Characteristics of the absorption recovery and the saturation of intersubband transition in GaN/AlN quantum wells are investigated for the purpose of applying these quantum wells to optical switches operating at a higher bit rate than 1 Tb/s. The pump-probe measurement verifies the absorption recovery time to be 150 fs at a wavelength of 4.5 μm. Dependence on the absorption on the input light intensity is examined at a wavelength of 1.48 μm for an optical pulse with a width of 130 fs. The characterization is performed with the Lorentzian fit of the absorption spectrum on the assumption of a two-level system. The result indicates that the recovery time is much less than 1 ps and the absorption saturation intensity is of the order of pJ/μm<sup>2</sup>. A ridge waveguide was fabricated and the onset of the intersubband absorption was confirmed. Finally, the switching performance is studied by means of the finite-difference time-domain (FDTD) simulation combined with three-level rate equations. Ridge waveguide structures with 3-QWs in the mid-layer are examined. Control and signal pulses are assumed to be the Gaussian pulses with a width of 250 fs. The results show that an extinction ratio of larger than 10 is achievable with an input control pulse energy of less than 1 pJ.
The intersubband transition (ISBT) in nitride quantum wells (QWs) is considered to be an excellent device mechanism for ultrafast optical switches capable of 1 Tb/s operation at room temperature. The 1.55-micrometers ISBT is feasible because of a large (approximately 2 eV) conduction band discontinuity in AlGaN/GaN QWs. The intersubband relaxation time in AlGaN/GaN QWs was calculated to be about 100 fs, which is 25 times shorter than that in AlAs/(In)GaAs QWs. The fast relaxation in nitride semiconductors is due to the strong interaction between electrons and LO-phonons. Intersubband absorption in the wavelength range of 3 - 7 micrometers was observed in MOCVD-grown AlGaN/GaN QWs, and the ultrafast response of the ISBT in nitrides was experimentally verified. The ISBT wavelength in the nitride QWs, however, was found to be affected by a strong built-in field (approximately MV/cm) caused by the spontaneous polarization and piezoelectric effect. A design to realize the ISBT at the communication wavelength in AlGaN/GaN QWs with a strong built-in field is discussed. Next, we report on an ultrashort pulse propagation model for nonlinear optical waveguides utilizing the intersubband absorption in AlGaN/GaN QWs. The finite-difference time-domain approach in conjunction with the rate equations describing the ISBT was adopted. Ultrafast optical gate operation in the waveguide was simulated.
The feasibility of the inter-subband transition (ISBT) in Al(Ga)N/GaN quantum wells (QWs) as a device mechanism for ultrafast optical switches is theoretically investigated. The 1.55-micrometers ISBT is shown to be feasible because of its large conduction band discontinuity. The inter-subband relaxation time at 1.55 micrometers 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.