Optical Trapping and Manipulation of Microparticles in Air

Kumar, Avinash (2015) Optical Trapping and Manipulation of Microparticles in Air. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Avinash Kumar (10MS42)) Avinash_10MS42_MSThesis.pdf - Submitted Version Restricted to Repository staff only Download (1MB)

Abstract

Brownian motion has played important roles in many areas of science. It is the manifestation of the fluctuation of the surrounding medium. It regulates various systems like physics, chemistry, biology and finance. Einstein’s prediction about Brownian motion is valid for longer time scales only. At short time scales, the motion is different and is no longer completely random due to the inertia of the surrounding medium and the particle itself. We use optical tweezers to study the Brownian motion of particle. Trapping has been conventionally done in liquid where it is easier to trap the particles. But here we report trapping in air which apart from being unconventional, is very difficult as well. We trap an absorbing dielectric particle using Gaussian beam and Hermite-Gaussian beam in air with the help of thermal forces (radiometric forces). Trapping is achieved with a minimum of 50-60 mW of laser power when Gaussian beam is used for trapping. But the power requirement reduces to 14-20 mW when the particle is trapped with an optical fiber generated Hermite Gaussian beam. We have observed the dynamics of the particles in the thermal trap and have successfully been able to reach the ballistic Brownian regime. We have also measured the time series of the position of the trapped particle which brings out some interesting information about the particle like its shape, surface morphology, rotation speed and so on. We also describe the technique that we have developed for generating higher order Gaussian beam, HG 1,0 using single mode optical fiber which as described earlier we have used to trap particles as well.

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