Controlling local temperature in water using femtosecond optical tweezer.

Dipankar Mondal,Debabrata Goswami

doi:10.1364/boe.6.003190

Dipankar Mondal, Debabrata Goswami

Open Access

https://doi.org/10.1364/boe.6.003190

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Abstract

A novel method of directly observing the effect of temperature rise in water at the vicinity of optical trap center is presented. Our approach relies on changed values of corner frequency of the optical trap that, in turn, is realized from its power spectra. Our two color experiment is a unique combination of a non-heating femtosecond trapping laser at 780 nm, coupled to a femtosecond infrared heating laser at 1560 nm, which precisely controls temperature at focal volume of the trap center using low powers (100-800 µW) at high repetition rate. The geometric ray optics model quantitatively supports our experimental data.

Highlights

Laser trapping of cells, viruses, bacteria, etc. using forces of coherent radiation pressure at benign infrared (IR) wavelength of 1064 nm [1,2,3] have increased the range of precise biomanipulation appliances
While the same advantages remain for benign wavelength operation at 780 nm as trapping laser, this is not true for IR laser trapping at 1560 nm, which has an increased water absorption [4, 5]
The strong vibrational combination band of hydroxyl group (OH) at 1560 nm [6] along with the non-radiative relaxation of water leads to a substantial temperature rise

Summary

Introduction

Viruses, bacteria, etc. using forces of coherent radiation pressure at benign infrared (IR) wavelength of 1064 nm [1,2,3] have increased the range of precise biomanipulation appliances. Femtosecond optical tweezers [12,13,14] have attracted more attention over conventional continuous wave (CW) laser tweezers [15, 16] Due to their ultra-high peak powers, femtosecond tweezers use very little average power to generate the requisite gradient force to overcome the scattering force to trap and manipulate micron to nano-sized particles. The low average power of femtosecond high repetition rate 1560 nm laser in presence of 780 nm femtosecond laser trap is able to increase temperature and decrease viscosity significantly [17]. Repetition rate IR laser with microwatt power is sufficient to change the trapping volume temperature significantly due to its high peak power. The environmental change can be sensed by the trapped bead, which is reflected in its Brownian motion [21, 22]

Experimental section

Theoretical model

Experimental observation

Conclusion