Light-induced rotation of absorbing microscopic particles by transfer of angular momentum from light to the material raises the possibility of optically driven micromachines. The phenomenon has been observed using elliptically polarized laser beams or beams with helical phase structure. But it is difficult to develop high power in such experiments because of overheating and unwanted axial forces, limiting the achievable rotation rates to a few hertz. This problem can in principle be overcome by using transparent particles, transferring angular momentum by a mechanism first observed by Beth in 1936, when he reported a tiny torque developed in a quartz waveplate due to the change in polarization of transmitted light. Here we show that an optical torque can be induced on microscopic birefringent particles of calcite held by optical tweezers. Depending on the polarization of the incident beam, the particles either become aligned with the plane of polarization (and thus can be rotated through specified angles) or spin with constant rotation frequency. Because these microscopic particles are transparent, they can be held in three-dimensional optical traps at very high power without heating. We have observed rotation rates in excess of 350 Hz.

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Optical alignment and spinning of laser-trapped microscopic particles

We report tightly focused characteristics of the auto-focusing linear polarized circular Pearcey Gaussian vortex beams (CPeGVBs) with on-axis and off-axis vortex pairs through high numerical aperture in this paper. Compared with the linear polarized Airy beam, we find that the autofocusing linear polarized CPeGVBs are rotating during the tightly focusing progress. We analyse the relationship between the autofocusing and the tightly focusing of CPeGVBs. At the same time, some cases of the flat-top beam and the quasi optical comb mode generated by the linear polarized CPeGVBs are analyzed in detail, and the changing of the focus depth with beam parameters is also discussed. To show the superiority of linear polarization CPeGVBs, we explore the tightly focusing intensity contrasts of the beams, and we characterize the resulting optical polarization singularity using zero-, one- and two-order topological structures. We present the spin density vector and the polarization ellipse distributions of the CPeGVBs in the x − y plane. Furthermore, the CPeGVBs present two kinds of the spin density vector on the optical axis that one is helix shaped and the other one is oscillating in the one axial direction of CPeGVBs by choosing the appropriate factors. Our results have the potential applications in reverse shaping of focal fields and optical trapping control.

Tightly focusing evolution of the auto-focusing linear polarized circular Pearcey Gaussian vortex beams

A transmissive all-silicon dielectric square metasurface with a side length of 12.4 mm is designed in this paper. At 2.52 terahertz, the normally incident x-polarized Gaussian beam can be shaped into the radially polarized narrow-width annular beam with Bessel-Gaussian amplitude distribution at a propagation distance of 45 mm away from the metasurface. Anisotropic rectangular silicon pillars with different dimensions and rotation angles are arranged on the silicon substrate to manipulate the beam. To suppress the reflection loss, a polarization-independent square silicon pillar array is also proposed, which is patterned at the other side of the substrate. In addition, the errors analysis is also discussed. It is expected that our study may have potential for applying terahertz wave in high-resolution microscopy and capturing particles in three dimensions.

Theoretical study on generation of radially polarized beam from linearly polarized beam with all-silicon metasurface in the terahertz regime

Tight-focusing properties of radially polarized chirped circular Airy Gaussian beam

In this article, we propose a non-orbital angular momentum (OAM) method for constructing the spiral twists of spin density (SD) vectors in three-dimensional (3D) optical fields. This is realized by generating strings of SD singularities in a longitudinal plane of a strongly focused HG10 mode beam with circular polarization. We demonstrate that in this plane, the SD singularities manifest themselves as the Gouy phase difference. Through observing the Gouy phase difference, the strings of the SD singularities and their topological reactions are examined. It is found that the spiral twists of SD vectors are constructed in the two-dimensional (2D) space between two strings and their twisting behaviors can be adjusted by the topological reactions of the strings. This finding shows that the spiral twists can be formed in a 2D space rather than the 1D space in previous studies, which provides a more flexible way to utilize this rotational degree of freedom. Our result also clarifies that although the spiral twists of SD vectors can be a result of spin-orbital angular momentum interaction, the OAM is not a necessary condition for the generation of these twists. Our theoretical proposal for structuring SD vectors may have applications in 3D optical manipulation.

Feng Ye

Papers

The Effect of the Spin Angular Momentum on the Tight-Focusing Vortex Hollow Gaussian Beams