Optically pumped lasing and gain formation properties in blue In _xGa_1-xN MQWs

Lasing and gain formation properties have been studied in an In _xGaN_1-x multiple quantum wells lasing at around 460 nm by employing the modified variable stripe length method (VSLM) and pump and probe spectroscopy (P&P). It was found that the spontaneous emission (E _sp) appeared far below absorption edge (E_a) that is observed as a photo-bleaching negative-peak in P&P, indicating the formation of localized tail states. Lasing peaks appeared in between E_sp and E_a with wide spectral distribution. This is consistent with the results where VSLM revealed the broad feature of optical gain spectra associated with rapid peak saturation of lasing even just above the threshold photo-pumping power density. Such mechanism observed in In-rich In _xGa_1-xN MQWs is contributed not only from the broad distribution of localized density-of-states but also from hot carrier distribution (determined by Maxwell-Boltzmann statistics), temperature of which is raised up due to long energy relaxation time to localized tail states.