Investigating The Effciency of Photophoretic Trapping With Different Intensity Structures of Light

Punse, Aman Anil (2021) Investigating The Effciency of Photophoretic Trapping With Different Intensity Structures of Light. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS Dissertation of Aman Anil Punse (16MS040)) 16MS040_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (4MB)

Abstract

Photophoretic force is an order of magnitude larger than the gradient and the scat- tering forces used in conventional optical tweezers, thus providing a way for trapping in air. In a conventional trap the Brownian motion is restricted due to the viscosity of the medium, whereas in air - due to low viscosity - the gradient and scattering forces are not enough for trapping. So, the photophoretic force is used to trap particles in air, with the help of simple experimental setups with no need of bulky microscopes and the need for tight focusing. In this thesis we trap a absorbing particle by illuminating light having different intensity structures. These intensity structures are generated by the use of the light output from different optical �bers and focused with a 25mm lens. The use of an optical �ber for photophoretic trapping provides clear simplicity in setup and align- ment. We further use different optical �bers for trapping and increase the trapping e�ciency by nearly 5 times with the use of a multimode �ber when compared to a single mode �ber. The other �ber used were a dual mode �ber and a four modes �ber which also increase the effciency compared to a single mode �ber. We try to explain the results by analysing the intensity pro�le of the multimode �ber and the trajectories the particle follows while being trapped in both the �ber. The analysis leads us to hypothesize that there exists a possible continuous interplay between the photophoretic force and gravity in the axial direction, which gives rise to an effective restoring force. It is well known that radial trapping is also due to a restoring force, and our measurements provide further proof towards that. The restoring forces in both axial and radial directions are signi�cantly higher for trapping using multimode �bers compared to modes having lesser intensity minimas as clearly demonstrated by our experimental results. ix

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