Femtosecond pump and probe spectroscopy of optical nonlinearities in an InGaN/GaN heterostructure
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Femtosecond pump and probe spectroscopy of optical nonlinearities in an InGaN/GaN heterostructure Fei Chen and A. N. Cartwright Department of Electrical Engineering, State University of New York at Buffalo, Buffalo, NY 14260, USA ABSTRACT The magnitudes and evolutions of two photoinduced absorption nonlinearities, absorption bleaching and field screening, were compared and investigated by employing electroabsorption and femtosecond pump-probe spectroscopy in a biased InGaN/GaN p-i-n double heterostructure. Steady state electroabsorption measurements indicate the field induced absorption coefficient changes in this structure are caused by the Franz-Keldysh effect. The temporal resolution of the absorption bleaching spectra suggests that the photoinduced carriers rapidly relaxed to the InGaN band edge within several picoseconds. As the applied reverse bias field was increased, the transition of the differential absorption spectral signature from the signature for absorption bleaching to the signature for field screening was observed. The magnitude of the change in absorption due to photoinduced carrier screening of the applied field is quantified and compared to absorption bleaching. INTRODUCTION The development of III-V nitride based laser devices has continued to advance since the first demonstration of an InGaN multiple quantum well (MQW) laser diode in 1996.1,2,3 Recently the design and performance of a two section InGaN MQW laser diode consisting of an electroabsorption (EA) modulator and amplifier section has been reported.4 Compared with the traditional direct current modulation, integrated EA modulator reduces the transient heating effects, which makes it attractive for printing applications. The improvements of the device performance of nitride based EA modulators and future applications in all-optical switching and computing, such as self-electro-optic-effect devices (SEEDs), require the detailed study on the absorption nonlinearities in these devices. Two fundamentally different types of absorption nonlinearities have been clarified in the studies on GaAs-based SEEDs and [111] oriented piezoelectric InGaAs devices: excitonic bleaching and field screening.5,6,7 Excitonic bleaching is due to various many-body mechanisms.8 It induces a decrease and broadening of the excitonic absorption and the corresponding change in absorption is dominated by a large negative peak centered at the excitonic peak energy. In field screening, the blue shift of the excitonic absorption due to reduction of the quantum-confined Stark effect and Franz-Keldysh effects (FKEs) produces a change in absorption coefficient which will consist of positive and negative peaks, with a zero crossing that blue shifts as the injected carrier density is increased. However, similar time-resolved studies of absorption nonlinearities on III-V nitride based devices are lacking. In this article, we present both steady state EA measurements and femtosecond pump-probe spectroscopy to identify different photoinduced changes in absorption in a bia
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