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1. High-performance electron-blocking-layer-free deep ultraviolet light-emitting diodes implementing a strip-in-a-barrier structure

   https://doi.org/10.1364/OL.400917  

   Velpula, Ravi Teja ; Jain, Barsha ; Velpula, Swetha ; Nguyen, Hoang-Duy ; Nguyen, Hieu Pham Trung ( September 2020 , Optics Letters)

   In this Letter, the electron-blocking-layer (EBL)-free AlGaN ultraviolet (UV) light-emitting diodes (LEDs) using a strip-in-a-barrier structure have been proposed. The quantum barrier (QB) structures are systematically engineered by integrating a 1 nm intrinsic${\mathrm{A}\mathrm{l}}_x{\mathrm{G}\mathrm{a}}_{(1-<\#comment/>x)}\mathrm{N}$strip into the middle of QBs. The resulted structures exhibit significantly reduced electron leakage and improved hole injection into the active region, thus generating higher carrier radiative recombination. Our study shows that the proposed structure improves radiative recombination by$\sim <\#comment/>220\mathrm{\%<\#comment/>}$, reduces electron leakage by$\sim <\#comment/>11$times, and enhances optical power by$\sim <\#comment/>225\mathrm{\%<\#comment/>}$at 60 mA current injection compared to a conventional AlGaN EBL LED structure. Moreover, the EBL-free strip-in-a-barrier UV LED records the maximum internal quantum efficiency (IQE) of$\sim <\#comment/>61.5\mathrm{\%<\#comment/>}$which is$\sim <\#comment/>72\mathrm{\%<\#comment/>}$higher, and IQE droop is$\sim <\#comment/>12.4\mathrm{\%<\#comment/>}$, which is$\sim <\#comment/>333\mathrm{\%<\#comment/>}$less compared to the conventional AlGaN EBL LED structure at$\sim <\#comment/>284.5\mathrm{n}\mathrm{m}$wavelength. Hence, the proposed EBL-free AlGaN LED is the potential solution to enhance the optical power and produce highly efficient UV emitters.

    

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2. Enhanced hole transport in AlGaN deep ultraviolet light-emitting diodes using a double-sided step graded superlattice electron blocking layer

   https://doi.org/10.1364/JOSAB.399773  

   Jain, Barsha ; Velpula, Ravi Teja ; Velpula, Swetha ; Nguyen, Hoang-Duy ; Nguyen, Hieu Pham Trung ( August 2020 , Journal of the Optical Society of America B)

   In this paper, deep ultraviolet AlGaN light-emitting diodes (LEDs) with a novel double-sided step graded superlattice (DSGS) electron blocking layer (EBL) instead of a conventional EBL have been proposed for$\sim <\#comment/>254\mathrm{n}\mathrm{m}$wavelength emission. The enhanced carrier transport in the DSGS structure results in reduced electron leakage into the$p$-region, improved hole activation and hole injection, and enhanced output power and external quantum efficiency. The calculations show that output power of the DSGS structure is$\sim <\#comment/>3.56$times higher and electron leakage is$\sim <\#comment/>12$times lower, compared to the conventional structure. Moreover, the efficiency droop at 60 mA in the DSGS LED was found to be$\sim <\#comment/>9.1\mathrm{\%<\#comment/>}$, which is$\sim <\#comment/>4.5$times lower than the regular LED structure.

