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1. 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 ( ${\text{c}}_{\text{p}}$) 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 ${\text{c}}_{\text{p}}$and ${\text{b}}_{\text{bp}}$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 ${\text{c}}_{\text{p}}$at the time of sampling, whereasProchlorococcusdynamics ( $\sim <\#comment/>0.5\text{µ<\#comment/>}\mathrm{m}$) were essential to reproducing temporal variability in ${\text{b}}_{\text{bp}}$. 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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2. Mid-IR spectroscopy of Er ³⁺ doped low-phonon CsCdCl ₃ and CsPbCl ₃ crystals

   https://doi.org/10.1364/JOSAB.470152  

   Brown, Ei_Ei; Fleischman, Zackery_D; McKay, Jason; Hommerich, Uwe; Kabir, Al_Amin; Riggins, Jazmine; Trivedi, Sudhir; Dubinskii, Mark (September 2022, Journal of the Optical Society of America B)

   The mid-IR spectroscopic properties of ${\mathrm{E}\mathrm{r}}^{3+}$doped low-phonon ${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{d}\mathrm{C}\mathrm{l}}_3$and ${\mathrm{C}\mathrm{s}\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}}_3$crystals grown by the Bridgman technique have been investigated. Using optical excitations at $\sim <\#comment/>800\mathrm{n}\mathrm{m}$and $\sim <\#comment/>660\mathrm{n}\mathrm{m}$, both crystals exhibited IR emissions at $\sim <\#comment/>1.55$, $\sim <\#comment/>2.75$, $\sim <\#comment/>3.5$, and $\sim <\#comment/>4.5\text{µ<\#comment/>}\mathrm{m}$at room temperature. The mid-IR emission at 4.5 µm, originating from the ${}^4{\mathrm{I}}_{9/2}\to <\#comment/>{}^4{\mathrm{I}}_{11/2}$transition, showed a long emission lifetime of $\sim <\#comment/>11.6\mathrm{m}\mathrm{s}$for ${\mathrm{E}\mathrm{r}}^{3+}$doped ${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{d}\mathrm{C}\mathrm{l}}_3$, whereas ${\mathrm{E}\mathrm{r}}^{3+}$doped ${\mathrm{C}\mathrm{s}\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}}_3$exhibited a shorter lifetime of $\sim <\#comment/>1.8\mathrm{m}\mathrm{s}$. The measured emission lifetimes of the ${}^4{\mathrm{I}}_{9/2}$state were nearly independent of the temperature, indicating a negligibly small nonradiative decay rate through multiphonon relaxation, as predicted by the energy-gap law for low-maximum-phonon energy hosts. The room temperature stimulated emission cross sections for the ${}^4{\mathrm{I}}_{9/2}\to <\#comment/>{}^4{\mathrm{I}}_{11/2}$transition in ${\mathrm{E}\mathrm{r}}^{3+}$doped ${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{d}\mathrm{C}\mathrm{l}}_3$and ${\mathrm{C}\mathrm{s}\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}}_3$were determined to be $\sim <\#comment/>0.14\times <\#comment/>{10}^{-<\#comment/>20}{\mathrm{c}\mathrm{m}}^2$and $\sim <\#comment/>0.41\times <\#comment/>{10}^{-<\#comment/>20}{\mathrm{c}\mathrm{m}}^2$, respectively. The results of Judd–Ofelt analysis are presented and discussed. 

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3. 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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4. Design of infrared microspectrometer based on phase-modulated axilenses

   https://doi.org/10.1364/AO.390610  

   Chen, Yuyao; Britton, Wesley_A; Dal_Negro, Luca (June 2020, Applied Optics)

   We design and characterize a novel axilens-based diffractive optics platform that flexibly combines efficient point focusing and grating selectivity and is compatible with scalable top-down fabrication based on a four-level phase mask configuration. This is achieved using phase-modulated compact axilens devices that simultaneously focus incident radiation of selected wavelengths at predefined locations with larger focal depths compared with traditional Fresnel lenses. In addition, the proposed devices are polarization-insensitive and maintain a large focusing efficiency over a broad spectral band. Specifically, here we discuss and characterize modulated axilens configurations designed for long-wavelength infrared (LWIR) in the 6 µm–12 µm wavelength range and in the 4 µm–6 µm midwavelength infrared (MWIR) range. These devices are ideally suited for monolithic integration atop the substrate layers of infrared focal plane arrays and for use as compact microspectrometers. We systematically study their focusing efficiency, spectral response, and cross-talk ratio; further, we demonstrate linear control of multiwavelength focusing on a single plane. Our design method leverages Rayleigh–Sommerfeld diffraction theory and is validated numerically using the finite element method. Finally, we demonstrate the application of spatially modulated axilenses to the realization of a compact, single-lens spectrometer. By optimizing our devices, we achieve a minimum distinguishable wavelength interval of $\mathrm{\Delta <\#comment/>}\mathrm{\lambda <\#comment/>}=240\mathrm{n}\mathrm{m}$at ${\mathrm{\lambda <\#comment/>}}_c=8\text{µ<\#comment/>}\mathrm{m}$and $\mathrm{\Delta <\#comment/>}\mathrm{\lambda <\#comment/>}=165\mathrm{n}\mathrm{m}$at ${\mathrm{\lambda <\#comment/>}}_c=5\text{µ<\#comment/>}\mathrm{m}$. The proposed devices add fundamental spectroscopic capabilities to compact imaging devices for a number of applications ranging from spectral sorting to LWIR and MWIR phase contrast imaging and detection. 

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5. Determination of size and complex index of refraction of single particles with elastic light scattering

   https://doi.org/10.1364/AO.413675  

   Waez, Mir_Seliman; Eckels, Steven_J; Sorensen, Christopher_M (January 2021, Applied Optics)

   We show that for spherical particles greater than ca. 5 µm, the differential scattering cross section is only weakly dependent on the real and imaginary parts of the refractive index ( $m=n+i\mathrm{\kappa <\#comment/>}$) when integrated over angle ranges near $37\pm <\#comment/>5^{\circ <\#comment/>}$and $115\pm <\#comment/>5^{\circ <\#comment/>}$, respectively. With this knowledge, we set up an arrangement that collects scattered light in the ranges $37\pm <\#comment/>5^{\circ <\#comment/>}$, $115\pm <\#comment/>5^{\circ <\#comment/>}$, and $80\pm <\#comment/>5^{\circ <\#comment/>}$. The weak functionality on refractive index for the first two angle ranges simplifies the inversion of scattering to the particle properties of diameter and the real and imaginary refractive indices. Our setup also uses a diamond-shaped incident beam profile that allows us to determine when a particle went through the exact center of the beam. Application of our setup to droplets of an absorbing liquid successfully determined the diameter and complex refractive index to accuracies ranging from a few to ten percent. Comparisons to simulated data derived from the Mie equations yielded similar results. 

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