Negative optical spin torque wrench of a non-diffracting non-paraxial fractional Bessel vortex beam

Abstract

-An absorptive Rayleigh dielectric sphere in a non-diffracting non-paraxial fractional Bessel vortex beam experiences a spin torque. The axial and transverse radiation spin torque components are evaluated in the dipole approximation using the radiative correction of the electric field. Particular emphasis is given on the polarization as well as changing the topological charge  and the beam's half-cone angle. When  is zero, the axial spin torque component vanishes. However, when  becomes a real positive number, the vortex beam induces left-handed (negative) axial spin torque as the sphere shifts off-axially from the center of the beam. The results show that a non-diffracting non-paraxial fractional Bessel vortex beam is capable to induce a spin reversal of an absorptive Rayleigh sphere placed arbitrarily in its path. Potential applications are yet to be explored in particle manipulation, rotation in optical tweezers, optical tractor beams, the design of optically-engineered metamaterials to name a few areas. Keywords: Negative spin torque, dielectric sphere, dipole approximation.The optical radiation torque [1] which arises from the transfer of angular momentum of light to a (lossy) dielectric sphere has received significant attention in optical levitation [2-4] and particle rotation applications in atomic physics [5,6], cell biology [7] and other areas [8,9]. In common optical laser beams, and under some conditions related to the particle absorptive properties, a spin torque causing the particle to rotate around its center of mass arises, in addition to an orbital component causing the particle to rotate around the beam's axis of wave propagation [1]. Note that the orbital torque component exerted on the sphere vanishes at the center of the beam. Moreover, the sphere should be absorptive in order to experience a spin torque [1]. Various standard types of laser beams have been suggested and investigated for this purpose [10,11]. Examples of beams also include optical vortices of Gaussian-like [12,13] and nondiffracting beams [14].A particular kind of vortices with fractional topological charge (or order) has received increasing attention due to their special In contrast, another class for factional vortex beams (which can be realized experimentally using blazed-phase hologram encoded in a programmable liquid crystal display illuminated with a He-Ne laser [19]) and displays limited-diffracting features during propagation exists [19,20]. The physically-realizable apodized beam carrying finite energy preserves the nondiffracting propagation property. It is also known as a high-order Bessel vortex beam of fractional type  (HOBVB-F) [21][22][23]. A HOBVB-F with fractional order  connects standard nondiffracting Bessel (vortex) beams (Fig. 1) of successive integer order in a smooth transition. It also generates individual vortices in the (lateral) plane perpendicular to the axis of wave propagation corresponding to a cross-section of the incident nondiffracting beam [19]. This specific feature provides ...

Optical Bessel beam illumination of a subwavelength prolate gold (Au) spheroid coated by a layer of plasmonic material: radiation force, spin and orbital torques

“…Notice that the rate of energy absorption by the subwavelength spheroid considered here is zero. It must be recalled here that the spin torque reversal has been observed initially in the context of a scalar (acoustical) HOBVB incident upon a viscous sphere located arbitrarily in space (figure 4 in [47]), as well as vector (optical) HOBVBs on dielectric absorptive and semiconducting spheres [45,48,76] satisfying energy conservation. Furthermore, the orbital torque sign reversal, initially observed in the context of a circularly polarized Gaussian laser beam incident upon an absorptive sphere ( figure 7 in [10]), and predicted previously for vector HOBVBs [46] on a semiconducting lossy sphere, is anticipated here on a subwavelength layered lossless spheroid as shown by the results of the present study.…”

“…Moreover, the HOBVB resists diffraction over an extended distance of several wavelengths, and has the capability to reform [35][36][37][38] after encountering an obstruction, as long as the whole beam is not blocked. In addition, a HOBVB has the capability of inducing a negative pulling radiation force [39][40][41][42][43][44] as well as spin [45,46] and orbital [46] torque reversal [45][46][47][48] depending on the beam parameters.…”

“…All these properties make the optical HOBVB an attractive candidate from the standpoint of particle manipulation and handling applications. Various investigations recognizing the importance and utility of the HOBVB considered the case of a sphere, and examined the resonance scattering [49], radiation forces [42,43,50,51] and torques [45,46,48]. Nonetheless, in some instances, the actual shapes of a variety of objects deviate significantly from the spherical geometry.…”

Enhanced Fractional Acoustic Vortices by an Annulus Acoustic Metasurface with Multi‐Layered Rings

“…Because of these special features, many efforts were first devoted into the fractional optical vortices (FOVs), which were explored for achieving the atomic trapping, [ 11 ] edge‐detection imaging, [ 12,13 ] spatial quantum entanglement, [ 14,15 ] and micro‐particle manipulation. [ 16–18 ] More recently, excited by these significant applications of FOVs, fractional acoustic vortices (FAVs) have received increasing attention because of their advantages of lower power consumption, deeper penetration depth in tissue, and weaker biological damage. [ 19–21 ]…”

Optical Force and Torque on a Graphene-Coated Gold Nanosphere by a Vector Bessel Beam

“…Bessel beams [ 5 , 6 , 7 , 8 ], a typical nondiffracting beam, can simultaneously trap and manipulate many particles in multiple planes because of their unique properties of nondiffraction and self-healing. In addition, by adjusting the beam parameters including half-cone angle, beam order, and polarization, Bessel beams can exert pulling force [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 ] and negative optical torque [ 3 , 37 , 38 , 39 , 40 , 41 ] on particles.…”