Optical trapping and manipulation of absorbing mesoscopic particles in air using fiber-optic tweezers

Saha, Tushar Kanti (2017) Optical trapping and manipulation of absorbing mesoscopic particles in air using fiber-optic tweezers. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Tushar Kanti Saha (12MS096)) 12MS096.pdf - Submitted Version Restricted to Repository staff only Download (965kB)

Abstract

Photophoretic forces, by virtue of their large magnitude and simplicity of implementation, are rapidly evolving to be a crucial facilitator in the field of optical trapping and micro-manipulation. Photophoretic tweezers are gradually being used widely, and hold great promise to be deployed in various fields like polymer science, biology and in general optics. In this thesis we develop a method to further simplify optical tweezers instrumentation by trapping particles using a single fiber. This significantly increases the simplicity and portability of optical tweezers. Optical trapping requires the creation of a three dimensional potential well to confine particles which is particularly challenging in air as the trapped particles undergo Brownian motion. So, conventional traps use viscous medium to reduce this Brownian motion. But photophoretic traps generate forces which are 4-5 orders of magnitude higher than conventional traps which makes them very convenient to trap particles in air. We thus use photophoretic forces to trap printer toner particles by illuminating them with light at a wavelength where the particles absorb. The light is coupled into the trap by means of an optical fiber and focused by a simple 25 mm lens, in contrast to the specialized high numerical aperture microscope objective lenses typically required for conventional optical trapping. We try to further increase the efficiency of the optical trap by using a dual mode fiber - that gives an output which is a superposition of Gaussian and Hermite-Gaussian modes instead of a pure Gaussian mode. This increases the trap efficiency by 80% compared to a Gaussian beam. Using these traps we were able to manipulate particles to distances of 12 mm at a velocity of 5 mm/s in the vertical direction. We could also manipulate particles in the radial direction at velocity of 0.73 mm/s using a Hermite-Gaussian trap and 0.43 mm/s using a Gaussian trap. We then replace the single mode fiber with a multimode fiber and study the trapping efficiency of the resulting optical trap. We observe that the multimode trap has even higher efficiency (~10 times) compared to the Hermite-Gaussian beam. The use of a single fiber for trapping particles increases the portability and simplicity of optical tweezers and promises a wider application space with new and novel ways of confining and studying single mesoscopic particles.

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