Calculation and measurement of trapping stiffness in femtosecond optical tweezers

Due to the characteristics of ultra-short pulse width and ultra-high peak power, femtosecond pulse laser can effectively induce nonlinear optical effects in trapped objects. As a result, it holds great value in the fields of micro and nano manipulation, microfluidics, and cell biology. However, the nonlinear optical effects on the stiffness of femtosecond optical traps remain unclear. Calibration of trap stiffness is crucial for accurately measuring forces and manipulating small particles. In this paper, we compare the stiffness between femtosecond optical traps and continuous wave optical traps. Experimental results demonstrate that the stiffness of the femtosecond optical trap in the splitting direction is greater than that in other directions and the stiffness of the continuous wave optical trap under the same laser power condition. Additionally, as the laser power increases, the stiffnesses of both the femtosecond optical trap and the continuous wave optical trap gradually increases. In contrast to a linear increase of the continuous wave optical trap, the stiffness of the femtosecond optical trap exhibits an exponential rise with increasing laser power. This research provides guidance and reference for improving the force measurement accuracy of femtosecond optical tweezer system.