Observation of ultrafast all-optical modulation based on intersubband transition in n-doped quantum wells by using free electron laser

A picosecond-range all-optical modulation based on the nonlinear interaction between interband- and intersubband-resonant light in GaAs/AlGaAs multiple quantum wells was demonstrated. A free electron laser (FEL) with a very short pulse width (<10 ps) was used for the intersubband excitation. The dependence of the modulation depth on the wavelength of the intersubband-resonant light was investigated by utilizing the wide tunability of the FEL. The result is in good agreement with the intersubband absorption spectrum of the multiple quantum wells measured by FT-IR spectroscopy, which indicates that the observed modulation is indeed based on the intersubband transition induced by the FEL.