Pump-probe measurement of ultrafast all-optical modulation based on intersubband transition in n-doped quantum wells

Modulation of interband-resonant light (similar to 800 nm) by intersubband- resonant light (5-7 mu m) was investigated in n-doped AlGaAs/GaAs multiple quantum wells by a two-color femtosecond pump-probe technique. Modulation w ith a recovery time of similar to 1 ps is observed in a plainer-type modula tion device at room temperature. The modulation of interband absorption coe fficient is similar to 1000 cm(-1) when the energy density of the intersubb and light pulse is similar to 4 fJ/mu m(2). The modulation efficiency indic ates that 99% modulation can be achieved with a control pulse energy of sim ilar to 1 pJ when a conventional waveguide-type device structure is utilize d. The mechanism which determines the modulation speed is discussed in term s of carrier relaxation process. It is shown that the modulation speed is m ainly determined by the inter- and intrasubband relaxation times, where the latter is influenced by hot phonon effects. (C) 2000 American Institute of Physics. [S0003-6951(00)02727-3].