Intersubband relaxation dynamics in short-wavelength InGaAs/AlAsSb quantum well structures

Intersubband transitions in semiconductor quantum wells (QW) are crucial for mid-infrared lasers, detectors, and modulators. New compound materials such as lattice matched InGaAs/AlAsSb and strain compensated InGaAs/AlAs, both grown on InP, feature large conduction band discontinuities (>1eV) and allow the extension of the available wavelength range into the near infrared. Such short wavelengths require narrow QWs (<3 nm) where the first excited state inside the QW may be raised above indirect (X or L) valleys within the Brillouin zone.
We have studied intersubband relaxation dynamics in In0.53Ga0.47As/AlAs0.56Sb0.44 multiple QWs with thicknesses between 2 and 4.6 nm (corresponding to absorption wavelengths of 1.9 to 3.2 µm) by femtosecond pump-probe experiments. The high repetition rate (78 MHz) of our 280 fs pulses in combination with a rapid-scanning technique results in a detectable transmission change as low as 10^-5. At early delay times, all samples show an exponential decay of the transient transmission occurring with time constants of 1 to 1.5 ps. The relaxation dynamics at later delay times strongly depends on the QW thickness and doping. For very narrow QWs the observed bi-exponential decay indicates several competing relaxation channels. Here transfer of electrons to X- and L-states in the wells or in the barriers is energetically possible. States localized in the barrier exist due to strong band bending resulting from the n-type modulation doping. Finally, we will also show rate-equation simulations to study possible relaxation scenarios within a three level system.