Resolution of Discrete Excited States in InGaN Multiple Quantum Wells   using Degenerate Four Wave Mixing
  We report on two pulse, degenerate four wave mixing (DFWM) measurements on shallow InGaN/GaN multi-quantum 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 energy levels and optical transition matrix elements. InGaN/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 
D.O. Kundys, J.-P.R. Wells, A.D. Andreev, S.A. Hashemizadeh, T. Wang,   P.J. Parbrook, A.M. Fox, D.J. Mowbray and M.S. Skolnick
physics.optics cond-mat.mes-hall