Mechanisms of optical angular momentum transfer to nematic liquid crystalline droplets

Abstract

A detailed study is presented that evaluates the relative importance of wave plate behavior, scattering processes and absorption phenomena in transferring optical torque from circularly polarized light to optically trapped nematic droplets. A wide range of parameters is considered: droplet diameters between 1 and 15 µm, birefringence values from 0.15 to 0.26 and trapping beam powers from 50 mW to 400 mW. Wave plate behavior is verified through the dependence of torque on droplet diameter and material birefringence. The dependence of the magnitude of the torque on material birefringence confirms the additional importance of the scattering mechanism. Absorption processes are found to be negligible

Optical Trapping, Manipulation, and 3D Imaging of Disclinations in Liquid Crystals and Measurement of their Line Tension

“…Laser tweezers experiments in anisotropic fluids (such as LCs) require some additional care because (a) the refractive index difference between the particle and the host medium depends on the local LC director field ) ( r n r and light polarization, (b) birefringence of LC results in light defocusing, and (c) the focused light beam can reorient the local ) ( r n r as demonstrated for the nematic LC in flat slabs [20] and spherical droplets [22]. These problems are intrinsic when applying laser tweezers for the study of anisotropic media; they can be mitigated by using LCs with small birefringence n ∆ and colloidal particles larger than the waist of the laser beam, Fig.1b, as we will show below.…”

“…Most of the trapping and manipulation experiments using laser tweezers were done for the isotropic fluids. The advantages of laser tweezers in the study of LCs only begin to be explored [16][17][18][19][20][21][22] and the important new features of laser trapping in these birefringent media begin to be realized [17,19]; for example, optical trapping allowed researches to study both dipolar [17,18] and quadrupolar [23] colloidal interactions of particles immersed in nematic LCs.…”

Precise switching control of liquid crystalline microgears driven by circularly polarized light

“…In 1936, circularly polarized light was shown to transfer both angular and linear momentum to quartz plates used as birefringent materials [1]. Laser tweezers can also be used to rotate liquid crystalline (LC) droplets, birefringent liquid particles that are surrounded by solvents, with circularly polarized light, although the shapes of such birefringent micro-objects have been limited to simple configurations of spheres or cylinders [4][5][6][7][8][9].…”

“…Micro-objects are optically rotated by light with inherent angular momentum, including lasers with a helical phase structure (e.g., the Laguerre-Gaussian mode) and circularly polarized light [1][2][3][4][5][6][7][8][9], because micro-objects can extract the angular momentum of such light due to their absorptive or birefringent optical properties [9]. In 1936, circularly polarized light was shown to transfer both angular and linear momentum to quartz plates used as birefringent materials [1].…”

“…In this letter, we impart birefringence to photopolymerizable resins for more precise switching control of microrotors. Although there have been many fundamental studies on the rotational behaviors of LC droplets, both theoretical and experimental [4][5][6][7], their potential as elaborate micromachines remains to be explored [8]. We used photopolymerizable LC materials instead of genuine LC droplets and fabricated microgears of various shapes using two-photon microstereolithography.…”