Optical trapping of absorbing mesoscopic particles in air using a single multimode fibre

Prabhakar, Kawale Mayuri (2018) Optical trapping of absorbing mesoscopic particles in air using a single multimode fibre. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Kawale Mayuri Prabhakar (13MS089)) 13MS089.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

The high portability of photophoretic traps compared to conventional optical traps involving microscopes holds great promise in trapping and manipulation of particles. Trapping particle using photophoresis holds capacity to be widely used in segregating organic and inorganic particles, in the study of aerosols, in various branches of chemistry, biology and physics. Using optical tweezers for trapping of particle requires a viscous medium as particle undergo Brownian motion, thus making trapping in air even more difficult using conventional methods. In this case, photophoretic force comes to the rescue since its magnitude is many orders higher. It works on the simple phenomenon of momentum exchange between the absorbing particles being trapped and the surrounding gas molecules. In the current set-up we use light coupled through an optical fibre to trap printer toner particles and a focused using a simple convex lens. It has been shown earlier in the case of photophoresis-based optical traps that the trap stiffness increases with increasing intensity and the particles are trapped offaxis. This was the motivation in our earlier work to show that the radial trapping force in case of a Hermite Gaussian beam is much larger compared to a Gaussian beam. The same results motivated us to check the trapping efficiency using the multimode fibre in which the higher modes would have more off-axis intensity distribution which could be controlled by changing the mode itself. We have indeed demonstrated that the magnitude of radial trapping force to be 10-15 orders of magnitude higher than the Gaussian beam. We also found the mode dependency of the radial trapping force. This arises due to the speckle pattern which arises due to the various modes interfering. In the present work, we have done initial speckle analysis for three different modes and anticipate that the efficiency does not depend on the beam spot size alone but also on the mean speckle size. We also anticipate that the efficiency is even higher for the mode for which the mean speckle size is close to the particle dimension.

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