Abstract
As an intrinsic attribute of light, the spin angular momentum (SAM) of photons has aroused considerable attention because of the fascinating properties emerging from light-matter interactions. We show that a diffraction-limited focal field with a steerable photonic spin structure in three dimensions can be produced under a 4π microscopic system. This is achieved by focusing two counter-propagating configurable vector beams produced in the coherent superposition of three different beams with x-polarization, y-polarization, and radial-polarization. By altering the amplitude factors of these resultant beams, the ratios between the three mutually orthogonal polarized components can be freely tuned within the focal plane, thereby allowing dynamic control over the spin orientation and ellipticity of the tightly focused optical field. The results demonstrated in this paper may find applications in spin-controlled nanophotonics.
Highlights
Since the seminal work of Poynting [1] and the first experimental demonstration by Beth [2], it is well acknowledged that light carries both linear and angular momenta (AM)
By varying the amplitude factors from 0 to 1, the direction angles α, β, and γ, which are used to quantify the orientation of the spin axis, take values in the ranges of [90°, 180°], [90°, 180°], and [0°, 90°], respectively [see Figs. 5(a), 5(c), and 5(e)]
We find that the focal spot obtained in this work is 3D super-resolved because all three dimensions are smaller than the diffraction limit (∼0.526λ)
Summary
Since the seminal work of Poynting [1] and the first experimental demonstration by Beth [2], it is well acknowledged that light carries both linear and angular momenta (AM). For a nonparaxial beam with 3D controllable elliptical polarization, the spin orientation directly represented by the normal to the polarization plane is unrestricted in 3D space Such polarization states, as recently demonstrated in tight focusing systems, can be synthesized by reversing the radiation patterns from two electric dipoles [23,24,25,26] or by the assistance of a liquid crystal variable retarder [27]. Ellipticity, as another important parameter of polarization, need to be controlled to achieve a full manipulation of photonic spin In this context, by focusing two counter-propagating configurable vector beams in the 4π microscopic system, we develop a novel method to control the spin orientation and ellipticity of a highly confined focused electric field.
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