Demonstration of an InGaN/GaN-based optically pumped multiquantum well distributed feedback laser using holographically defined third-order gratings

Abstract

We demonstrate an optically pumped InGaN/GaN-based multiquantum well distributed feedback laser in the blue spectral region. The third-order grating providing feedback was defined holographically and dry etched into the upper waveguiding layer by chemically assisted ion-beam etching. When aligning the stripe-shaped pump beam either parallel or perpendicular to the grating grooves, we found a considerably lower pumping threshold, higher slope efficiency, a slightly longer emission wavelength, and a much narrower linewidth for the geometry with the pump beam orthogonal to the grating lines. A nearly constant emission wavelength of 400.85 nm and a linewidth of 0.7 Å were observed under various pump intensities. To the best of our knowledge, this is the narrowest linewidth ever reported for an optically pumped device in this material system.The fabrication of an electrically pumped laser in the blue spectral region has attracted a lot of attention in the past couple of years. Both pulsed and continuous-wave operation of InGaN/GaN-based devices have been demonstrated at room temperature. 1,2 One of the main concerns in nitride lasers is the fabrication of high-quality mirrors. Up to now, most research groups use sapphire substrates for GaN growth; however, the misorientation between the sapphire and the GaN cleavage planes does not readily permit cleaving of the facets. Polishing of the laser mirrors also does not entirely satisfy because it is a very time-consuming process. Although dry-etched mirrors with high reflective coatings seem to work well in this material system, there is still a certain demand to improve the cavity properties of nitride lasers as far as mirror loss and especially mode selection are concerned. One of the possible solutions of this problem is the implementation of distributed feedback ͑DFB͒, which eliminates the need for excellent cavity mirrors. A relatively straight forward way to explore this idea is the fabrication of an optically pumped device. According to this, Hofmann et al. 3 reported an optically pumped DFB laser in the InGaN/ GaN material system. However, their device was a simple double heterostructure, employed a second-order diffraction grating for surface emission, and the grating was generated using e-beam direct writing. In this letter, we demonstrate the fabrication of a nitride-based multiquantum well ͑MQW͒ DFB laser with a holographically defined third-order grating, which results in edge emission. When compared to the Fabry-Perot-type emission observed from the same material, a reduced threshold pump intensity, a higher slope efficiency, and a much narrower linewidth were seen.The fabrication of these devices relied on growing a 4 m thick GaN layer on C-face sapphire. On top of this layer, we grew a 500 nm thick Al 0.08 Ga 0.92 N lower cladding layer, a 100 nm thick GaN lower waveguiding layer, a 40 nm thick active region with five In 0.15 Ga 0.85 N quantum wells ͑QWs͒ and GaN barriers, and a 180 nm thick GaN upper waveguiding layer. The third-order grating with a perio...

Realization of a complex-coupled InGaN/GaN-based optically pumped multiple-quantum-well distributed-feedback laser

“…The use of distributed feedback ͑DFB͒ has already been explored somewhat for the purpose of improved mode and wavelength stability. Consequently, optically pumped 4,5 and electrically injected DFB lasers 6 in the InGaN/GaN material system have been reported recently. These devices were based on index coupling between the forward and backward propagating modes, which usually leads to additional loss through scattering at the corrugated interface between the waveguide and the cladding layer.…”

Laterally Coupled InGaN/GaN DFB Laser Diodes

“…Furthermore, the stability of the laser emission and the, in principle, small linewidth of DFB lasers enables the application of blue lasers beyond their present use in displays or in optical memories. Experiments with optically pumped laser structures in the past have demonstrated DFB laser emission for lasers with second- [1,2] as well as third-order gratings [3]. Approaches based on epitaxial overgrowth [4] also demonstrated the possibility of overcoming epitaxial re-growth problems of DFB gratings in the nitride material system.…”

Characterization of InGaN/GaN-Based Multi-Quantum Well Distributed Feedback Lasers

“…Moreover, the longitudinal mode structure, which, due to the very close mode spacing in these devices, is very competitive and noisy, can be markedly improved by using the DFB mechanism for mode selection. A straightforward way to explore this idea is the fabrication of optically pumped DFB lasers [5], [6].…”