We present cumulative perturbation effects of femtosecond laser pulses on an optical tweezer. Our experiments involve a dual wavelength high repetition rate femtosecond laser, one at the non-heating wavelength of 780 nm while the other at 1560 nm to cause heating in the trapped volume under low power (100-800 μW) conditions. The 1560 nm high repetition rate laser acts as a resonant excitation source for the vibrational combination band of the hydroxyl group (OH) of water, which helps create the local heating effortlessly within the trapping volume. With such an experimental system, we are the first to observe direct effect of temperature on the corner frequency deduced from power spectrum. We can, thus, control and measure temperature precisely at the optical trap. This observation has lead us to calculate viscosity as well as temperature in the vicinity of the trapping zone. These experimental results also support the well-known fact that the nature of Brownian motion is the response of the optically trapped bead from the temperature change of surroundings. Temperature rise near the trapping zone can significantly change the viscosity of the medium. However, we notice that though the temperature and viscosity are changing as per our corner frequency calculations, the trap stiffness remains the same throughout our experiments within the temperature range of about 20 K.
Dipankar Mondal and Debabrata Goswami, "Calibration of femtosecond optical tweezers as a sensitive thermometer," Proc. SPIE 9548, Optical Trapping and Optical Micromanipulation XII, 95481N (Presented at SPIE Nanoscience + Engineering: August 12, 2015; Published: 25 August 2015); https://doi.org/10.1117/12.2187097.
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