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Single-beam optical tweezers that use continuous wave (CW) lasers for trapping microscopic particles can be understood in terms of force-balancing light pressure from a tightly focused laser beam. High-repetition-rate femtosecond lasers for single-beam optical trapping research have matured as a technique and have garnered increasing interest. There are important differences between the theoretical models for femtosecond laser tweezers and the CW tweezers, e.g., in the sensitive detection of background-free two-photon fluorescence. The instantaneous trapping potential is due to the high peak power of each laser pulse, while the sustained stable trapping regime is a consequence of the high repetition rate of successive pulses. Simulating real-time scenarios for predicting optical trapping behavior continues to be a challenging problem. However, the capability and usefulness of optical tweezers setups with both CW and pulsed lasers are well established. For a tightly focused beam as used in an optical tweezer, cumulative heating can occur despite the minimal absorption cross-section of the trapping medium or the trapped particle, which reaches its maximum value near the focus. A temperature gradient from the laser focal spot is thus generated outwards from the laser focus in the medium, creating a refractive index gradient across the focusing region. The refractive index attains its minimum value at the focus, gradually increasing as a function of increasing distance from it. Since the trapping force and potential depend on the refractive index of the medium, the thermal effect impacts the force and potential of the trapped particle significantly. With CW lasers, computational evidence of temperature rise at the focus of optical tweezers has been posited, which, unfortunately, is not a feasible approach for ultrafast lasers, given their inherent computational complexities. A better understanding of high photon-flux induced processes and a working model of the single-beam optical tweezers that could address both CW and pulsed lasers would be ideal for elucidating the effects of this inherent thermal gradient of the optical tweezers. We demonstrate a framework that includes all possible nonlinear effects arising from high photon flux interactions and validate this with experimental results. Our approach allows a coherent and consistent treatment for both CW and ultrafast cases. We have the purely thermal nonlinear effects for the CW laser case, while for the ultrafast laser case, we include both the thermal and the Kerr type nonlinearities. Such a source-sensitive model is amenable to high throughput computations when coupled with a suitable paradigm for modeling experimental conditions as well.
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