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We report a numerical and experimental investigation of fabrication tolerances and outcoupling in optically pumped III-nitride nanolasers operating near λ = 460 nm, in which feedback is provided by a one-dimensional photonic crystal nanobeam cavity and gain is supplied by a single InGaN/GaN quantum well. Using this platform, we and others previously demonstrated single-μW lasing thresholds due to the high βQ-product inherent to the nanobeam geometry (β is spontaneous emission coupling fraction into desired mode). In this work, we improved the fraction of emission emitted into our microscope's light cone by combining a redesigned photonic crystal cavity (c.f. [3]) with a cross-grating coupler with period approximately twice the photonic crystal lattice constant. The samples were fabricated in epitaxial III-nitride layers grown on (111) silicon substrates using metal organic vapor phase epitaxy. The photonic crystal and output couplers were patterned using a single electron beam lithography exposure and subsequently transferred to the underlying III-nitride layers using dry etching. The nanobeams were then suspended via vapor phase etching of silicon in XeF2. Scanning electron microscopy cross-sections revealed high-aspect ratio (>5), sub-70 nanometer diameter holes with near-vertical sidewalls. Fabrication-induced geometry errors were characterized by processing scanning electron micrographs with custom critical dimension software.
Using UV micro-photoluminescence spectroscopy at room temperature, we measured the nanobeams' emission intensity, far-field profile, and quality factor. By comparing more than ten nominally identical nanobeams for each geometry with finite-difference time-domain simulations taking into account the geometrical deviations measured during fabrication, we characterized the role of fabrication-induced imperfections. Finally, we explored the trade-off between the quality factor and collected signal via lithographic variations of the output coupler grating amplitude. Our results demonstrate the robustness of III-nitrides for short-wavelength photonic crystal applications, such as photonic integrated circuits, optoelectronics, and cavity quantum electrodynamics.
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