Modulation spectroscopy for determination of integral excitonic absorption in single quantum-well structure

Abstract

Photoreflectance spectra of InGaAs/GaAs single quantum well structures are studied at pump power densities from 0.5 to 300 mW/cm 2 . The spectra measured at different pump powers were found to differ not only in amplitude but also in spectral position. Moreover, a linear correlation between the amplitude and the spectral position of the modulation spectra takes place. We explain this dependence by means of the conventional scheme of modulation spectra modeling, taking into account the second order expansion of the quantum well refraction and absorption indices with respect to the excitonic resonance frequency. Plotting spectral positions of modulation spectra features versus modulation spectra amplitudes we can find the spectral position limit when the amplitude tends to zero. This limit is an excitonic resonance frequency in the absence of perturbation induced by pumping light. We have found that the observed spectral displacement of the modulation spectrum with respect to the nonperturbated excitonic frequency is equal to a half of the excitonic frequency shift induced by external modulation. Using the pumping-induced excitonic frequency shift, derivatives (delta) R/(delta) n and (delta) R/(delta) k, and modulation spectrum (Delta) R/R itself, it is easy to find the absolute value of the excitonic absorption (the excitonic oscillator strength) in a quantum well.