A comprehensive radiative transfer model is used to calculate surface spectral ultraviolet irradiance under cloud‐free conditions. The results are compared with measurements made at Lauder, New Zealand (45°S, 170°E) before and after the eruption of Mount Pinatubo and including a snow‐covered surface. There is reasonable agreement between measured and calculated diffuse, direct, and global irradiances over the range 300 to 450 nm. Discrepancies may be due to calibration errors in the UV measurements, or in the extra terrestrial irradiances input to the model. Ratios of diffuse/direct irradiances are independent of such calibration uncertainties and therefore provide a sensitive test of the model. If appropriate ozonesonde data, surface albedo, and aerosol optical properties are used, the model ratios are in satisfactory agreement with measurements over a wide range of observing conditions. For cases in which the atmospheric optical properties are best known the agreement is better than 8% in the UV‐B range, and for wavelengths 320 to 450 nm the deviation is smaller. The comparison suggests that the ultraviolet radiation exposure can be computed with confidence for clear sky conditions if the appropriate atmospheric molecular density profiles, ozonesonde data, surface albedo, and aerosol optical properties are available. The UV radiation model is used to investigate the impact of changes in solar zenith angle, ozone abundance, surface albedo, and aerosol loading on UV radiation reaching the surface of the Earth. The ratios of diffuse to direct irradiance depend critically on solar zenith angle, surface albedo, and aerosol extinction. Ozone changes have pronounced effects on the global UVB irradiance but have only a minor effect on these ratios.
Vortex beams with helical phase, carrying phase singularity and orbital angular momentum, have attract great attention in the past decades due to their wide applications in optical communications, optical manipulation, super-resolution imaging and so on. Vortex beams with low spatial coherence, i.e. partially coherent vortex beams, carrying correlation singularity, display some unique properties during propagation, e.g. self-shaping, selfsplitting and self-reconstruction. Partially coherent vortex beams exhibit some advantages over coherent vortex beams in some applications, such as remote sensing, laser radar and free-space optical communications. This review summarizes research progress on partially coherent vortex beams, including theoretical models, propagation properties, generation and topological charge determination.
Zero-order and higher-order Bessel beams are well-known nondiffracting beams. Namely, they propagate with invariant profile (intensity) and carry a fixed orbital angular momentum. Here, we propose and experimentally study an anomalous Bessel vortex beam. Unlike the traditional Bessel beams, the anomalous Bessel vortex beam carries decreasing orbital angular momentum along the propagation axis in free space. In other words, the local topological charge is inversely proportional to the propagation distance. Both the intensity and phase patterns of the generated beams are measured experimentally, and the experimental results agree well with the simulations. We demonstrate an easy way to modulate the beam’s topological charge to be an arbitrary value, both integer and fractional, within a continuous range. The simplicity of this geometry encourages its applications in optical trapping and quantum information, and the like.
As an indispensable complement to an integer vortex beam, the fractional vortex beam has unique physical properties such as radially notched intensity distribution, complex phase structure consisting of alternating charge vortex chains, and more sophisticated orbital angular momentum modulation dimension. In recent years, we have noticed that the fractional vortex beam was widely used for complex micro-particle manipulation in optical tweezers, improving communication capacity, controllable edge enhancement of image and quantum entanglement. Moreover, this has stimulated extensive research interest, including the deep digging of the phenomenon and physics based on different advanced beam sources and has led to a new research boom in micro/nano-optical devices. Here, we review the recent advances leading to theoretical models, propagation, generation, measurement, and applications of fractional vortex beams and consider the possible directions and challenges in the future.
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