Constant power optical tweezers with controllable torque

Single-Crystal Rutile TiO2 Nanocylinders are Highly Effective Transducers of Optical Force and Torque

Optical trapping of (sub)micron-sized particles is broadly employed in nanoscience and engineering. The materials commonly employed for these particles, however, have physical properties that limit the transfer of linear or angular momentum (or both). This reduces the magnitude of forces and torques, and the spatiotemporal resolution, achievable in linear and angular traps. Here, we overcome these limitations through the use of single-crystal rutile TiO₂, which has an exceptionally large optical birefringence, a high index of refraction, good chemical stability, and is amenable to geometric control at the nanoscale. We show that rutile TiO₂ nanocylinders form powerful joint force and torque transducers in aqueous environments by using only moderate laser powers to apply nN·nm torques at kHz rotational frequencies to tightly trapped particles. In doing so, we demonstrate how rutile TiO₂ nanocylinders outperform other materials and offer unprecedented opportunities to expand the control of optical force and torque at the nanoscale.

Single-Molecule Angular Optical Trapping for Studying Transcription Under Torsion

Optical tweezers are flexible and powerful single-molecule tools that have been extensively utilized in biophysical studies. With their ability to stretch and twist DNA, and measure its force and torque simultaneously, they provide excellent opportunities to gain novel insights into the function of protein motors and protein-DNA interactions. Recently, a novel DNA supercoiling assay using an angular optical tweezers (AOT) has been developed to investigate torque generation during transcription. Here, we provide a detailed and practical guide to performing this technique. Using bacterial RNA polymerase (RNAP) as an example, we present protocols for constructing and calibrating an AOT instrument, preparing DNA templates, and acquiring and analyzing real-time data for transcription under DNA supercoiling. While these protocols were initially developed with E. coli RNAP, they can be readily adapted to study other DNA-based motor proteins.

Torque measurement at the single-molecule level

Methods for exerting and measuring forces on single molecules have revolutionized the study of the physics of biology. However, it is often the case that biological processes involve rotation or torque generation, and these parameters have been more difficult to access experimentally. Recent advances in the single-molecule field have led to the development of techniques that add the capability of torque measurement. By combining force, displacement, torque, and rotational data, a more comprehensive description of the mechanics of a biomolecule can be achieved. In this review, we highlight a number of biological processes for which torque plays a key mechanical role. We describe the various techniques that have been developed to directly probe the torque experienced by a single molecule, and detail a variety of measurements made to date using these new technologies. We conclude by discussing a number of open questions and propose systems of study that would be well suited for analysis with torsional measurement techniques.

Controlling total spot power from holographic laser by superimposing a binary phase grating

By superimposing a tunable binary phase grating with a conventional computer-generated hologram, the total power of multiple holographic 3D spots can be easily controlled by changing the phase depth of grating with high accuracy to a random power value for real-time optical manipulation without extra power loss. Simulation and experiment results indicate that a resolution of 0.002 can be achieved at a lower time cost for normalized total spot power.

Soft magnetic tweezers: a proof of principle

We present here the principle of soft magnetic tweezers which improve the traditional magnetic tweezers allowing the simultaneous application and measurement of an arbitrary torque to a deoxyribonucleic acid (DNA) molecule. They take advantage of a nonlinear coupling regime that appears when a fast rotating magnetic field is applied to a superparamagnetic bead immersed in a viscous fluid. In this work, we present the development of the technique and we compare it with other techniques capable of measuring the torque applied to the DNA molecule. In this proof of principle, we use standard electromagnets to achieve our experiments. Despite technical difficulties related to the present implementation of these electromagnets, the agreement of measurements with previous experiments is remarkable. Finally, we propose a simple way to modify the experimental design of electromagnets that should bring the performances of the device to a competitive level.

Passive torque wrench and angular position detection using a single-beam optical trap

The recent advent of angular optical trapping techniques has allowed for rotational control and direct torque measurement on biological substrates. Here we present a method that increases the versatility and flexibility of these techniques. We demonstrate that a single beam with a rapidly rotating linear polarization can be utilized to apply a constant controllable torque to a trapped particle without active feedback, while simultaneously measuring the particle angular position. In addition, this device can rapidly switch between a torque wrench and an angular trap. These features should make possible torsional measurements across a wide range of biological systems.

Stable trapping and manually controlled rotation of an asymmetric or birefringent microparticle using dual-mode split-beam optical tweezers

Inserting a coverslip into half of a Gaussian laser beam at a suitable tilting angle can make the single-mode laser beam become closely spaced dual light spots at the laser focus. In this way, we can reform the conventional single-beam optical tweezers easily and construct a set of dual-mode split-beam optical tweezers, which can be used to manually rotate a trapped and twisted red blood cell around the optical axis. Furthermore, we demonstrate that the split-beam optical tweezers can also stably trap and orient a birefringent polystyrene micro strip particle, which otherwise will self rotate at a varying speed along the structural principal axes, fast spin about the optical axis in a tilting pose, or precess like a gyroscope, in the original linearly polarized single-beam optical tweezers.

Theory of optical trapping by an optical vortex beam

We propose a theory to explain optical trapping by optical vortices (OVs), which are emerging as important tools to trap mesoscopic particles. The common perception is that the trapping is solely due to the gradient force and that it may be characterized by three real force constants. However, we show that the OV trap can exhibit complex force constants, implying that the trapping must be stabilized by ambient damping. At different damping levels, particles exhibit remarkably different dynamics, such as stable trapping and periodic and aperiodic orbital motions.

Highly birefringent vaterite microspheres: production, characterization and applications for optical micromanipulation

This paper reports on a simple synthesis and characterization of highly birefringent vaterite microspheres, which are composed of 20-30 nm sized nanocrystalls. Scanning electron microscopy shows a quite disordered assembly of nanocrystals within the microspheres. However, using optical tweezers, the effective birefringence of the microspheres was measured to be Deltan = 0.06, which compares to Deltan = 0.1 of vaterite single crystals. This suggests a very high orientation of the nanocrystals within the microspheres. A hyperbolic model of the direction of the optical axis throughout the vaterite spherulite best fits the experimental data. Results from polarized light microscopy further confirm the hyperbolic model.

Rotation of birefringent particles in optical tweezers with spherical aberration

Birefringent particles rotate when trapped in elliptically polarized light. When an infinity corrected oil-immersion objective is used for trapping, rotation of birefringent particles in optical tweezers based on an infinity optical microscope is affected by the spherical aberration at the glass-water interface. The maximum rotation rate of birefringent particles occurs close to the coverslip, and the rotation rate decreases dramatically as the trapped depth increases. We experimentally demonstrate that spherical aberration can be compensated by using a finite-distance-corrected objective to trap and rotate the birefringent particles. It is found that the trapped depth corresponding to the maximum rotation rate is 50 microm, and the rotation rates at deep trapped depths are improved.