Superluminescence in Green Emission GaInN/GaN Quantum Well Structures under Pulsed Laser Excitation
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1040-Q05-05
Superluminescence in Green Emission GaInN/GaN Quantum Well Structures under Pulsed Laser Excitation Jayantha Senawiratne1,2, Stephanie Tomasulo2, Theeradetch Detchprohm1,2, Mingwei Zhu1,2, Yufeng Li1,2, Wei Zhao1,2, Yong Xia1,2, Zihui Zhang1,2, Peter Persans2, and Christian Wetzel1,2 1 Future Chips Constellation, Rensselaer Polytechnic Institute, Troy, NY, 12180 2 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, NY, 12180 ABSTRACT We report nonlinear optical investigation of green emission GaInN/GaN multi-quantum structures grown along c- and m-axes on sapphire and bulk GaN substrates, respectively. Under intense pulsed photo excitation, we observed strong superluminescence near the lasing condition in c-plane grown quantum well structures with full width at half maximum of 6 nm. The superluminescence couples out of the edge of the sample in a mode pattern consistent with gain in a high mode of the waveguide. The wavelength of the superluminescence is 474 nm. The threshold intensity of superluminescence was found to be 156 kW/cm2. Increasing pump intensity leads to a strong photoluminescence blueshift as large as 380 meV in samples grown along the c-axis on sapphire substrate, while under the same excitation conditions, the blue shift for the m-axis grown structure on bulk GaN substrate is less than 10 meV. The large emission blueshift is hereby attributed to the internal piezoelectric field in the c-axis grown structure. Its absence in the m-axis structure could enable low threshold current visible laser diodes.
INTRODUCTION GaInN/GaN quantum well (QW) structures are the system of choice for UV and blue laser diodes (LDs) and also hold the biggest promise for green laser diodes [1,2]. The successful development of this material system, however, requires identification of the underling physical mechanisms for spontaneous and stimulated emission. In particular, for structures with high In content GaInN layers, large piezoelectric fields in c-axis oriented QWs are known to play a crucial role in the spontaneous emission process. Such a phenomenon also must be expected for the stimulated emission process [3,4]. By means of the quantum confined Stark effect (QCSE) in the fields of the strong piezoelectric polarization, electrons and holes tend to become separated in real space and so reduce the interband transition matrix element [4]. It is expected that even under high carrier injection conditions, the optical gain can detrimentally be affected in the consequence. On the other hand, potential fluctuations in the active layers are known to lead to a broadening of the optical emission band, further reducing the optical gain. Here we report the optical properties of green emitting GaInN/GaN multi-QW (MQW) structures near the lasing conditions under pulsed and continuum (cw) laser excitation. The effect of the polarization field on the optical properties is evaluated by comparing the emission properties of GaInN/GaN QW structures grown along the polar c-axis on c-
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