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1. 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.
2. The Simons Observatory: metamaterial microwave absorber and its cryogenic applications

   https://doi.org/10.1364/AO.411711  

   Xu, Zhilei ; Chesmore, Grace E. ; Adachi, Shunsuke ; Ali, Aamir M. ; Bazarko, Andrew ; Coppi, Gabriele ; Devlin, Mark ; Devlin, Tom ; Dicker, Simon R. ; Gallardo, Patricio A. ; et al ( January 2021 , Applied Optics)

   Controlling stray light at millimeter wavelengths requires special optical design and selection of absorptive materials that should be compatible with cryogenic operating environments. While a wide selection of absorptive materials exists, these typically exhibit high indices of refraction and reflect/scatter a significant fraction of light before absorption. For many lower index materials such as commercial microwave absorbers, their applications in cryogenic environments are challenging. In this paper, we present a new tool to control stray light: metamaterial microwave absorber tiles. These tiles comprise an outer metamaterial layer that approximates a lossy gradient index anti-reflection coating. They are fabricated via injection molding commercially available carbon-loaded polyurethane (25% by mass). The injection molding technology enables mass production at low cost. The design of these tiles is presented, along with thermal tests to 1 K. Room temperature optical measurements verify their control of reflectance to less than 1% up to${65}^{\circ <\#comment/>}$angles of incidence, and control of wide angle scattering below 0.01%. The dielectric properties of the bulk carbon-loaded material used in the tiles is also measured at different temperatures, confirming that the material maintains similar dielectric properties down to 3 K.
3. GHz Ultrasonic Chip-Scale Device Induces Ion Channel Stimulation in Human Neural Cells

   https://doi.org/10.1038/s41598-020-58133-0  

   Balasubramanian, Priya S. ; Singh, Ankur ; Xu, Chris ; Lal, Amit ( February 2020 , Scientific Reports)

   Emergent trends in the device development for neural prosthetics have focused on establishing stimulus localization, improving longevity through immune compatibility, reducing energy re-quirements, and embedding active control in the devices. Ultrasound stimulation can single-handedly address several of these challenges. Ultrasonic stimulus of neurons has been studied extensively from 100 kHz to 10 MHz, with high penetration but less localization. In this paper, a chip-scale device consisting of piezoelectric Aluminum Nitride ultrasonic transducers was engineered to deliver gigahertz (GHz) ultrasonic stimulus to the human neural cells. These devices provide a path towards complementary metal oxide semiconductor (CMOS) integration towards fully controllable neural devices. At GHz frequencies, ultrasonic wavelengths in water are a few microns and have an absorption depth of 10–20 µm. This confinement of energy can be used to control stimulation volume within a single neuron. This paper is the first proof-of-concept study to demonstrate that GHz ultrasound can stimulate neuronsin vitro. By utilizing optical calcium imaging, which records calcium ion flux indicating occurrence of an action potential, this paper demonstrates that an application of a nontoxic dosage of GHz ultrasonic waves$$(\ge 0.05\frac{W}{c{m}^{2}})$$$\left(\ge 0.05\frac{W}{c{m}^2}\right)$caused an average normalized fluorescence intensity recordings >1.40 for the calcium transients. Electrical effects due to chip-scale ultrasound delivery wasmore »discounted as the sole mechanism in stimulation, with effects tested atα = 0.01 statistical significance amongst all intensities and con-trol groups. Ionic transients recorded optically were confirmed to be mediated by ion channels and experimental data suggests an insignificant thermal contributions to stimulation, with a predicted increase of 0.03^oCfor$$1.2\frac{W}{c{m}^{2}}\cdot $$$1.2\frac{W}{c{m}^2}\cdot$This paper paves the experimental framework to further explore chip-scale axon and neuron specific neural stimulation, with future applications in neural prosthetics, chip scale neural engineering, and extensions to different tissue and cell types.

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4. Shape of particle backscattering in the North Pacific Ocean: the χ factor

   https://doi.org/10.1364/AO.414695  

   Zhang, Xiaodong ; Hu, Lianbo ; Gray, Deric ; Xiong, Yuanheng ( February 2021 , Applied Optics)

   Volume scattering functions were measured using two instruments in waters near the Ocean Station Papa (50°N 145°W) and show consistency in estimating the$\mathrm{\chi <\#comment/>}$factor attributable to particles (${\mathrm{\chi <\#comment/>}}_p$). While${\mathrm{\chi <\#comment/>}}_p$in the study area exhibits a limited variability, it could vary significantly when compared with data obtained in various parts of the global oceans. The global comparison also confirms that the minimal variation of${\mathrm{\chi <\#comment/>}}_p$is at scattering angles near 120°. With an uncertainty of$<<\#comment/>10\mathrm{\%<\#comment/>}$,${\mathrm{\chi <\#comment/>}}_p$can be assumed as spectrally independent. For backscatter sensors with wide field of view (FOV), the averaging of scattering within the FOV reduces the values of${\mathrm{\chi <\#comment/>}}_p$needed to compute the backscattering coefficient by up to 20% at angles$<<\#comment/>{130}^{\circ <\#comment/>}$.
5. Aerogel scattering filters for cosmic microwave background observations

   https://doi.org/10.1364/AO.390408  

   Essinger-Hileman, Thomas ; Bennett, Charles L. ; Corbett, Lance ; Guo, Haiquan ; Helson, Kyle ; Marriage, Tobias ; Meador, Mary Ann B. ; Rostem, Karwan ; Wollack, Edward J. ( June 2020 , Applied Optics)

   We present the design and performance of broadband and tunable infrared-blocking filters for millimeter and submillimeter astronomy composed of small scattering particles embedded in an aerogel substrate. The ultralow-density (typically$<<\#comment/>150\mathrm{m}\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$) aerogel substrate provides an index of refraction as low as 1.05, removing the need for antireflection coatings and allowing for broadband operation from DC to above 1 THz. The size distribution of the scattering particles can be tuned to provide a variable cutoff frequency. Aerogel filters with embedded high-resistivity silicon powder are being produced at 40 cm diameter to enable large-aperture cryogenic receivers for cosmic microwave background polarimeters, which require large arrays of sub-Kelvin detectors in their search for the signature of an inflationary gravitational-wave background.