Light-induced rotations of chiral birefringent microparticles in optical tweezers

Abstract

We study the rotational dynamics of solid chiral and birefringent microparticles induced by elliptically polarized laser light in optical tweezers. We find that both reflection of left circularly polarized light and residual linear retardance affect the particle dynamics. The degree of ellipticity of laser light needed to induce rotations is found. The experimental results are compared with analytical calculations of the transfer of angular moment from elliptically polarized light to chiral birefringent particles.

Optical tweezers and their applications

“…Further applications of optically trapped of microbubbles include Raman spectroscopy of the gas enclosed by the bubble [159], measuring the microbubble shell thickness [160], and positioning the bubble adjacent to cells prior to it being irradiated by ul- [170,171,172,173,174]. In particular, lefthanded CLC solid microparticles have been synthesized [175], optically trapped, and chiral rotations observed for the corresponding left-handed circularly polarised light [176,177]. In fact, the left-handed CLC particles behave as chiral 710 mirrors, which reflects only left-handed light, while maintaining its handedness [176].…”

“…A solution to this demand can be provided by structured optical beams, carrying orbital and spin angular momentum (SAM and OAM), generated by holographic optical tweezers (HOT) or similar techniques [195]. In particular, microrotators 740 and micropumps have been realized by transferring SAM and OAM to microparticles [196,166,197,176,177].…”

Spin-orbit optomechanics of optically levitated chiral Bragg microspheres

“…Here we explore the direct optomechanical observation of such a spin-orbit torque by using laser-levitated chiral liquid-crystal droplets that exhibit the circular Bragg reflection phenomenon [12]. More generally, we point out that the total angular momentum transfer per photon may depart from the spin-only ±2h contribution assumed so far [13,14]. In the context of applied chiral optomechanics [15][16][17][18], this may have implications towards the elaboration of optical enantioseparation techniques, which so far have been exploiting only spin-driven discriminatory optical forces [19][20][21][22][23].…”

“…This results in −2χh spin angular momentum transfer per photon, in stark contrast to reflection off a conventional mirror that is associated with no net transfer of angular momentum. So far it is the above spin-only angular momentum balance that has been used to describe the rotational consequences of the circular Bragg reflection on cholesteric droplets [13,14,18]. However, liquid crystals may exhibit a rich variety of textures and topological defects [24].…”

Superchiral Surface Waves for All-Optical Enantiomer Separation

“…[5][6][7] A variety of solutions, based on plasmonic and photonic nanostructures, have been designed to obtain superchiral light fields exploitable in realistic scenarios, with a particular focus on designing solutions capable of analyzing small amounts of chiral molecules. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] In parallel with the search for efficient superchiral sensing platforms, the research community devoted significant attention to the study and design of electromagnetic fields for the generation of enantioselective optical forces, [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] i.e. optical forces that can trap, separate, or more in general discriminate between the two different enantiomers of a chiral chemical compound.…”