Research On The Physical Mechanism Of Optical Trapping

In the past 50 years,optical micromanipulation has been an indispensable tool in the fields of physics,chemistry and biology.Especially in the biological field,optical micro-manipulation can be carried out at the particle size level of nanometers to micrometers,which shows that it has the ability to study biological processes at the single-molecule level or cellular and subcellular level,and does not destroy cell.Stable optical trapping of atoms and nanoparticles is one of the most important achievements of optical micromanipulation.Optical tweezers are one of the most basic and commonly used approaches to achieve stable optical trapping.Corresponding to the extensive experimental application of optical trapping,optical tweezer has a relatively imperfect theoretical research.Since Ashkin proposed the analysis of small-sized Rayleigh particles in 1983 and the analysis of large-sized geometric particles in 1992,there has been no suitable theoretical explanation for the intermediate Mie particles.The main reason is that it is difficult to separate the gradient force and scattering force of Mie particles.The gradient force and scattering force play a crucial role in explaining the physical mechanism of optical trapping.This article will discuss the mechanism of optical trapping in the optical tweezers system through numerical and analytical methods.Optical trapping has always been considered as a competition of conservative gradient force and non-conservative scattering force.The understanding comes from the study of the optical trapping of Rayleigh particles.And Mie particles have always been considered an extension of Rayleigh particles,but our research has found that this is different.Based on the separation of gradient force and scattering force,we found that the maximum value of the scattering force is greater than that of gradient force in some directions,but this does not lead to the failure of optical trapping.Because the maximum value of gradient force and scattering force are not in the same spatial distribution,and this creates an “optical force well” of optical force.Particles approaching the boundary of the "optical force well" will be pulled back to the center of the “optical force well” by the gradient force and the strongest scattering force exists outside the “optical force well”.The optical trapping of Mie particles should be attributed to the construction of the “optical force well” rather than the competition between gradient force and scattering force.We also studied the geometric properties of the “optical force wells” and found that the “optical force well” is a threedimensional spherical space located at the center of the beam.At the same time,we quantitatively analysis and found that the radius of the “optical force well” is roughly equal to the radius of the trapped particles.It is a universal characteristic for Mie particles and this is a scalable system.Our studies offer an insight into the optical micromanipulation of Mie particles such as trapping,cooling,and applications on optomechanics.