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

We have discovered that below bandgap 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-bandgap absorption follows a spectral Urbach's rule. In addition, proton-bombarded samples show and Urbach energy correlated with bombardment- induced defects. This sensitive technique has enabled us to study samples as thin as 1micron at energies where the absorption coefficient is ~10 cm(-1).