Resolution of Discrete Excited States in InGaN Multiple Quantum Wells using Degenerate Four Wave Mixing

摘要

We report on two pulse, degenerate four wave mixing (DFWM) measurements onshallow InGaN/GaN multi-quantum wells (MQWs) grown on sapphire substrates.These reveal pulse length limited signal decays. We have found a 10:1 resonantenhancement of the DFWM signal at the excitonic transition frequencies whichthereby give a sharp discrimination of the discrete excitonic contributionswithin the featureless distribution seen in absorption spectra. The excitonresonances have peak positions, which yield good overall agreement with a fullk.P model calculation for the quantum well energy levels and optical transitionmatrix elements. InGaN/GaN MQWs generally exhibit strongly inhomogeneouslybroadened excitation spectra due to indium fluctuation effects; this approachtherefore affords a practical method to extract information on the excitedexcitonic states not available previously