Stimulated emission and ultrafast carrier relaxation in InGaN multiple   quantum wells
  Stimulated emission (SE) was measured from two InGaN multiple quantum well (MQW) laser structures with different In compositions. SE threshold power densities (I_th) increased with increasing QW depth (x). Time-resolved differential transmission measurements mapped the carrier relaxation mechanisms and explained the dependence of I_th on x. Carriers are captured from the barriers to the QWs in < 1 ps, while carrier recombination rates increased with increasing x. For excitation above I_th an additional, fast relaxation mechanism appears due to the loss of carriers in the barriers through a cascaded refilling of the QW state undergoing SE. The increased material inhomogeneity with increasing x provides additional relaxation channels outside the cascaded refilling process, removing carriers from the SE process and increasing I_th. 
Umit Ozgur, Henry O. Everitt, Stacia Keller and Steven P. DenBaars