Speaker: Prof Monika Ritsch-Marte
Director of the Institute of Biomedical Physics, Department of Physiology and Medical Physics, Medical University of Innsbruck, Austria

Title: Measurement of optical forces and torques acting on trapped particles

Abstract:
Optical tweezers have become so widespread in micro-manipulation, because their application goes beyond simple confinement of a particle in an optical trap: They provide quantitative information on the optical forces in action.
It will be shown how the local forces acting on trapped particles can be inferred from a single interference pattern created by the incident trapping beam and the scattered light. Our method is based on accounting for changes of the optical momentum by refraction or scattering of light by the particles, and therefore can be applied to objects of arbitrary shape, in contrast to many other force-measurement approaches, which only work for spherical particles. Moreover, it allows one to simultaneously measure all components of the force applied to an individual particle in a trapped ensemble [1].
For force measurements we exploit the conservation of optical momentum, but the approach can be modified to measure also optical torque, based on conservation of optical angular momentum [2]. Again, local torques acting on individual particles are accessible, as well as separate spin or orbital parts of the total torque.
Micron-size objects in liquid solution will be discussed as demonstration examples, yet the technique is not limited to typical biomedical contexts, but may also be used in air or vacuum.

[1] F. Strasser, S. Moser, M. Ritsch-Marte, and G.Thalhammer, Direct measurement of individual optical forces in ensembles of trapped particles, Optica 8, 79-87 (2021).
[2] F. Strasser, S. Barnett, M. Ritsch-Marte, and G. Thalhammer, Generally applicable holographic torque measurement for optically trapped particles, Physical Review Letters 128, 213604 (2022).

Zoom:
Meeting URL: https://oist.zoom.us/j/96510844590?pwd=bDN3ai9lM24zd2M4WXVmT04yQ0dtdz09&from=addon

Meeting ID: 965 1084 4590
Passcode: 475082