Time‐resolved gain saturation dynamics in InGaN multi‐quantum well structures

Abstract

Transient gain spectra were measured for an In 0.02 Ga 0.98 N / In 0.16 Ga 0.84 N multiple quantum well using the variable stripe length method (VSLM) in combination with the ultrafast optical Kerr gate (OKG) technique. Gain dynamics were measured for a range of excitation lengths from short (50 µm) to long (350 µm) stripes with the sample under femtosecond photoexcitation. Analysis of the temporal behaviour of gain and chemical potential suggests that stimulated emission originates from a photoexcited electronhole plasma at early times; at later times localized states dominate as the electron-hole plasma becomes exhausted. Gain reduction at early times is attributable to coupling of the electron-hole plasma with photons along the stripe, whilst localized states are less susceptible to gain saturation. . However, the physics of optical gain and stimulated emission has not been clearly resolved. Analysis of gain spectra is very useful in understanding the mechanisms responsible for stimulated emission and the VSLM is commonly used for this purpose. Conventional analysis had been limited to short stripe length regimes as it has proved difficult to find appropriate fitting functions [3]. In preliminary work, we presented an analysis method for VSLM data [5] that enables the calculation of stripe-length-dependent gain for longer stripe lengths. This analysis has been further extended by time-resolving the edge emission for different stripe lengths, from which it is possible to calculate the time-resolved gain spectrum. It has been reported that the primary mechanism for stimulated emission depends on the degree of electron/hole localization, which is mainly determined by the Inmole fraction [6]. In high In-content devices (>20 %), localized excitons play an important role in stimulated emission, while an electron-hole plasma becomes the dominant mechanism in low In-content devices. The sample investigated here is in the intermediate indium content range, which is likely to result in complicated dynamics since both localized states and electron hole plasma will play a role in stimulated emission. In this work, these two effects are investigated using transient gain spectra measurements. In particular, the carrier dynamics related to gain saturation effects in a longer stripe are discussed in terms of many-body effects and compared with the dynamics in a shorter stripe.