Abstract ：

The differences in spatially optical properties between blue and green quantum wells (QWs) in a monolithic dual-wavelength semipolar (20-21) structure were investigated by scanning near-field optical microscopy (SNOM). The shortest wavelength for green QWs and the longest wavelength for blue QWs were both discovered in the region with the largest stress. It demonstrated that In composition, compared to stress, plays a negligible role in defining the peak wavelength for blue QWs, while for green QWs, In composition strongly affects the peak wavelength. For green QWs, significant photoluminescence enhancement was observed in the defect-free region, which was not found for blue QWs. Furthermore, the efficiency droop was aggravated in the defect-free region for green QWs but reduced for blue QWs. It indicates that carrier delocalization plays a more important role in the efficiency droop for QWs of good crystalline quality, which is experimentally pointed out for the first time.

Keyword ：

localization states scanning near-field optical microscopy semipolar LEDs

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Abstract ：

We investigated the excitation-dependent spatially resolved luminescence properties of InGaN/GaN light-emitting diodes (LEDs) with air-cavity arrays using scanning near-field optical microscopy (SNOM), and quantified the effect of the air-cavity structure on the spatial distributions of light-extraction efficiency, internal quantum efficiency, and external quantum efficiency through experiments and numerical simulations. We found that higher light-extraction efficiency and less stress were always observed in the air-cavity areas. Compared to flat areas, the average light-extraction efficiency of the air-cavity areas was improved by 65%, and the residual stress of the air-cavity center was reduced by 89 MPa, which resulted in increased PL intensity, reduced wavelength shift, and narrowed full width at half maximum. The highest external quantum efficiency and aggravated efficiency droop were found in the air-cavity center. At 3.1 W/cm2, the external quantum efficiency and internal quantum efficiency of the air-cavity center were enhanced by 255% and 223%, respectively.

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As a common clinical problem, the therapy of SSTIs is facing growing challenges due to an increase in the number of drug-resistant bacteria and fungi. UV C (UVC) light sterilization has been widely used in all aspects of daily life, but there are very few reports about in vivo therapy using UVC light. Due to the changes in pathogenic species and the absence of research on topical skin antibiotics, the therapy of skin and soft tissue infections (SSTIs) is facing more and more severe challenges. It is particularly urgent to look for alternative therapies without induction of drug resistance. UV C (UVC) light within the range of 200 to 280 nm is one of the most common techniques used to kill and/or inactivate pathogenic microorganisms. However, the traditional most commonly used wavelength of 254 nm irradiated from a low-pressure mercury lamp is hazardous to human health, being both carcinogenic and damaging to eye tissues, which limits its applications in vivo. This research aimed to investigate the antimicrobial properties and influence of 275-nm UVC light from a light-emitting diode (UVC-LED light) on wound healing time. Five bacteria, three fungi, and scalded-mouse models combined with SSTIs were used to evaluate the antimicrobial effect in vitro and in vivo. 275-nm UVC-LED light inactivated both bacteria and fungi with a very short irradiation time in vitro and induced neither DNA damage nor epidermal lesions in the mice's skin. Furthermore, in mouse models of SSTIs induced by either methicillin-resistant Staphylococcus aureus (MRSA) or Candida albicans, the 275-nm UVC-LED light showed significant antimicrobial effects and shortened the wound healing time compared with that in the no-irradiation group. UVC-LED light at 275 nm has the potential to be a new form of physical therapy for SSTIs.IMPORTANCE As a common clinical problem, the therapy of SSTIs is facing growing challenges due to an increase in the number of drug-resistant bacteria and fungi. UV C (UVC) light sterilization has been widely used in all aspects of daily life, but there are very few reports about in vivo therapy using UVC light. It is well known that prolonged exposure to UVC light increases the possibility of skin cancer. In addition, it is also very harmful for eyes. UV irradiation with 254-nm UVC light can cause corneal damage, like thinning of the corneal epithelial layer, superficial punctate keratitis, corneal erosion, etc. In this study, we focused on looking for a more accessible light source and safer UVC wavelength, and 275-nm UVC LED light was chosen. We investigated its applicability for SSTIs therapy with relative skin safety and expected that it could be used as a new physical therapy method for SSTIs.

