Abstract
Focusing systems with high numerical aperture can be used to convert spin angular momentum into orbital angular momentum with efficiencies of 50%, while for low numerical apertures this conversion vanishes. In this paper, based on the properties of binary Fresnel zone plates, we propose a structure that is achieved by making an accurate selection of the width and the depth of the rings. This allows us to obtain a large increase in the spin to orbital angular momentum conversion of the resulting focusing fields, and it also has the special characteristic that the obtained conversion is higher for low numerical aperture structures, where standard focusing systems do not work. The ability of the system to perform this extraordinary conversion is demonstrated by FDTD methods and an analytical model developed using a combination of guided mode theory for the structure and Stratton–Chu diffraction theory.
Highlights
Wavelengths) and the far field, since despite that the conversion efficiency is lower in the near field than for the far field, the values are much greater than that obtained from tightly focusing systems; and may be the most important advantage, since our system is completely scalable to all the electromagnetic spectrum adapting the dimensions to the corresponding wavelength, from the RX to the microwaves, while the Q-plates are limited to the working spectra of the liquid crystals
We are going to show the different electromagnetic properties for the fields generated with the GFZP as a function of design parameters, such as the focal length and structure thickness, paying special attention to the extraordinary spin-to-orbital angular momentum conversion
The effect of the guiding characteristics of the structure is studied for two different values of the structure depth: one with β = 0.1 μm, which is about ten times lower than the wavelength, the so-called short guided Fresnel zone plate (SGFZP); and the other with β = 1 μm, which we call the long guided Fresnel zone plate (LGFZP)
Summary
We are going to show the different electromagnetic properties for the fields generated with the GFZP as a function of design parameters, such as the focal length and structure thickness, paying special attention to the extraordinary spin-to-orbital angular momentum conversion. The SAM to OAM conversion is analyzed through the corresponding integral ratios of the axial component of the spin and orbital angular momentum in the focal plane, which can be considered the values per photon The designed GFZP structures generate an extraordinary SAM to OAM conversion, much greater than the one described for standard high numerical aperture focusing systems, and with the special characteristic that the conversion factor is greater for higher focal distances, while the conversion for the other focusing systems vanishes
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