Intersubband transitions in InGaAs/AlAs QWs on GaAs and application to ultrafast all-optical modulators

Abstract

Shortening of intersubband transition (ISB-T) wavelength is carried out by introducing InGaAs / AlAs quantum wells (QWs) grown on GaAs substrates, which have a large conduction band offset larger than 1.1 eV. Near-infrared ISB-T up to 1.9 μm has been achieved in the QWs. In advance of this as one of the most important applications of the ISB-T, ultrafast all-optical modulators based on a nonlinear coupling between interband resonant light and ISB resonant light in n-doped QWs are explained. Here, modulation by mid-infrared ISB-T in GaAs/AlGaAs QWs is demonstrated by using a free-electron laser system as an ISB light source. To apply the near-infrared ISB-T to the ultrafast modulators, it is investigated whether the relaxation time is still very fast as in the case of the mid-infrared ISB-T or not. Femtosecond one color pump-probe experiment is carried out and the relaxation time is shown to be as fast as 2∼3 ps. The increase of the relaxation time with the increase in the ISB energy spacing is considered to mainly originate from the increase of the intrasubband energy relaxation time.