Analysis Of Trapping Forces And Motion Of Particles In The Focused Field Of Gaussian Beams

Since Ashkin experimentally observed the trapping on micro-particles of focused beams,optical tweezers technology has been greatly developed and has been widely used in research of physics,chemistry,life science and so on,optical tweezers can be used to manipulate the biological particles with complex structure especially in biological area.Up to now,a variety of methods have been developed to calculate the trapping force,the T matrix method,as an efficient numerical method,has received much attention,it can be used in the calculation of the radiation force on multilayered particles with complex structure.The principle of T matrix is introduced,the trapping efficiency of three different beams,including plane wave,Gaussian beam and 1st order vector Bessel-Gaussian(BG)beam,on a double-layered spherical(DLS)particle are calculated.We also analyze the particles' motion in the focused filed and do some relevant experiments.At first,we introduce the background of optical tweezers technology and its development in the aspects of structure,application,theory and so on,we mainly introduce some methods which are used to computer the trapping force.Then,the null-field method that is used to calculate the T matrix of single-layered(SL)and double-layered(DL)particle is introduced in detail,we also introduce the vector diffraction integral which is used for calculating the focusing field and the Maxwell stress tensor integral which can computer the trapping force.In the aspect of theory,this paper obtains the detailed expression for the calculation of T matrix of DLS particles,it can be extended to multilayer spherical particles;When plane wave,Gauss beam or 1st-order vector Bessel Gauss beams is used as the light source,the formulas for calculating the expansion coefficients of the incident field is also given;The trapping efficiency is calculated when a DLS particle locates in different place in the focused field of three different beams.The results show that there are two equilibrium points(one stable and one unstable)on the beam axis if the inner layer's refractive index is lower than the out layer's,when plane wave or Gauss beam is used;The stability of the trap for hollow particles(the inner layer's refractive index is one)formed by plane wave,Gauss beam and vector beamsdecreases sequentially;The capture effect than the plane wave and Gauss light to be poor,this result is beyond all expectations;Besides,both the inner layer's refractive index and size will affect the trapping efficiency.With the increase of the refractive index,the trapping efficiency increases first and then decreases to zero,and with the inner layer's size of the hollow particles increases,the capture efficiency decreases to zero.In the aspect of experiment,this paper measures the corrosion rate of fiber core and cladding in hydrofluoric acid(HF);Probe with double-cone is made,the angle of the first-cone is influenced by the type of sealing fluid,and the etching time and the concentration of HF will affect the second-cone's angle;We also set up an optical tweezers trapping system,and do some experiments about trapping and manipulating yeast,the trapping force is measured roughly,and the results is compared with the theoretical outcomes,we find that the force is obviously small than the theoretical results.