Ultrafast interband-light modulation by intersubband light (free-electron laser) in quantum wells

Ultrafast interband-resonant light modulation induced by an intersubband transition in GaAs/AlGaAs multiple quantum wells is observed by using a free-electron laser (FEL) for intersubband excitation. The FEL operates with a picosecond pulse width and a wide wavelength tunability. The dependence of the modulation depth on the wavelength of the intersubband-resonant light shows good agreement with the intersubband absorption spectrum of the multiple quantum wells measured by Fourier transform infrared (FTIR) spectroscopy. We also investigated the relaxation time of the intersubband transition by a pump-probe experiment and found that it was less than the FEL micropulse width (similar to 3 ps). (C) 1998 Elsevier Science B.V. All rights reserved.