Research On Dynamic Analysis Of Trapped Microsphere In Non-liquid Optical Tweezers

Optical trap is a spatial potential well which uses laser beam to capture and manipulate particles.With the advantages of non-contact levitation,high sensitivity and easy integration,optical trap sensing system has shown some unique technical advantages and wide application prospects in the fields of precision sensing and fundamental physics.The dynamic property of microsphere determines the sensing property of an optical trap,and it is also the basis to study frontier physical problems.Therefore,the dynamic analysis of micro sphere is fundamental to understand the mechanism of optical trap and expand its application.In this paper,aiming at studying dynamics of microsphere in optical trap,the optical force calculation method and microsphere dynamical model are introduced.Besides,the experimental platform of optical trap is designed and built,so the dynamics of microsphere under several special situations can be analyzed.This paper focused on the following aspects(1)The dynamic model of microsphere in dual beam laser trap is established.Based on the theory of optical force,combining with the Brownian motion analysis of microsphere in fluid environment,the motion of microsphere in optical trap is described by utilizing Langevin equation;the response of microsphere to external force is analyzed by using spring vibration model.In order to understand the mechanism of optical trap and realize high sensitivity,the dynamics of micro sphere was analyzed by considering several conditions(various vacuum pressure,misalignment of dual beam and temperature of microsphere.)(2)A dual beam vacuum optical trap setup was built to detect microsphere motion,and experiment was carried out to study the dynamics of microspheres under vacuum condition.Through the detection of microsphere motion signals under air and vacuum conditions,the microsphere dynamic was analyzed under different vacuum pressures.The microsphere motion was analyzed both in time and frequency domain,and the optical trap stiffness and viscosity were obtained through the power spectrum of the microsphere motion.Because the microsphere is easy to escape at high vacuum,the feedback control of microsphere movement is necessary.We use the feedback control to cool down the temperature of micro sphere movement in low vacuum.This method is expected to be used for the stable capture of microsphere in ultra-high vacuum.(3)The dynamics of microsphere in the misaligned dual beam optical trap is studied,and the classical force analysis and motion state simulation of a 10 ?m microsphere in the dual beam optical trap are analyzed.An optical trap with adjustable beam alignment precision is designed and built.By precisely controlling the misalignment of dual optical trap,the movement state of microsphere at various misalignment is detected in experiment.The critical conditions of static levitation,rotation and escaping are compared in theory and experiment,this work provides the criterion for the alignment accuracy in dual beam optical trap system.(4)The thermal dynamics of microsphere in a single beam optical trap system was studied.A carbon dioxide laser was used to heat the microspheres in vacuum,and reasons for heated microspheres under different vacuum conditions were studied.In the experiment,it was confirmed that the photophoresis effect produced by the temperature gradient is the main reason for the escape of microspheres under the pressure of 20-30 mbar.In the higher vacuum environment,the reason for its escape of the microsphere is to intensified Brownian motion after heating.