Understanding femtosecond optical tweezers: the critical role of nonlinear interactions

Abstract

Abstract Typical single-beam optical tweezers use continuous wave (CW) lasers, which can be explained through force balancing the light pressure from a tightly focused laser beam used for trapping microscopic particles. Recent years have also seen a surge in single-beam optical trapping research with high-repetition-rate femtosecond lasers that has shown certain differences from the CW tweezers, one of which is its sensitive detection capability of the ultrashort pulse induced background free two-photon fluorescence signals. The high peak power of each laser pulse is enough to provide instantaneous trapping potential, while the high repetition rate ensures sustained stable trapping from the successive pulses. Though the capability and usefulness of the optical-tweezers are well established, for both CW and pulsed lasers, simulating real-time scenarios to predict optical trapping behaviour remains a challenging problem. This is especially true for femtosecond laser tweezers since high peak powers are involved when the laser is tightly focused for achieving the tweezing action. The nonlinear optical effect and thermal nonlinearity become much more significant for femtosecond optical trapping. We demonstrate the importance of including these nonlinear interactions for femtosecond pulsed laser mediated optical trapping via their effect in scattering and gradient forces in the Rayleigh regime. Our optical-tweezers model includes thermal and optical nonlinear interactions, making it easier to predict the optical-trap stability in real optical trapping scenarios for both CW and pulsed lasers. Our model provides predictive metrics for choosing solvents, probes, and several optical parameters, which can be validated from our experiments.