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1. Light Scattering Study of Highly Absorptive, Non-fractal, Hematite Aggregates

   https://doi.org/doi.org/10.1016/j.jqsrt.2020.106919  

   Gautam, P.; Maughan, J.; Ilavsky, J.; Sorensen, C. M. (January 2020, Journal of quantitative spectroscopy and radiative transfer)

   We present measurements of the scattered light intensity by aerosolized hematite aggregate particles. The measurements were made at a wavelength of 532 nm in the scattering angle range from 0.32 °to 157 °. Hematite has high values of the real and imaginary parts of the refractive index m = n + i κ= 3 + i0.5 at the studied wavelength. Scanning electron micrographs (SEM) indicated that the particles were aggregates whereas the optical microscope pictures showed that the aerosol had a bimodal distribution with effective mean diameters of roughly 1 and 10 μm. This is consistent with the light scattering results which displayed two Guinier regimes. The aggregates were composed of smaller grains with an approximate size of 200 nm. Ultra Small-Angle X-ray Scattering (USAXS) indicate that the aggregates were uniform and non-fractal. Mie calculations for a sphere equivalent to the aggregate size were compared to the experimentally observed results. The observed results showed an enhanced backscattering, whereas the Mie calculations did not due to the large imaginary part of the refractive index. Hematite aggregates were simulated by assuming they were composed of spherical monomers inside a spherical volume. Then the light scattering was calculated using the T-matrix method for these simulated aggregates. The calculated results show an enhanced backscattering. We present a dimensional analysis to estimate the extent of multiple scattering within the aggregate and find a correlation between the average number of scattering events within the aggregate and the enhancement in the backscattering. 

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2. The role of the medium in the effective-sphere interpretation of holographic particle characterization data

   https://doi.org/10.1039/c9sm01916b  

   Odete, Mary Ann; Cheong, Fook Chiong; Winters, Annemarie; Elliott, Jesse J.; Philips, Laura A.; Grier, David G. (January 2020, Soft Matter)

   The in-line hologram of a micrometer-scale colloidal sphere can be analyzed with the Lorenz–Mie theory of light scattering to obtain precise measurements of the sphere's diameter and refractive index. The same technique also can be used to characterize porous and irregularly shaped colloidal particles provided that the extracted parameters are interpreted with effective-medium theory to represent the properties of an equivalent effective sphere. Here, we demonstrate that the effective-sphere model consistently accounts for changes in the refractive index of the medium as it fills the pores of porous particles and therefore yields quantitative information about such particles' structure and composition. In addition to the sample-averaged porosity, holographic perfusion porosimetry gauges the polydispersity of the porosity. We demonstrate these capabilities through measurements on mesoporous spheres, fractal protein aggregates and irregular nanoparticle agglomerates, all of which are noteworthy for their industrial significance. 

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3. Light confinement in low-index particles with all-dielectric anisotropic metamaterial shell (Conference Presentation)

   https://doi.org/10.1117/12.2545314  

   Jahani, Saman; Bahng, Joong Hwan; Montjoy, Douglas; Yao, Timothy A.; Kotov, Nicholas A.; Marandi, Alireza (January 2020, SPIE OPTO, 2020)

   We propose and demonstrate low-refractive-index particles with all-dielectric metamaterial shell which lead to formation of high intensity photonic nanojets. We show that the extra degree of freedom because of the anisotropy of the shell gives rise to an increase in the photonic jet intensity inside the metamaterial shell without a need to increase the size of the particle. The anisotropy of the shell can also control the spectral and spatial location of the Mie-type multipolar resonances to achieve the desired scattering. In experiments, the metamaterial shell is composed of strong nonlinear materials leading to enhanced nonlinear wavelength conversion at nanoscale. 

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4. Porous Dust Particles in Protoplanetary Disks: Application to the HL Tau Disk

   https://doi.org/10.3847/1538-4357/acdb4e  

   Zhang, Shangjia; Zhu, Zhaohuan; Ueda, Takahiro; Kataoka, Akimasa; Sierra, Anibal; Carrasco-González, Carlos; Macías, Enrique (August 2023, The Astrophysical Journal)

   Abstract Dust particle sizes constrained from dust continuum and polarization observations by radio interferometry are inconsistent by at least an order of magnitude. Motivated by porous dust observed in small solar system bodies (e.g., from the Rosetta mission), we explore how the dust particle’s porosity affects the estimated particle sizes from these two methods. Porous particles have lower refractive indices, which affect both opacity and polarization fraction. With weaker Mie interference patterns, the porous particles have lower opacity at millimeter wavelengths than the compact particles if the particle size exceeds several hundred microns. Consequently, the inferred dust mass using porous particles can be up to a factor of six higher. The most significant difference between compact and porous particles is their scattering properties. The porous particles have a wider range of particle sizes with high linear polarization from dust self-scattering, allowing millimeter- to centimeter-sized particles to explain polarization observations. With a Bayesian approach, we use porous particles to fit HL Tau disk’s multiwavelength continuum and millimeter-polarization observations from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Very Large Array (VLA). The moderately porous particles with sizes from 1 mm–1 m can explain both continuum and polarization observations, especially in the region between 20 and 60 au. If the particles in HL Tau are porous, the porosity should be from 70%–97% from current polarization observations. We also predict that future observations of the self-scattering linear polarization at longer wavelengths (e.g., ALMA B1 and ngVLA) have the potential to further constrain the particle’s porosity and size. 

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5. Detectability of unresolved particles in off-axis digital holographic microscopy

   https://doi.org/10.1364/AO.507375  

   Johnston, Nikki; Dubay, Megan_Marie; Serabyn, Eugene; Nadeau, Jay_L (February 2024, Applied Optics)

   Off-axis digital holographic microscopy (DHM) provides both amplitude and phase images, and so it may be used for label-free 3D tracking of micro- and nano-sized particles of different compositions, including biological cells, strongly absorbing particles, and strongly scattering particles. Contrast is provided by differences in either the real or imaginary parts of the refractive index (phase contrast and absorption) and/or by scattering. While numerous studies have focused on phase contrast and improving resolution in DHM, particularly axial resolution, absent have been studies quantifying the limits of detection for unresolved particles. This limit has important implications for microbial detection, including in life-detection missions for space flight. Here we examine the limits of detection of nanosized particles as a function of particle optical properties, microscope optics (including camera well depth and substrate), and data processing techniques and find that DHM provides contrast in both amplitude and phase for unresolved spheres, in rough agreement with Mie theory scattering cross-sections. Amplitude reconstructions are more useful than phase for low-index spheres and should not be neglected in DHM analysis. 

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