Ultrafast all optical modulation based on intersubband transition in semiconductor quantum wells

Ultrafast modulation of interband-resonant light by intersubband-resonant l ight in n-doped GaAs/AlGaAs and GaN/AlGaN quantum wells was investigated by femtosecond pump-probe technique. A planar-type AlGaAs/GaAs modulation dev ice shows a modulation speed of similar to1 ps at room temperature. The obs erved modulation efficiency indicates that 99% modulation can be achieved w ith a control pulse energy of similar to1 pJ when a waveguide-type device s tructure is utilized. The feasibility of the all-optical modulation in GaN/ AlGaN quantum wells is also investigated. The intersubband carrier relaxati on time, which mainly determines the modulation speed, is measured and is f ound to be extremely fast (130-170 fs). The results indicate that the optic al modulation at a bit rate of over 1 Tb/s will be possible by utilizing th e intersubband transition in GaN/AlGaN quantum wells. The modulation effici ency in GaN/AlGaN quantum wells is also discussed in comparison with that i n GaAs/AlGaAs quantum wells.