The Optical Rotation Of Mesoscopic Particle And Its Speed Measurement Technology

The optical rotation is an angular manipulation technology for microparticles suspended in the optical trap.In this technology,the angular momentum of light is transferred to the trapped particles,causing them to rotate.Apart from the original three Dimensional optical manipulation,the optical rotation technology archives the rotation control of the trapped particles.It has greatly extended the application of optical trap.In this paper,several mesoscopic particles including polystyrene microspheres,red blood cells,and vaterites were trapped and rotated in the optical trap.The rotation speed was accurately measured using Rotational Doppler Effect.The main content of this pater consists of two parts.Firstly,the orbital rotations of polystyrene microspheres and red blood cells were firstly achieved by introducing a transverse offset to the dual-beam fiber-optic trap.The influence of offset distance on the dual-beam fiber-optic trap was discussed in detail.Then,the vaterite microsphere was trapped and rotated by a circularly polarized beam.The rotation speed of the vaterite microsphere was accurately measured using Rotational Doppler Effect.The research of this paper has greatly enriched the implementations and the experimental phenomena of the optical rotation of mesoscopic particles,and tremendously extended the application of the optical rotation technology.The main contents and results of this paper are listed as follows:1.The optical force exerted on the microsphere trapped by the dual-beam fiber-optic trap was simulated,and then the motion state of the microsphere was systematically analyzed by the Runge-Kutta method.As the offset distance between two counterpropagating beams increases,the motion type of the microsphere starts with capture,then spiral motion,then orbital rotation,and ends with escape.2.The orbital rotation of polystyrene microsphere was realized by introducing a transverse offset to the dual-beam fiber-optic trap.The orbital rotation perimeter and frequency are evaluated as functions of the offset distance d,the beam waist separation S,the light power P,and the radius of the microsphere r₀ both experimentally and numerically.It was found that,when the other parameters are invariable,the orbital rotation perimeter increases with increased d,decreased S,and decreased r₀,while it is invariable with P.The orbital rotation frequency increases with decreased d,increased P,increased S,and increased r₀.The theoretical model results about the orbital rotation perimeter and frequency agreed well with the experiments.Both the experimental and simulation results showed that the offset interval corresponding to the orbital rotation was getting to narrow down with the decreasing of r₀.3.The vectorial ray-tracing method was used to simulate the optical force and torque exerted on the ellipsoid in the dual-beam fiber-optic trap with transverse offset.The motion state of the trapped ellipsoid was analyzed.The simulation showed that the ellipsoid trapped in vacuum would spin in the trap center.The rotation speed is approximately sinusoidal over time.4.The spin and orbital rotation of the red blood cell were achieved simultaneously by introducing a transverse offset to the dual-beam fiber-optic trap.The motion state of the red blood cell for various offset distance was analyzed.When the trapping beam was strictly aligned,the red blood cell would be stably suspended in the trap center,with the long axis paralleling to optic axis.When the offset distance is smaller than a critical offset distance,the RBC is observed to spin in the trap center.When the offset distance is above the critical value,the RBC rotated along an elliptic orbit in the optical trap.In addition,along with the orbital rotation,the RBC still spin itself in the orbit.The spin frequency decreases with the offset distance.5.The angular velocity of a vaterite microsphere spinning in the optical trap is measured using Rotational Doppler Effect.The perfectly spherical vaterite microspheres were synthesized via coprecipitation in the presence of silk fibroin nanospheres.When trapped by a circularly polarized beam,the vaterite microsphere is uniformly rotated in the trap center.The probe beams containing two Laguerre-Gaussian beams of l=±7,l=±8 and l=±9 were respectively illuminated on the vaterite microsphere.The rotation frequencies in the three cases were measured to be?15.85±0.12?Hz,?15.78±0.05?Hz,and?15.78±0.01?Hz.The final error in the three cases could be controlled within 0.4percent.This approach has much higher accuracy for the rotation frequency measurement than the transmission method.Furthermore,the larger the topological number,the higher the measurement accuracy.