Pump and probe spectroscopy of InGaN multi quantum well based laser diodes

Dynamical behavior of dense carriers has been assessed at room temperature (RT) in the InGaN multi quantum well (MQW) based laser diodes (LDs) by employing pump and probe (P&P) spectroscopy under the pulse width of 150 fs. The LDs are composed of InxGal−xN-InyGal−yN MQWs [(a), x=0.1, y=0.02; (b), x=0.2, y=0.05 and (c), x=0.3, y=0.05], whose stimulated emissions correspond to near ultraviolet (390 nm), violet (420 nm) and blue (440 nm), respectively. The optical gain was contributed from the nearly delocalized states [the lowest-quantized MQW levels (LQL)] in the sample (a), while it was from highly localized levels with respect to LQL by 250 meV for the sample (b), and by 500 meV for the sample (c). It was found that the photo-generated carriers rapidly (less than 1 ps) transferred to LQL, and then relaxed to the localized tail within the time-scale of several ps, giving rise to the optical gain. Such gain spectra were saturated and other bands appeared in the vicinity of LQL under higher photo-excitation.