Research On Single Fiber Optical Tweezers With Negative Cone Angle Structure

Optical tweezers is a technology that uses the radiation pressure of light to capture and manipulate micro-particles.It has been widely used in several fields,ranging from cell biology,colloid science,physics,and even optical sensing,because of its no-contact,no-damaging and flexible operation characteristics.The research of optical tweezers is gradually advancing towards the direction of more complex optical trap.The fiber probes with negative cone angles are innovatively applied in the fields of optical sensing and imaging due to the cavity structure.In this paper,we used Finite Element Method to simulate the force of spherical particles in singlefiber optical tweezers with cavity structure.The trapping efficiency of two kinds of fiber tips(negative cone angle and positive cone angle)were measured and compared experimentally.At first,we introduce the background of optical tweezers technology and its development in the aspects of structure,application,fabrication methods and so on.Then the theoretical principles of far-field optical tweezers,near-field optical tweezers and optical-fiber tweezes are explained by comparison.Meanwhile,the corresponding computational models and computing methods of force on three types of particles in optical tweezers are presented.In the section of simulation analysis,we firstly simulated the output filed of optical fiber probe with negative cone angle by Finite Element Method.Then we analyzed the trapping for particles by optical fiber probe with different structure parameters(including the cavity radius and negative cone angle)and finally obtain the optimum cavity radius and negative cone angle of fiber probe which exert the best trapping effect.The influence of the radius of the particles and the refractive index on the force was also discussed.When the radius of the particles was 2.5 μm and the refractive index was 1.4,they corresponded to the best axial capture efficiency.In addition,considering the cavity structure of fiber probe,the effect of particle modification on the output field and trapping efficiency was studied,which can be used to adjust the axial trapping range of the probe.Compared with the positive cone angle fiber probe,the cavity structure of the negative cone angle probe can effectively improve the stability of the trapped particles.In the aspect of experiment,an improved tube etching method to fabricate highquality fiber probes for optical tweezers was proposed and implemented.This method retained the smooth surface of conventional tube etching method,can improve the controllability of the etching result and overcome the dependence on the fiber types.In the part of cavity fiber probe fabrication,we explore the factors,such as the length of bare fiber,the etching time and the dipping length of HF acid.Finally,we set up the manipulation system of single optical fiber probe with negative cone angle to achieve the trap and manipulate of yeast cells.The trapping efficiency was measured and compared with the theoretical results.The differences of the trapping efficiency of three probes were analyzed from the aspects of the output light field distribution and fiber transmission mode.