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Optical tweezers have become an important tool for various applications over the past couple of decades. However, as their range of applications increases, the calibration and sensitivity of detection schemes need to keep pace. Additionally, practical optical tweezer setups become very complex if there is more than one laser-beam trap. When an optical tweezer setup uses a high-repetition-rate femtosecond laser beam to immobilize the trapping object, the technique is known as a Femtosecond optical tweezer (FOT). The instantaneous trapping potential is due to the high peak power of each laser pulse. In contrast, the sustained stable trapping regime results from the high repetition rate of successive pulses. FOTs provide critical advantages in sensitivity through in situ two-photon detection capabilities due to the sensitive detection of background-free two-photon fluorescence. 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. FOTs offer an interesting balance of thermal aspects with inherent nonlinearities. In addition to providing sensitive measurements with super-resolution capabilities, FOTs also allow for sensitive monitoring of the colloidal aggregation processes, which is presented in some detail here.
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