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1. 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 reconstructed${\mathrm{\beta <\#comment/>}}_p(\mathrm{\pi <\#comment/>})$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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2. Sparse decomposition light-field microscopy for high speed imaging of neuronal activity

   https://doi.org/10.1364/OPTICA.392805  

   Yoon, Young-Gyu ; Wang, Zeguan ; Pak, Nikita ; Park, Demian ; Dai, Peilun ; Kang, Jeong Seuk ; Suk, Ho-Jun ; Symvoulidis, Panagiotis ; Guner-Ataman, Burcu ; Wang, Kai ; et al ( October 2020 , Optica)

   One of the major challenges in large scale optical imaging of neuronal activity is to simultaneously achieve sufficient temporal and spatial resolution across a large volume. Here, we introduce sparse decomposition light-field microscopy (SDLFM), a computational imaging technique based on light-field microscopy (LFM) that takes algorithmic advantage of the high temporal resolution of LFM and the inherent temporal sparsity of spikes to improve effective spatial resolution and signal-to-noise ratios (SNRs). With increased effective spatial resolution and SNRs, neuronal activity at the single-cell level can be recovered over a large volume. We demonstrate the single-cell imaging capability of SDLFM within vivoimaging of neuronal activity of whole brains of larval zebrafish with estimated lateral and axial resolutions of$\sim <\#comment/>3.5\text{µ<\#comment/>}\mathrm{m}$and$\sim <\#comment/>7.4\text{µ<\#comment/>}\mathrm{m}$, respectively, acquired at volumetric imaging rates up to 50 Hz. We also show that SDLFM increases the quality of neural imaging in adult fruit flies.

    

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3. 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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4. Phase-factor spectra of turbulent phase screens

   https://doi.org/10.1364/JOSAA.429928  

   Muschinski, Andreas ( August 2021 , Journal of the Optical Society of America A)

   The optical phase$\mathrm{\varphi <\#comment/>}$is a key quantity in the physics of light propagating through a turbulent medium. In certain respects, however, the statistics of the phasefactor,$\mathrm{\psi <\#comment/>}=\exp <\#comment/>(i\mathrm{\varphi <\#comment/>})$, are more relevant than the statistics of the phase itself. Here, we present a theoretical analysis of the 2D phase-factor spectrum$F_{\mathrm{\psi <\#comment/>}}(\mathit{\kappa <\#comment/>})$of a random phase screen. We apply the theory to four types of phase screens, each characterized by a power-law phase structure function,$D_{\mathrm{\varphi <\#comment/>}}(r)=(r/r_c{)}^{\mathrm{\gamma <\#comment/>}}$(where$r_c$is the phase coherence length defined by$D_{\mathrm{\varphi <\#comment/>}}(r_c)=1{\mathrm{r}\mathrm{a}\mathrm{d}}^2$), and a probability density function$p_{\mathrm{\alpha <\#comment/>}}(\mathrm{\alpha <\#comment/>})$of the phase increments for a given spatial lag. We analyze phase screens with turbulent ($\mathrm{\gamma <\#comment/>}=5/3$) and quadratic ($\mathrm{\gamma <\#comment/>}=2$) phase structure functions and with normally distributed (i.e., Gaussian) versus Laplacian phase increments. We find that there is a pronounced bump in each of the four phase-factor spectra$F_{\mathrm{\psi <\#comment/>}}(\mathrm{\kappa <\#comment/>})$. The precise location and shape of the bump are different for the four phase-screen types, but in each case it occurs at$\mathrm{\kappa <\#comment/>}\sim <\#comment/>1/r_c$. The bump is unrelated to the well-known “Hill bump” and is not caused by diffraction effects. It is solely a characteristic of the refractive-index statistics represented by the respective phase screen. We show that the second-order$\mathrm{\psi <\#comment/>}$statistics (covariance function, structure function, and spectrum) characterize a random phase screen more completely than the second-order$\mathrm{\varphi <\#comment/>}$counterparts.

    

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5. 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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