Keyword ：

275 nm Candida albicans MRSA skin and soft tissue infections UVC-LED

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The selective area growth (SAG) technique enables the creation of multi-color micro-structures. However, the substrate pattern to obtain multi-color and mass transfer of these micro-structures are complicated to achieve. Moreover, high dislocation density as well as large residual stress in c-plane quantum wells still remain challenges. Here, an improved SAG strategy was proposed to achieve completely merged asymmetric pyramids on closely connected deep-concave holes via the lateral overgrowth of adjacent pyramids. These asymmetric pyramids were directly grown on the sapphire substrate, making mass transfer be possible. Growth rate distinction appears at the different positions of the overgrowth region, resulting in a multi-wavelength emission. More importantly, the residual stress and the dislocation density can be suppressed by utilizing the deep-hole pattern and lateral overgrowth, contributing to a lower piezoelectric polarization field and a higher spontaneous emission rate. These ultrafast and high-efficient multi-color micro-LEDs are suitable for the application in highresolution display, flexible devices, and high-speed visible-light-communication.

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Deep-hole pattern Lateral overgrowth Microstructures Stress and defect

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Modulation bandwidth and the emission region are essential features for the widespread use of visible light communications (VLC). This paper addresses the contradictory requirements to achieve broadband and proposes ultrafast, asymmetric pyramids grown on adjacent deep concave holes via lateral overgrowth. Multicolor emission with an emission region between 420 nm and 600 nm is obtained by controlling the growth rate at different positions on the same face, which also can provide multiple subcarrier frequency points for the employment of wavelength division multiplexing technology. The spontaneous emission rate distinction is narrowed by lowering the number of the crystal plane, ensuring a high modulation bandwidth over broadband. More importantly, the residual stress and dislocation density were minimized by employing a patterned substrate, and lateral overgrowth resulted in a further enhancement of the recombination rate. Finally, the total modulation bandwidth of multiple subcarriers of the asymmetric pyramids is beyond GHz. These ultrafast, multicolor microLEDs are viable for application in VLC systems and may also enable applications for intelligent lighting and display. (C) 2021 Chinese Laser Press

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Multimode and random directionalities are major issues restricting the application of whispering gallery mode microcavity lasers. We demonstrated a 40 mu m 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. (C) 2021 Chinese Laser Press

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Abstract ：

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.

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localization surface plasmon micro-LEDs near-filed scanning optical microscopy

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A full structure 290-nm ultraviolet light-emitting diode (UV-LED) with a nanoporous n-AIGaN underlayer was fabricated by top via hole formation followed by high-voltage electrochemical etching. The 20 to 120 nm nanopores were prepared in regular doped n-AIGaN by adjusting the etching voltage. The comparison between the Raman spectrum and the photoluminescence wavelength shows that the biaxial stress in the nanoporous material is obviously relaxed. The photoluminescence enhancement was found to be highly dependent on the size of the pores. It not only improves the extraction efficiency of top-emitting transverse-electric (TE)-mode photons but also greatly improves the efficiency of side-emitting transverse-magnetic (TM)-mode photons. This leads to the polarization change of the side-emitting light from -0.08 to -0.242. The intensity of the electroluminescence was increased by 36.5% at 100 mA, and the efficiency droop at high current was found to decrease from 61% to 31%. (C) 2020 Chinese Laser Press

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To reduce the threshold and achieve unidirectional lasing emission in a whispering gallery mode microcavity, we propose and demonstrate a Galsl-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 degrees 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. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Subwavelength freestanding InGaN/GaN quantum well microtubes with a wall thickness of about 50nm are formed by selective release of a coherently strained bilayer heterostructure from a hosting substrate. Highly polarized spontaneous and stimulated emissions due to whispering gallery mode oscillation were observed in photoluminescence spectra at room temperature. The quality factor was increased by a factor of 3 by elevating the microtube from the substrate and reducing light leakage before threshold. A lasing threshold was characterized as low as 3mJ/cm(2). Microtubes were immersed in different liquids with refractive index variation as small as 0.01 and tested below and above threshold. The peak positions shift obviously while exhibiting excellent repeatability. Using a simple optical probe-detect method, a 5-mu m diameter microtube cavity achieves a sensitivity of 40 nm/refractive index unit (RIU) and a detection limit of 6 x 10(-3) RIU. Published under license by AIP Publishing.

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