On the spatiotemporal control with a single beam femtosecond optical tweezer

Abstract

Spatiotemporal control refers to the simultaneous control over the position and time variables during the dynamics of an event. The monitoring of fundamental processes at microscopic lengths and ultrashort timescales is typically intertwined with the interaction of light with matter. Optical trapping for spatial control results in the immobilization of microscopic objects with light pressure from a tightly focused laser beam and has wide-ranging applications. Conventional single-beam optical tweezers use continuous-wave lasers for spatial control through photon flux but are lacking in temporal control aspects. Pulsed lasers can control the dynamics of the process being studied and thus provide a temporal control knob by enabling the circumvention of relaxation processes. The tradeoff, however, is that this spatial control knob becomes more dependent on the frequency of the light source. The high photon flux requirement of stable optical tweezers with pulsed lasers necessitates femtosecond pulse shaping at a rapid repetition rate, which has been a barrier until recently. We present a generalized model to describe thermal effects for optical tweezers, capable of treating pulsed lasers systems (which differ in instantaneous interaction effects) as well as CW systems (which can be understood as a limiting case of our model). The model has nonlinear optical (NLO) interactions included prima-facie and can describe pulsed laser tweezers in areas where they excel, like the two-photon-fluorescence-based detection. A key result obtained from our model is the fact that NLO interactions can be used to balance out thermal effects.