Observation of ultrafast interband-light modulation by intersubband-light in n-doped quantum wells by using free electron laser

An intersubband-transition (ISB-T) in a semiconductor quantum well (QW) has been drawing much attention due to its unique characteristics such as ultrafast energy relaxation time (/spl sim/1 ps) and large transition probability. In this paper, we report for the first time a picosecond range all-optical modulation by using a free electron laser (FEL) as the ISB-light. This has become possible by the attractive features ofthe FEL such as ultra-short pulse operation, wide tunability of the lasing wavelength, and high peak power keeping the average power low. The sample utilized for the experiment has a multiple quantum well structure with 30 periods of GaAs wells and Al/sub 0.3/Ga/sub 0.7/As barrier layers grown by molecular-beam epitaxy on a GaAs substrate, where the barrier layers were selectively n-doped with Si.