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In this work we show a novel method based on a local two-port interferometer to distinguish the topological charge of radio-vortices at 30 GHz by using a small portion of the entire wavefront only. The experimental investigation of the amplitude and phase properties of the interference pattern with a pure Gaussian beam (l = 0) and a l = 1 radio vortex is carried out, and results are compared with the theory based on Laguerre-Gauss modes. Experiments were performed both with the interferometer and with single antenna to highlight the effective benefits of the interferometric approach, sensitive to the azimuthal phase of the vortex field. Method is also extendable at higher topological charges for applications to high-density millimetric communications.

We exploit the properties of differential geometry of minimal surfaces to introduce a novel approach for characterizing wavefronts. Since Gaussian and mean curvatures describe global and local properties of any differentiable surface, a method for characterizing wavefronts endowed with non-trivial topological features has been introduced. We provide experimental evidence that the wavefront of an l = 1 radio-vortex at 30 GHz can be fully characterized by exploiting the wavefront phase in the far field of the source, accessing a small portion of the beam only. A particular care is dedicated to distinguish diffraction effects from the intrinsic curvature of the helicoidal wavefront. Results are applicable to the local measurement of the topological charge and to the local detection of orbital angular momentum radiation at the millimetric wavelengths.

We describe a robust, portable, deployable instrument for multiparametric optical characterization of single airborne particles. It is based on the Single Particle Extinction and Scattering method with additional sensors at 45° and 90° angles. Four independent optical parameters are associated to each particle. Basically, it provides a rigorous measurement of the extinction cross section and the complex amplitude of the forward scattered field. Moreover, thanks to the multiparametric single particle approach, it is possible to roughly classify the particles within a size range from a few hundreds of nanometers to some micrometers. By assigning a reasonable single scattering albedo for each population, our data are enough to fit the phase function with acceptable uncertainties. We report here the results of tests performed with water droplets, generating well controlled data without any free parameters. Data analysis is described in detail. We also report measurements performed on urban aerosol collected in the city of Milan by recovering the optical properties and feeding radiative transfer models. The findings reported here support the importance of an accurate measurement of the phase function, as already established by the Community. Copyright © 2020 American Association for Aerosol Research.

We describe a light scattering technique for characterizing colloidal samples under constant flow. It exploits the properties of speckles in the deep Fresnel region—the so-called near field speckles—providing absolute scattering measurements of the static form factor of the sample, as described extensively by Mazzoni et al. [Rev. Sci. Instrum. 84, 043704 (2013)] for static samples. We exploit a strongly astigmatic beam for illuminating the scattering volume with a light sheet a few microns thick. This largely improves the sensitivity of the method to small signals. Moreover, by flowing the sample in the direction perpendicular to the light sheet, the transit times are reduced to a minimum, allowing for fast measurements. We tested the instrument with suspensions of calibrated colloidal polystyrene spheres with a size comparable to the light wavelength. In particular, we recovered the static form factors of suspensions of spherical particles and the phase lag of the zero-angle scattering amplitude, which both compare well to Mie theory predictions. We then applied the method to colloidal fractal aggregates of sub-wavelength particles and measured their fractal dimension. The instrument is designed to be operational in continuous flow analysis systems.

Mineral dust aerosol in ice cores is one of the most important proxies for paleoclimate research. Under certain conditions, in the deeper part of ice cores, the pristine paleoclimate signal can be altered by in situ formation of dust aggregates, following the relocation of the impurities. Thus, aggregate detection is a critical indication for post-depositional processes. Clues for the presence of aggregates have been provided by anomalously large dust size distributions, while small aggregates were basically invisible to conventional dust analysis techniques. In this paper, we propose an optical approach to this problem based on the single-particle extinction and scattering (SPES) method, which allows researchers to distinguish between compact and non-compact particles through the analysis of samples populated by isometric particles contained in the core samples. This method can potentially be used during continuous flow analyses of ice cores. It allows for the detection of even the tiniest aggregates, falling within the typical size interval of aeolian mineral aerosol. This approach will potentially provide key evidence for assessing the integrity of paleoclimate records

Mineral dust aerosol (dust) is widely recognized as a fundamental component of the climate system and is closely coupled with glacial-interglacial climate oscillations of the Quaternary period. However, the direct impact of dust on the energy balance of the Earth system remains poorly quantified, mainly because of uncertainties in dust radiative properties, which vary greatly over space and time. Here we provide the first direct measurements of the aerosol optical thickness of dust particles windblown to central East Antarctica (Dome C) during the last glacial maximum (LGM) and the Holocene. By applying the Single Particle Extinction and Scattering (SPES) technique and imposing preferential orientation to particles, we derive information on shape from samples of a few thousands of particles. These results highlight that clear shape variations occurring within a few years are hidden to routine measurement techniques. With this novel measurement method the optical properties of airborne dust can be directly measured from ice core samples, and can be used as input into climate model simulations. Based on simulations with an Earth System Model we suggest an effect of particle non-sphericity on dust aerosol optical depth (AOD) of about 30% compared to spheres, and differences in the order of ∼10% when considering different combinations of particles shapes.

Characterizing nano- and micro-particles in fluids still proves to be a significant challenge for both science and industry. Here, we show how to determine shape and size distributions of polydisperse water suspensions of micron-sized particles by the analysis of the field scattered in the forward direction by single particles illuminated by a laser beam. We exploit the novel Single Particle Extinction and Scattering method in connection with shear conditions which give preferred orientations to the particles passing through the scattering volume. Water suspensions of calibrated non-spherical particles, polydisperse standard monophasic mineral samples of quartz and kaolinite, and a mixture of quartz and illite are studied in detail. Application and limitation of the method are discussed.

We discuss the design and expected performance of STRIP (STRatospheric Italian Polarimeter), an array of coherent receivers designed to fly on board the LSPE (Large Scale Polarization Explorer) balloon experiment. The STRIP focal plane array comprises 49 elements in Q band and 7 elements in W-band using cryogenic HEMT low noise amplifiers and high performance waveguide components. In operation, the array will be cooled to 20 K and placed in the focal plane of a ~0.6 meter telescope providing an angular resolution of ~1.5 degrees. The LSPE experiment aims at large scale, high sensitivity measurements of CMB polarization, with multi-frequency deep measurements to optimize component separation. The STRIP Q-band channel is crucial to accurately measure and remove the synchrotron polarized component, while the W-band channel, together with a bolometric channel at the same frequency, provides a crucial cross-check for systematic effects.