Intersubband relaxation dynamics in narrow InGaAs/AlAsSb and InGaAs/AlAs quantum well structures using pump-probe spectroscopy

Intersubband (ISB) transitions in semiconductor quantum wells (QWs) can be employed for various mid-infrared optoelectronic devices. Presently there is strong interest to extend the available wavelength range into the near infrared, by using materials with a large conduction band offset. To achieve such short wavelengths thin QWs are required, where the first excited state inside the QW may lie higher than some state related to indirect valleys. Examples for such material systems are strained InGaAs/AlAs or lattice matched InGaAs/AlAsSb, both grown on InP. We have studied the ISB relaxation dynamics in multi QWs of both material systems by femtosecond pump-probe measurements. The transient transmission as a function of the pump-probe delay does not show a single-exponential decay, indicating a more complicated relaxation dynamics. This can be caused by transfer of electrons to X- or L- states in the QWs or the barriers. We will show results on samples with different QW thicknesses and compare them to simulations based on rate equations.