Study Of Optoelectronic Tweezers On Lithium Niobate Crystals

Optoelectronic tweezers?OET?could flexibly manipulate a large number of particles by dielectrophoresis forces,such as cells,virus,macromolecules.A space charge field,without using an additional voltage supply,can be induced on the surface of lithium niobate?LiNbO₃,LN?which has large values of electro-optic,nonlinear optical properties,piezoelectric,photorefractive as well as acousto-optical coefficients.Trapping particles by OET is a core technology for the Lab-on-A-Chip on the surface of LiNbO₃ substrates.In this paper,we set up a platform for a real-time trapping particles by OET to study the trapping dynamic and the influence of suspension medium of the particles,anisotropy of the substrate,time and power of the laser beam.Firstly,we study the influence of suspension medium of particles.We find that the behaviors of trapping particles,equably dispersed in non-electrolyte solution and electrolyte solution,have the features of optoelectronic tweezers and optical tweezers,respectively,on the surface of LiNbO₃ samples.Compared these behaviors with the case on quartz,we point out that the positive and negative ions ionizing from electrolyte solution shield the space charge field.As a result,the trapping behaviors are unachievable with the electrolyte medium.Secondly,we study the influence of the LiNbO₃ sunstrates with different crystal orientations.There are some differences between the+c-face,-c-face and y-cut LiNbO₃:Fe for the trapping dynamic and particle distribution.A plenty of particles are radially and promptly attracted from the distance in the case of+c-face,showing a nearly-centrosymmetry at and around the beam spot,local distribution.Particles on the-c-face are firstly trapped to the supernatant medium around the beam spot,and then attracted to the medium-sample interface.As most of the trapped particles are ejected from the beam spot at a high rate of speed,instead of adhering to the surface of crystal.The particles on the y-cut substrate will not cover the beam spot,while,they are attached on its two sides demonstrating an obvious directional,nonlocal distribution along the c-axis,and finally they are attracted to the+c-side.The force condition of particles is analyzed from the point of convection of the medium,velocity of the particle and light-induced scattering of the crystal.We suggest the particles are drived by several kinds of electric field forces.Lastly,we study the trapping effiency of OET.We measure the curves of change of the trapped particles number with time and the power of laser beam.We suggest that time and the power of laser beam affect the trapping effiency through the accumulation of photo-excited charges.Besides,trapping range is measured with different samples.This result means that under the similar experimental conditions?i.e.laser power and sample thickness?there is a critical distance,below which y-cut LiNbO3 shows better trapping ability,while,above which c-cut LiNbO3 starts to demonstrate superiority on trapping.Finite element method is utlized for simulating the dielectrophoresis forces ratio of c-cut to y-cut and vertifying the existance of the critical distance.