinearity in GaN QWs is little affected by the delay in intra-subband energy relaxation caused by non-equilibrium phonons. These characteristics suggest that the ISBT in nitride QWs is a promising mechanism for multi-terabit/s optical switches,