  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.
