Nondegenerate Optical Pump-Probe Spectroscopy of Highly Excited Group III Nitrides
- PDF / 452,534 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 30 Downloads / 175 Views
ABSTRACT We report the results of nondegenerate optical pump-probe absorption experiments performed on GaN and InGaN thin films and quantum wells under the conditions of strong optical band to band excitation. The evolution of the band edge in these materials was monitored as the number of photoexcited free carriers was increased beyond that required to achieve population inversion and observe stimulated emission. The band edge of InGaN is shown to exhibit markedly different high excitation behavior than that of GaN, explaining in part the reduction in stimulated emission threshold that typically accompanies the incorporation of indium into GaN to form InGaN. A comparison of the band edge absorption changes observed in pump-probe experiments to the gain spectra measured in variable-stripe gain experiments is also given. INTRODUCTION GaN and its respective alloys (InGaN and AIGaN) have been attracting an ever increasing amount of attention due to their physical hardness, inert nature, and large direct band gaps, making them promising materials for UV-Blue-Green light emitting devices and detectors.' Current technological advances have made high brightness light-emitting diodes (LED's) and cw laser diodes based on these materials a reality.2 Their nonlinear properties are now becoming a focus for many research groups. Femtosecond four-wave-mixing (FWM) experiments have been used to study the dephasing times of the A and B free excitons in GaN, 3' 4 and femtosecond pump-probe transient transmission experiments have been used to study the ultrafast carrier dynamics in InGaN expitaxial films. 5 Picosecond FWM experiments have shown strong optical nonlinearities below 6 and at the band edge of GaN, 7 as have nanosecond FWM experiments. 8 However, much information is still unknown about the optical phenomena exhibited by these materials at the high carrier densities at which practical devices operate. Recently, nanosecond pump-probe transmission experiments have shown exciton saturation due to resonant and below resonance optical excitation of the excitonic transitions of GaN. 7 Nanosecond pumpprobe experiments with optical excitation above the band gap of GaN have also been reported9"° and have shown large values of induced transparency and induced absorption in the band gap region with increasing optical excitation. The time evolution of these band edge changes have been studied on femtosecond," picosecond,' 2 and nanosecond' 3 time scales. Similar experiments have been performed on InGaN thin films and multiple quantum wells (MQWs).14'15 A better understanding of the optical phenomena associated with high carrier concentrations in this material system is important, not only for general physical insight, but also as an aid in designing practical devices. We present here a direct comparison of the absorption properties of GaN and InGaN-based structures in their highly excited state. The magnitude of the nonlinearities studied in this work suggests the possibility of new photonic devices based on the group HI nitrides as
Data Loading...