    

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3. Controlled carrier mean free path for the enhanced efficiency of III-nitride deep-ultraviolet light-emitting diodes

   https://doi.org/10.1364/AO.418603  

   Jain, Barsha ; Velpula, Ravi Teja ; Patel, Moulik ; Nguyen, Hieu Pham Trung ( April 2021 , Applied Optics)

   Electron overflow from the active region confines the AlGaN deep-ultraviolet (UV) light-emitting diode (LED) performance. This paper proposes a novel approach to mitigate the electron leakage problem in AlGaN deep-UV LEDs using concave quantum barrier (QB) structures. The proposed QBs suppress the electron leakage by significantly reducing the electron mean free path that improves the electron capturing capability in the active region. Overall, such an engineered structure also enhances the hole injection into the active region, thereby enhancing the radiative recombination in the quantum wells. As a result, our study shows that the proposed structure exhibits an optical power of 9.16 mW at$\sim <\#comment/>284\mathrm{n}\mathrm{m}$wavelength, which is boosted by$\sim <\#comment/>40.5\mathrm{\%<\#comment/>}$compared to conventional AlGaN UV LED operating at 60 mA injection current.

    

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4. Diel variability of bulk optical properties associated with the growth and division of small phytoplankton in the North Pacific Subtropical Gyre

   https://doi.org/10.1364/AO.394123  

   Henderikx Freitas, Fernanda ; Dugenne, Mathilde ; Ribalet, François ; Hynes, Annette ; Barone, Benedetto ; Karl, David M. ; White, Angelicque E. ( July 2020 , Applied Optics)

   Cross-platform observing systems are requisite to capturing the temporal and spatial dynamics of particles in the ocean. We present simultaneous observations of bulk optical properties, including the particulate beam attenuation (${\mathit{\text{c}}}_{\mathit{\text{p}}}$) and backscattering ($b_{\text{bp}}$) coefficients, and particle size distributions collected in the North Pacific Subtropical Gyre. Clear and coherent diel cycles are observed in all bulk and size-fractionated optical proxies for particle biomass. We show evidence linking diurnal increases in${\mathit{\text{c}}}_{\mathit{\text{p}}}$and${\mathit{\text{b}}}_{\text{bp}}$to daytime particle growth and division of cells, with particles$<<\#comment/>7\text{µ<\#comment/>}\mathrm{m}$driving the daily cycle of particle production and loss within the mixed layer. Flow cytometry data reveal the nitrogen-fixing cyanobacteriumCrocosphaera($\sim <\#comment/>4-<\#comment/>7\text{µ<\#comment/>}\mathrm{m}$) to be an important driver of${\mathit{\text{c}}}_{\mathit{\text{p}}}$at the time of sampling, whereasProchlorococcusdynamics ($\sim <\#comment/>0.5\text{µ<\#comment/>}\mathrm{m}$) were essential to reproducing temporal variability in${\mathit{\text{b}}}_{\text{bp}}$. This study is a step towards improved characterization of the particle size range represented byin situbulk optical properties and a better understanding of the mechanisms that drive variability in particle production in the oligotrophic open ocean.

    

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5. GaN/AlN quantum-disk nanorod 280 nm deep ultraviolet light emitting diodes by molecular beam epitaxy

   https://doi.org/10.1364/OL.45.000121  

   Wei, Tongbo ; Islam, S. M. ; Jahn, Uwe ; Yan, Jianchang ; Lee, Kevin ; Bharadwaj, Shyam ; Ji, Xiaoli ; Wang, Junxi ; Li, Jinmin ; Protasenko, Vladimir ; et al ( December 2019 , Optics Letters)

   We report optically and electrically pumped$\sim <\#comment/>280\mathrm{n}\mathrm{m}$deep ultraviolet (DUV) light emitting diodes (LEDs) with ultra-thin GaN/AlN quantum disks (QDs) inserted into AlGaN nanorods by selective epitaxial regrowth using molecular beam epitaxy. The GaN/AlN QD LED has shown strong DUV emission distribution on the ordered nanorods and high internal quantum efficiency of 81.2%, as a result of strain release and reduced density of threading dislocations revealed by transmission electron microscopy. Nanorod assembly suppresses the lateral guiding mode of light, and light extraction efficiency can be increased from 14.9% for planar DUV LEDs to 49.6% for nanorod DUV LEDs estimated by finite difference time domain simulations. Presented results offer the potential to solve the issue of external quantum efficiency limitation of DUV LED devices.

    

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