Absorption coefficients and exciton oscillator strengths in AlGaAs-GaAs superlattices

We have experimentally determined the absolute optical-absorption coefficients and exciton oscillator strengths in AlGaAs-GaAs superlattices at liquid-helium temperatures. We find that although the magnitude of the band-to-band absorption coefficient is essentially independent of the quantum-well width, the absorption coefficient due to exciton absorption is greatly enhanced as the well width becomes smaller. The absorption energies and exciton oscillator strengths are compared to theoretical calculations and shown to be in very good agreement. We find that the oscillator strength per unit volume is proportional to 1/$L^2$ in narrow wells. The effects due to coupling between wells was also investigated by the comparison of structures with barriers of 100 Å to those with barriers of only 25 Å. The subband energies in the coupled wells decreased from their values in uncoupled wells in quantitative agreement with our calculations while the oscillator strength also decreased.