Roadmap for Optical Tweezers 2023

Optical tweezers are tools made of light that enable contactless pushing, trapping, and manipulation of objects ranging from atoms to space light sails. Since the pioneering work by Arthur Ashkin in the 1970s, optical tweezers have evolved into sophisticated instruments and have been employed in a broad range of applications in life sciences, physics, and engineering. These include accurate force and torque measurement at the femtonewton level, microrheology of complex fluids, single micro- and nanoparticle spectroscopy, single-cell analysis, and statistical-physics experiments. This roadmap provides insights into current investigations involving optical forces and optical tweezers from their theoretical foundations to designs and setups. It also offers perspectives for applications to a wide range of research fields, from biophysics to space exploration

An interpretation and guide to single-pass beam shaping methods using SLMs and DMDs

Exquisite manipulations of light can be performed with devices such as spatial light modulators (SLMs) and digital micromirror devices (DMDs). These devices can be used to simulate transverse paraxial beam wavefunction eigenstates such as the Hermite-Laguerre-Gaussian mode families. We investigate several beam shaping methods in terms of the wavefunctions of scattered light. Our analysis of the efficiency, behaviour and limitations of beam shaping methods is applied to both theory and experiment. The deviation from the ideal output from a valid beam shaping method is shown to be due to experimental factors which are not necessarily being accounted for. Incident beam mode shape, aberration, and the amplitude/phase transfer functions of the DMD and SLM impact the distribution of scattered light and hence the effectiveness and efficiency of a beam shaping method. Correcting for these particular details of the optical system accounts for all differences in efficiency and mode fidelity between experiment and theory. We explicitly show the impact of experimental parameter variations so that these problems may be diagnosed and corrected in an experimental beam shaping apparatus. We show that several beam shaping methods can be used for the production of beam modes in a single pass and the choice is based on the particular experimental conditions

High-speed transverse and axial optical force measurements using amplitude filter masks

Direct optical force measurement is a versatile method used in optical tweezers experiments, providing accurate measurements of forces for a wide range of particles and trapping beams. It is based on the detection of the change of the momentum of light scattered by a trapped object. A digital micromirror device can be used to selectively reflect light in different directions using an appropriately defined mask. We have developed position-sensitive masked detection (PSMD) for measuring transverse (radial) and axial forces. The method is comparable in performance to the fastest split detectors, while maintaining the linearity and customizability similar to duo-lateral position-sensitive detectors (PSD) and cameras. We show an order of magnitude increase in the bandwidth compared to a conventional PSD for radial forces. We measure axial force and verify the measurement using the Stokes drag for the particle. Combining both detectors (PSMD and PSD), we can perform full 3-D optical force measurements in real time

Microscope images of strongly scattering objects via vectorial transfer matrices: modeling and an experimental verification

We present an accurate and efficient method for calculating the image of a mesoscopic particle that is captured using a high-numerical-aperture objective lens. We test various scattering models of silica, plastic, and birefringent vaterite spheres in water. We show that the calculated images accurately replicate experimental observations. This method uses the idea that the optical system can be represented as a product of matrices acting on the electromagnetic field in a truncated Hilbert space representation. A general reusable matrix encapsulating the polarization, limited capture angle, or beam shaping in the microscope can be applied to find the image and not be limited to a particular particle shape or medium. We show that the image obtained from this method can be used to determine and match particle properties. We also use incoherent averaging on multiple T-matrices to produce polychromatic images. Data obtained using this method could be used as an input to track the general behavior of particles in suspension or within an optical trap

Swimming force and behavior of optically trapped micro-organisms

We demonstrate how optical tweezers combined with a three-dimensional force detection system and high-speed camera are used to study the swimming force and behavior of trapped micro-organisms. By utilizing position sensitive detection, we measure the motility force of trapped particles, regardless of orientation. This has the advantage of not requiring complex beam shaping or microfluidic controls for aligning trapped particles in a particular orientation, leading to unambiguous measurements of the propulsive force at any time. Correlating the direct force measurements with position data from a high-speed camera enables us to determine changes in the particle’s behavior. We demonstrate our technique by measuring the swimming force and observing distinctions between swimming and tumbling modes of the Escherichia coli (E. coli) strain MC4100. Our method shows promise for application in future studies of trappable but otherwise arbitrary-shaped biological swimmers and other active matter

Calibration of force detection for arbitrarily shaped particles in optical tweezers

Force measurement with an optical trap requires calibration of it. With a suitable detector, such as a position-sensitive detector (PSD), it is possible to calibrate the detector so that the force can be measured for arbitrary particles and arbitrary beams without further calibration; such a calibration can be called an "absolute calibration". Here, we present a simple method for the absolute calibration of a PSD. Very often, paired position and force measurements are required, and even if synchronous measurements are possible with the position and force detectors used, knowledge of the force-position curve for the particle in the trap can be highly beneficial. Therefore, we experimentally demonstrate methods for determining the force-position curve with and without synchronous force and position measurements, beyond the Hookean (linear) region of the trap. Unlike the absolute calibration of the force and position detectors, the force-position curve depends on the particle and the trapping beam, and needs to be determined in each individual case. We demonstrate the robustness of our absolute calibration by measuring optical forces on microspheres as commonly trapped in optical tweezers, and other particles such a birefringent vaterite microspheres, red blood cells, and a deformable "blob"

Optically driven rotating micromachines

We review the basic theory and principles of optically driven micromachines, and present a series of simple heuristic principles for designing such micromachines. We discuss the relationship between symmetry and optical torque, and consider techniques to enhance or reduce reflection. Finally, we briefly survey some applications, and present a prototypical optically driven micromachine for use in microfluidic devices