Precise Control and Measurement of Temperature with Femtosecond Optical Tweezers

Abstract

Optical traps have often been used for physical manipulation and transport within liquids for studying bio-systems. In this connection, though a lot of work has focused on the property of the trapped particle, there is little effort on utilizing the effect of the trapping environment. Here we demonstrate a novel method for exploiting the effect of trapping environment in observing the temperature rise in liquids directly at the vicinity of an optical trap center. Our approach utilizes the photo-thermal effect at micro-volume dimension to measure temperature, which could eventually be extended to in-vivo conditions. Our two-color experiment is a unique combination of a non-heating femtosecond trapping laser at 780 nm, which is coupled to a femtosecond infrared heating laser at 1560 nm. Femtosecond pulsed laser heating is much more effective than the traditional continuous-wave laser, which increases the sensitivity of our measurements. The heating laser precisely controls temperature at the focal volume of the trap center using low powers at high repetition rate. The changed values of corner frequency of the optical trap due to this local heating is recognized from its power spectra. The solvent having hydroxyl group is very sensitive to 1560 nm laser due to non-radiative relaxation from their higher excited state. Since most bio-systems contain hydroxyl group, they would be highly responsive to our heating laser, while our trapping laser is highly transparent for such systems. Our method could be used to specifically heat a particular cell to evaluate its faster cell division and also can be utilized for the possible phototherapy of cancer cells. The advantage of this technique over the conventional phototherapy is that this is highly localized due to focusing through high numerical aperture resulting in a high gradient of temperature.