Motional Resonances in Absorbing Mesoscopic Particles Trapped in Air using Photophoretic Forces

Basak, Prithviraj (2019) Motional Resonances in Absorbing Mesoscopic Particles Trapped in Air using Photophoretic Forces. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Prithviraj Basak (14MS128)) 14MS128.pdf - Submitted Version Restricted to Repository staff only Download (15MB)

Abstract

Trapping of micron sized absorbing particles in air is a newer field of research compared to conventional optical tweezers. In the latter, gradient and scattering forces are at play which arise due to the difference in refractive index between a particle and its surrounding and can be achieved only by using a tightly focused Gaussian beam. This mechanism is extremely difficult to implement for trapping particles in air due to the low viscosity of the medium which leads to large Brownian jumps of particles in air. Thus, trapping in air is facilitated by photophoretic forces which are much higher in magnitude than the corresponding optical dipole forces in air and results from momentum exchange between an absorbing particle and the surrounding gas which have different temperatures. This force depends upon the temperature gradient and surface properties (ability to exchange thermal energy with the surrounding air molecules) of a particle. In our case, we heat a suitable absorbing particle which is falling under gravity, by using a loosely focused Gaussian beam that travels in the direction opposite to the particle. This generates a photophoretic force on the particle, the vertical component of which balances the weight of the particle and the radial component acts as a restoring force on the particle to keep it trapped radially. In this thesis, we have presented a detailed account of an experimental method in determining the stiffness of a photophoretic trap. We have developed our experimental system entirely in-house to study the motion of a photophoretically trapped particle precisely and accurately. Thus, we employ three cameras in three spatially orthogonal directions to measure the trajectory and spatial location of a trapped particle with respect to the center of the trapping laser beam. We have then studied the radial restoring force and in turn the trapping potential acting on the particle by modulating the trapping potential spatially and observing the amplitude and phase response of the particle. To achieve this, we have performed a lock-in detection of the particle using an indigenously-developed lock-in amplifier based on a digital platform using Matlab. The result shows that for small oscillations the system acts as a damped driven harmonic oscillator and shows a motional resonance as a function of frequency. Also the resonance frequency appears to be proportional to the power of trapping laser which does not directly follow from the present theoretical models. The physical origin of this observation requires further investigation. We also observe a second motional resonance in the direction transverse to the direction of excitation, both in terms of amplitude and direction of motion of the trapped particle. Since a particle trapped by photophoretic forces are known to execute rotational motion in the radial direction due to the body forces acting on it, we hypothesize that this motion may be due to a translation of the center of mass in a direction that is a vectorial resultant of the superposition of the angular momentum vector and the vector representing the direction of excitation of the particle. We intend to quantify our understandings on this in future work.

Actions (login required)

View Item