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1. In-vacuum measurements of optical scatter versus annealing temperature for amorphous Ta ₂ O ₅ and TiO ₂ :Ta ₂ O ₅ thin films

   https://doi.org/10.1364/JOSAA.415665  

   Capote, Elenna M. ; Gleckl, Amy ; Guerrero, Jazlyn ; Rezac, Michael ; Wright, Robert ; Smith, Joshua R. ( March 2021 , Journal of the Optical Society of America A)

   Optical coatings formed from amorphous oxide thin films have many applications in precision measurements. The Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and Advanced Virgo use coatings of$\mathrm{S}\mathrm{i}{\mathrm{O}}_2$(silica) and$\mathrm{T}\mathrm{i}{\mathrm{O}}_2:\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$(titania-doped tantala) and post-deposition annealing to 500°C to achieve low thermal noise and low optical absorption. Optical scattering by these coatings is a key limit to the sensitivity of the detectors. This paper describes optical scattering measurements for single-layer, ion-beam-sputtered thin films on fused silica substrates: two samples of$\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$and two of$\mathrm{T}\mathrm{i}{\mathrm{O}}_2:\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$. Using an imaging scatterometer at a fixed scattering angle of 12.8°, in-situ changes in the optical scatter of each sample were assessed during post-deposition annealing to 500°C in vacuum. The scatter of three of the four coated optics was observed to decrease during the annealing process, by 25–30% for tantala and up to 74% for titania-doped tantala, while the scatter frommore »the fourth sample held constant. Angle-resolved scatter measurements performed before and after vacuum annealing suggest some improvement in three of the four samples. These results demonstrate that post-deposition, high-temperature annealing of single-layer tantala and titania-doped tantala thin films in vacuum does not lead to an increase in scatter, and may actually improve their scatter.

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2. 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 opticalmore »properties and a better understanding of the mechanisms that drive variability in particle production in the oligotrophic open ocean.

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3. N-polar InGaN/GaN nanowires: overcoming the efficiency cliff of red-emitting micro-LEDs

   https://doi.org/10.1364/PRJ.450465  

   Pandey, A. ; Malhotra, Y. ; Wang, P. ; Sun, K. ; Liu, X. ; Mi, Z. ( March 2022 , Photonics Research)

   A high efficiency, high brightness, and robust micro or sub-microscale red light emitting diode (LED) is an essential, yet missing, component of the emerging virtual reality and future ultrahigh resolution mobile displays. We report, for the first time, to our knowledge, the demonstration of an N-polar InGaN/GaN nanowire sub-microscale LED emitting in the red spectrum that can overcome the efficiency cliff of conventional red-emitting micro-LEDs. We show that the emission wavelengths of N-polar InGaN/GaN nanowires can be progressively shifted from yellow to orange and red, which is difficult to achieve for conventional InGaN quantum wells or Ga-polar nanowires. Significantly, the optical emission intensity can be enhanced by more than one order of magnitude by employing anin situannealing process of the InGaN active region, suggesting significantly reduced defect formation. LEDs with lateral dimensions as small as$\sim 0.75\mathrm{\mu m}$, consisting of approximately five nanowires, were fabricated and characterized, which are the smallest red-emitting LEDs ever reported, to our knowledge. A maximum external quantum efficiency$\sim 1.2\%$was measured, which is comparable to previously reported conventional quantum well micro-LEDs operating in this wavelength range, while our device sizes are nearly three to five orders of magnitude smaller in surface area.
4. Variability of relationship between the volume scattering function at 180° and the backscattering coefficient for aquatic particles

   https://doi.org/10.1364/AO.383229  

   Hu, Lianbo ; Zhang, Xiaodong ; Xiong, Yuanheng ; Gray, Deric J. ; He, Ming-Xia ( February 2020 , Applied Optics)

   Properly interpreting lidar (light detection and ranging) signal for characterizing particle distribution relies on a key parameter,${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$, which relates the particulate volume scattering function (VSF) at 180° (${\mathrm{\beta <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$) that a lidar measures to the particulate backscattering coefficient ($b_{\mathit{\text{bp}}}$). However,${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$has been seldom studied due to challenges in accurately measuring${\mathrm{\beta <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$and$b_{\mathit{\text{bp}}}$concurrently in the field. In this study,${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$, as well as its spectral dependence, was re-examined using the VSFs measuredin situat high angular resolution in a wide range of waters.${\mathrm{\beta <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$, while not measured directly, was inferred using a physically sound, well-validated VSF-inversion method. The effects of particle shape and internal structure on the inversion were tested using three inversion kernels consisting of phase functions computed for particles that are assumed as homogenous sphere, homogenous asymmetric hexahedra, or coated sphere. The reconstructed VSFs using any of the three kernels agreed well with the measured VSFs with a mean percentage difference$<<\#comment/>5\mathrm{\%<\#comment/>}$at scattering angles$<<\#comment/>{170}^{\circ <\#comment/>}$. At angles immediately near or equal to 180°, the reconstructedstretchy='false'>)depends strongly on the inversion kernel.${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$derived with the sphere kernels was smaller than those derived with the hexahedra kernel but consistent with${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$estimated directly from high-spectral-resolution lidar andin situbackscattering sensor. The possible explanation was that the sphere kernels are able to capture the backscattering enhancement feature near 180° that has been observed for marine particles.${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$derived using the coated sphere kernel was generally lower than those derived with the homogenous sphere kernel. Our result suggests that${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$is sensitive to the shape and internal structure of particles and significant error could be induced if a fixed value of${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$is to be used to interpret lidar signal collected in different waters. On the other hand,${\mathrm{\chi <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$showed little spectral dependence.

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5. Twist-free ultralight two-photon fiberscope enabling neuroimaging on freely rotating/walking mice

   https://doi.org/10.1364/OPTICA.422657  

   Li, Ang ; Guan, Honghua ; Park, Hyeon-Cheol ; Yue, Yuanlei ; Chen, Defu ; Liang, Wenxuan ; Li, Ming-Jun ; Lu, Hui ; Li, Xingde ( June 2021 , Optica)

   Lightweight and head-mountable scanning nonlinear fiberscope technologies offer an exciting opportunity for enabling mechanistic exploration of ensemble neural activities with subcellular resolution on freely behaving rodents. The tether of the fiberscope, consisting of an optical fiber and scanner drive wires, however, restricts the mouse’s movement and consequently precludes free rotation and limits the freedom of walking. Here we present the first twist-free two-photon fiberscope technology for enabling neuroimaging on freely rotating/walking mice. The technology equips a scanning fiberscope with active rotational tracking and compensation capabilities through an optoelectrical commutator (OEC) to allow the animal to rotate and walk in arbitrary patterns during two-photon fluorescence (TPF) imaging of neural activities. The OEC provides excellent optical coupling stability ($<<\#comment/>\pm <\#comment/>1\mathrm{\%<\#comment/>}$fluctuation during rotation) and an extremely high torque sensitivity ($<<\#comment/>8\mathrm{m}\mathrm{N}\cdot <\#comment/>\mathrm{m}$). In addition, the new technology is equipped with a custom grating and prism to effectively manage the temporal properties of the femtosecond excitation pulses through the fiber-optic system, which improved neuroimaging signal by more than$2\mathrm{X}$. This TPF fiberscope imaging platform has been tested forin vivoimaging, and the results demonstrate that it enablesmore »reliable recording of calcium dynamics of more than 50 neurons simultaneously in the motor cortices of freely behaving mice in a twist-free fashion. With active tracking function of the OEC enabled, we observed considerable increase in both behavior and neural activities in the motor cortices of the mice during freely behaving neuroimaging experiments.

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