Spectroscopy of Narrow Minibands in the Continuum of Multi Quantum Wells

We report on the optical studies of intersubband transitions in InGaAs/InP multi-quantum-well superlattices. Absorption, photoinduced absorption and time-resolved photoinduced absorption are used in a complementary way in our studies. By comparing with a multi-band k-p theoretical model we spectroscopically identify all the observed transitions, explain their relative strength, and spectral shape. We show that for energies below the InP barrier’s conduction band, the spectrum is composed of inhomogeneously broadened transition between the first to the second confined electronic level. At low temperature the broadening is due to monolayer fluctuations in the well width. We measure characteristic time of a few nano-seconds for carriers to reach thermal equilibrium in the broadened first electronic level. Above the InP conduction band the spectrum is composed of well defined narrow minibands with minigaps between them. The optical transitions to these newly observed minibands, their polarization selection rules, temperature dependence, and characteristic lifetime are measured and discussed below.