Resolution of discrete excited states in InxGa1−xN multiple quantum wells using degenerate four-wave mixing

We report on two pulse, degenerate four-wave mixing DFWM measurements on shallow InxGa1−xN/GaN multiquantum wells MQWs grown on sapphire substrates. These reveal pulse length limited signal decays. We have found a 10:1 resonant enhancement of the DFWM signal at the excitonic transition frequencies which thereby give a sharp discrimination of the discrete excitonic contributions within the featureless distribution seen in absorption spectra. The exciton resonances have peak positions, which yield good overall agreement with a full k·p model calculation for the quantum well QW energy levels and optical transition matrix elements. InxGa1−xN/GaN MQWs generally exhibit strongly inhomogeneously broadened excitation spectra due to indium fluctuation effects; this approach therefore affords a practical method to extract information on the excited excitonic states not available previously.