Optical vortex tweezers induced orbital motion of cholesteric liquid crystal microdroplets

Abstract

This thesis uses optical vortex tweezers (OVTs) with spin and orbital angular momentum (SAM and OAM) to capture cholesterol liquid crystal microdroplets (CLC MDs) with Bragg onion-like structure The experiment mainly observes and studies the orbital motion behavior of the CLC MDs under the influence of the two angular momenta of the OVTs The first part of the experiment is to study the difference of the orbital speed of the CLC MDs when the handedness of the CLC MDs and the sense of the circular polarization (or SAM) of the OVTs are the same or opposite The second part of the experiment studies the difference between the orbital motion speeds of the CLC MDs caused by the manipulation of the OVTs before and after the CLC MDs are exposed to UV light The first part of the experimental results shows that when the sense of the circular polarization of the OVTs is the same as that of the handedness of the CLC MDs the orbital speed of the MDs is faster than when the circular polarization of the OVTs is opposite to the CLC MDs This is because when the circular polarization of the OVTs is opposite to the handed sense of the MDs the photons can be refracted into the MDs and the OAM are transferred to the MDs because the photon impacts the MDs with azimuthal scattering force which will induce the orbital rotation of the MDs However if the circular polarization of the eddy current clamp is the same as the handed sense of the MDs the photons will be Bragg-reflected by the MDs The reflected photons will give the MDs twice the incident OAM (i e twice the incident azimuthal scattering force) causing the MDs to rotate at a faster speed This study further uses Classical Mechanics to establish an approximate formula for the terminal speed of the orbital motion of the MDs with a simple physical model to explain the experimental results Experiments have also found that the direction of rotation of the MDs is dominated by the direction of the OVTs OAM The second part of the experimental results show that when azo LCs are added to the CLC MDs the UV light will cause the azo LCs to undergo trans-cis isomerization reaction causing the CLC MDs to isothermally transform into isotropic phase Comparing the cases before and after UV irradiation it is found that the orbital motion of the MDs caused by the right- and left-handed OVTs both have significantly decreased to the same angular velocity This result shows that the isotropic MDs are no longer selective for the circular polarization of the MDs This study further uses the approximate formula of the terminal rotation speed of the MDs to explain the possible reasons for the large slowdown of the MDs in the isotropic state (1) The decrease in the equivalent refractive index of the MDs after the phase change of the MDs causes the scattering force (torque) given by the OVTs photon to decrease Additionally because the position of the donut-like ring of the OVTs on the MDs remains unchanged the effective exposure volume of the optical ring on the MDs decreases when each MD enlarges also leading to the decrease of the scattering force (torque) given by the OVTs photon (2) Due to the fair enlargement of each MD after the phase change the contact surface area between the MDs and the environment becomes larger resulting in an increase in drag torque Based on the above reasons the rotation speed of the MDs after the phase transition from CLC to isotropic state is greatly reduced

Date of Award	2019
Original language	English
Supervisor	Chia-Rong Lee (Supervisor)