<title>Intersubband transition in AlGaN-GaN quantum wells for ultrafast all-optical switching at communication wavelength</title>

Abstract

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.