To reduce the threshold and achieve unidirectional lasing emission in a whispering gallery mode microcavity, we propose and demonstrate a GaN-based eccentric microring with an inner hole located off the center. Compared to microdisk with the same outer diameter, the eccentric microring structure exhibits a remarkable reduction of lasing threshold by up to 53%. The introduction of the hole disturbs and eventually suppresses the field distribution of the higher order modes. Laser emission with high unidirectionality with a far-field divergence angle of about 40° has been achieved, meanwhile the Q factor of the whispering gallery modesis remains high as 6388. Finite-difference time-domain numerical simulation is carried out to prove that the far-field profile of the eccentric microring structure can be controlled by the position and the size of the hole. The properties of the whispering gallery mode microcavities are improved greatly through a simple structure and process, which has an important guiding significance to the research and development of the microcavity lasers.

Multimode and random directionalities are major issues restricting the application of whispering gallery mode microcavity lasers. We demonstrated a 40 μm diameter microring with an off-centered embedded hole and warped geometry from strained III-nitride quantum well multilayers. Single-mode directional whispering gallery mode lasing was achieved by the warped structure and high-order mode suppression induced by the off-centered hole. In addition, the introduction of the off-centered hole reduced the lasing threshold from 3.24 to 2.79    MW / cm 2 compared with the warped microdisk without an embedded hole while maintaining a high-quality factor of more than 4000. Directional light emission in 3D was achieved and attributed to the warped structure, which provides a vertical component of the light emission, making it promising for building multifunctional coherent light sources in optoelectronic integration.

The size of the V-defects in the GaN/InGaN-based quantum wells blue light-emitting diode (LED) was intentionally modified from 50 nm to 300 nm. High resolution photoluminescence and electroluminescence of a single large V-defect were investigated by near-field scanning optical microscopy. The current distribution along the {10-11} facets of the large defect was measured by conductive atomic force microscopy. Nearly 20 times the current injection and dominant emission from bottom quantum wells were found in the V-defect compared to its vicinity. Such enhanced current injection into the bottom part of quantum wells through V-defect results in higher light output power. Reduced external quantum efficiency droops were achieved due to more uniform carrier distribution. The un-encapsulated fabricated chip shows light output power of 172.5 mW and 201.7 mW at 400 mA, and external quantum efficiency drop of 22.3% and 15.4% for the sample without and with large V-defects, respectively. Modified V-defects provide a simple and effective approach to suppress the efficiency droop problem that occurs at high current injection, while improving overall quantum efficiency.

The microcave array with extreme large aspect ratio was fabricated on the p-GaN capping layer followed by Ag nanoparticles preparation. The coupling distance between the dual-wavelength InGaN/GaN multiple quantum wells and the localized surface plasmon resonance was carefully characterized in nanometer scale by scanning near-field optical microscopy. The effects of coupling distance and excitation power on the enhancement of photoluminescence were investigated. The penetration depth was measured in the range of 39-55 nm depending on the excitation density. At low excitation power density, the maximum enhancement of 103 was achieved at the optimum coupling distance of 25 nm. Time-resolved photoluminescence shows that the recombination life time was shortened from 5.86 to 1.47 ns by the introduction of Ag nanoparticle plasmon resonance.Nanomaterials 2020, 10, 751 2 of 10 given by Z = λ/2π[(ε GaN -ε metal )/ε metal 2 ] 1/2 where ε GaN and ε metal are the real parts of the dielectric

A GaN ultraviolet photodetector with a petal-like β-Ga2O3 microcrystalline layer was prepared on the GaN template using electrochemical anodizing and annealing processes. The petal-like β-Ga2O3 microcrystalline layer was found to enhance the absorption of ultraviolet light and suppress the dark current, and a high responsivity from 230 nm (responsivity 8.5 A/W) to 400 nm (responsivity 0.1 A/W) was achieved by the photodetector. The rejection ratio of ultraviolet–visible light is greater than three orders of magnitude representing a high selectivity of ultraviolet light detection. The responsivity slopes of the photodetector under different biases were found to be strongly correlated with the wavelength of light, and the responsivity is much higher than that of conventional metal/insulator/metal wavelength identification photodetectors. This effective method of synthesizing β-Ga2O3 crystallites on GaN can be used to enhance the ultraviolet absorption of GaN photodetectors and improve the detection performance.