Direct measurement of optical forces and torque with optical tweezers
Manipulating particles with optical tweezers in combination with the ability to measure force and torque provides a powerful way to investigate objects on the microscale.
Recently we have developed an optical-tweezers-based generally applicable holographic measurement method that overcomes many limitations and issues of existing techniques. Specifically, in the presence of several trapping spots it enables one to simultaneously measure all components of the optical force and the optical (spin and orbital) torque applied at an individual trapping site. Our approach uses the scattered light in the far-field to deduce the force and torque from the change of linear and angular momentum of light induced by the trapped particle. The holographic measurement relies on a detailed analysis of a single interference pattern formed in the back focal plane of a high-NA condenser lens to recover amplitude and phase of the far-field by means of an iterative algorithm, using the known positions of the optical traps as constraints.
With our all optical approach we do not require information about the shape, size or optical properties of the particles. We demonstrate this by measuring the forces on a trapped red blood cell shown in figure 1 as well as with a torque measurement on a barbell shaped object shown in figure 2.
Figure1: (a) Schematic drawing of our four trap configuration to hold and stretch a RBC. (b)-(d) Image of cell for increasing trap distances along the stretching direction x. The arrows correspond to the individual retrieved forces.
Figure 2: Total optical torque applied to a pair of silica microspheres (diameter ∼ 3 μm) by two traps moved along a circular path shown in the inset. The torque from the simultaneous measurement using our method (solid lines) is compared to the sequential ’force times lever’ single trap measurement (dashed lines).
Funding
“Direct measurement of optical torque” FWF Project P29936-N36, 2017 – 2022.
Publications
[1] Strasser, F., Moser, S., Ritsch-Marte, M. & Thalhammer, G. Direct measurement of individual optical forces in ensembles of trapped particles. Optica 8, 79. https://opg.optica.org/optica/abstract.cfm?URI=optica-8-1-79 (2021).
[2] Strasser, F., Barnett, S. M., Ritsch-Marte, M. & Thalhammer, G. Generally Applicable Holographic Torque Measurement for Optically Trapped Particles. Physical Review Letters 128, 213604. https://link.aps.org/doi/10.1103/PhysRevLett.128.213604 (2022).
Article in austrian newspaper "Die Presse": https://www.diepresse.com/5960977/wie-von-zauberhand-bewegt-lichtstrahlen-dehnen-zellen?from=rss