Two-wavelength absorption modulation spectroscopy of bandtail absorption in GaAs quantum wells

Abstract

We have discovered that below-band-gap photoexcitation produces large bleaching of the exciton absorption in GaAs quantum well heterostructures. We have used this effect to perform the first investigation of room-temperature bandtail absorption in these structures. We find that the below-band-gap absorption follows a spectral Urbach’s rule. In addition, proton-bombarded samples show an Urbach energy correlated with bombardment-induced defects. This sensitive technique has enabled us to study samples as thin as 1 μm at energies where the absorption coefficient is ∼10 cm−1.