Effect of the thermal–optical nonlinearity on optical trapping Rayleigh particles

Abstract In femtosecond-pulsed optical tweezers, the high peak intensity of each laser pulse results in the instantaneous trapping of a particle, while the high-repetition-rate ensures the repetitive trapping of a particle by pulse trains. Therefore, the high peak intensity and high-repetition-rate inevitably lead to the third-order optical nonlinearity and the thermal–optical nonlinearity, respectively. Herein, we investigate the optical forces on the Rayleigh particle using a high-repetition-rate femtosecond-pulsed Gaussian beam when the Kerr nonlinearity and thermal–optical nonlinearity are considered simultaneously. With the help of thermally induced nonlinearity of the surrounding medium to change the relative refractive index of particles, the three-dimensional stable trapping of slightly low-refractive-index particles is realized by the use of high-repetition-rate femtosecond-pulsed Gaussian beams. Besides, we study the influence of thermal–optical nonlinearity on optical trapping of high-refractive-index particles. It is shown that the trapping stiffness increases nonlinearly with the increase of power due to the thermal–optical​ nonlinearity. This work opens up the possibility of trapping the low-refractive-index beyond the linear optics regime, successfully explains the reported experimental observations, and provides the theoretical support for tapping nonlinear optical particles in femtosecond optical tweezers.