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Africa has a long history in optics, but decades of turmoil have seen optical science in Africa advance only slowly, punching far below its weight. But a younger generation of scientists hold promise for the brighter future, addressing continental issues with photonics. In this Feature Issue on Optics in Africa we capture some of the exciting optical research from across the continent in 51 research reports, covering both fundamental and applied topics. The issue is supplemented by invited review articles that offer authoritative perspectives on the historical development of key research fields, from early advances in lasers to present-day progress in photonic materials. To encourage the exploration of new research directions, the issue has several tutorial articles that lower the entry barrier for emerging researchers, while highlighting the scope of research on the continent and its international context.Instrumentation design for Fourier transform spectroscopy has until now been hindered by a seemingly fundamental tradeoff between the étendue of the analyzed light source on one hand and the spectral resolution on the other. For example, if a freespace scanning Michelson interferometer is to achieve a spectral resolution of 4cm-1, it can have a maximum angular field of view of roughly 1° for wavelengths in the neighborhood of λ=800nm, where the general tradeoff for this instrument is that the quotient θm2/Δk of the square of the angular field of view θm and the minimum resolvable wavenumber difference Δk is a constant. This paper demonstrates a method to increase the angular field of view allowable for a given resolution by a full order of magnitude, and thus to increase the étendue and, with it, the potential power gathered from an extended source and potential measurement signal-to-noise ratio, by two orders of magnitude relative to the performance of a freespace Michelson interferometer. Generalizing this example, we argue that there may be no fundamental thermodynamic grounds for the tradeoff and that a scanning Fourier transform spectrometer can accept an arbitrarily high étendue field and still, in theory, achieve an arbitrarily narrow spectral resolution.Diffraction calculations are widely used in applications that require numerical simulation of optical wave propagation. Different numerical diffraction calculation methods have their own transform and sampling properties. In this study, we provide a unified analysis where five popular fast diffraction calculation methods are analyzed from the perspective of phase space optics and the sampling theorem single fast Fourier transform-based Fresnel transform, Fresnel transfer function approach, Fresnel impulse response approach, angular spectrum method, and Rayleigh-Sommerfeld convolution. The evolutions of an input signal's space-bandwidth product (SBP) during wave propagation are illustrated with the help of a phase space diagram (PSD) and an ABCD matrix. It is demonstrated that all of the above methods cannot make full use of the SBP of the input signal after diffraction; and some transform properties have been ignored. Each method has its own restrictions and applicable range. The reason why different methods have different applicable ranges is explained with physical models. After comprehensively studying and comparing the effect on the SBP and sampling properties of these methods, suggestions are given for choosing the proper method for different applications and overcoming the restrictions of corresponding methods. The PSD and ABCD matrix are used to illustrate the properties of these methods intuitively. Numerical results are presented to verify the analysis, and potential ways to develop new diffraction calculation methods are also discussed.We report on a theoretical and numerical study of a Gaussian beam modulated by several optical vortices (OV) that carry same-sign unity topological charge (TC) and are unevenly arranged on a circle. The TC of such a multi-vortex beam equals the sum of the TCs of all OVs. If the OVs are located evenly along an arbitrary-radius circle, a simple relationship for the normalized orbital angular momentum (OAM) is derived for such a beam. It is shown that in a multi-vortex beam, OAM normalized to power cannot exceed the number of constituent vortices and decreases with increasing distance from the optical axis to the vortex centers. We show that for the OVs to appear at the infinity of such a combined beam, an infinite-energy Gaussian beam is needed. On the contrary, the total TC is independent of said distance, remaining equal to the number of constituent vortices. We show that if TC is evaluated not along the whole circle encompassing the singularity centers, but along any part of this circle, such a quantity is also invariant and conserves on propagation. Besides, a multi-spiral phase plate is studied for the first time to our knowledge, and we obtained the TC and OAM of multi-vortices generated by this plate. When propagated through a random phase screen (diffuser) the TC is unchanged, while the OAM changes by less than 10% if the random phase delay on the diffuser does not exceed half wavelength. Such multi-vortices can be used for data transmission in the turbulent atmosphere.This paper presents an approach to design the all-dielectric metasurface with multi-function in the near-infrared range of 1.5-1.6 µm. Based on the geometric phase principle, the all-dielectric metasurface is composed of the Si nanopillar and the SiO2 substrate as an emitter unit distributed in a 21×21 array. Under the incidence of the circularly polarized light at 1550 nm, the metasurface works as a vortex-beam generator with high performance which generates the vortex beam with topological charges of ±1, and the mode purity of the vortex beam is 90.66%. Under the incidence of the linearly polarized light at 1550 nm, the metasurface also works as the azimuthally/radially polarized beam generator with high performance, and the purities of the azimuthally and the radially polarized beams are 92.52% and 91.02%, respectively. Smad3 phosphorylation Moreover, the metasurface generates different output spots under the different incident lights which can be applied to optical encryption, and the metasurface with the phase gradient also can be used as the dual-channel encoder/decoder in optical communication.
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