ANALYSIS OF TRANSVERSE AND LONGITUDINAL OPTICAL TRAP STIFFNESS AND ITS APPLICATION IN A NOVEL MEASUREMENT METHOD OF MICRO-PARTICLE REFRACTIVE INDEX

Based on the model of the single TEM00 Gaussian beam optical tweezers, the transverse and longitudinal optical trapping forces of micro-particles are calculated by ray-optics theory. The similarities and differences of the longitudinal optical trap shapes, which are computed by two different methods, are analyzed. We studied the relation between the optical trap stiffness and the particle parameters which include the size and refractive index (RI), and found that the ratio of the transverse and longitudinal stiffness is a constant when the refractive indices of the particles are the same. Through the analysis, the results show that there is an important relationship between the optical stiffness and the product of particle RI and radius. With this relationship, we can obtain the particle RI by measuring the optical stiffness and the particle radius. This novel micro-particle RI measurement method is of great usefulness in atmospheric environmental science, polymer chemistry science, identification of mineral science and bio-medical science.