Direct measurement of optical torque

Optical tweezers employ a laser beam to trap small particles, such as single cells or micro- organisms, without actually touching them. The light exerts forces on the particles, which are used to move or rotate the trapped particles. For fundamental research as well as for certain applications it is important to precisely and completely measure the forces. The goal of the research project ``Direct measurement of optical torque`` is to develop and characterize novel methods to measure not only optical forces, but also optical torques, which lead to a rotation of the trapped particle. For this we use the fact that the laser beam is deflected by the particle, and by analyzing the measured change of the direction of the trapping beam the exerted force and torque can be directly measured. However, measuring optical torque is significantly more demanding, as a much more detailed knowledge of the light field is required. We use digital holography to obtain a complete information on the light field. This also enables us to determine the individual forces and torques acting on several, simultaneously trapped particles. This method has the great advantage that for force and torque measurements no information about size, shape or material of the particle is required, as it is the case for conventional methods. This enables new applications, in particula in medicine and biology, where typically particles of irregular, differing shape are studied.

The goal of the project "Direct measurement of optical torque" was to develop novel methods to measure forces and torques that occur with optical trapping of small particles. Optical forces, which arise when a focussed laser beam is deflected by a particle, allow one to hold and manipulate small particles such as single cells or micro-organisms. The so called optical tweezers have found widespread applications, especially in the life sciences, due to its ability to precisely handle particles in a contact-less manner, and 2018 the Nobel Prize in Physics was awarded to Arthur Ashkin for its invention. The key idea of this project was to measure force and torque by observing how the direction of the trapping light is changed by interaction with a trapped particle. This directly yields the applied optical force and torque - no further information about the particle properties such as size, shape, and material is needed. Torque measurements based on this idea are more demanding, as a very detailed knowledge about the light wavefront is required. This challenge was overcome by using an approach taken from holography, whereby complete information about the light field is obtained. This allowed us to realize for the first time in a simple and robust manner a generally applicable method to measure optical torque that works with particles of arbitrary shape and material. The complete information about the trapping light that our method delivers also allowed us to deduce all components of the individual forces and torques when several traps are simultaneously engaged to hold one or more particles. Major efforts went into guaranteeing precise measurements, also for dynamically changing situations. The peculiar features of this force and torque measurement methods make them well suited for applications in the life sciences, where the complex and variable shape and properties of biological matter poses a large hurdle for previously available methods. As a demonstration of possible worthwhile applications of this method we measured the mechanical response of red blood cells when stretching them with four optical traps.