Realisation of pitch-rotational torque wrench in two-beam optical tweezers

3D Pitch (out-of-plane) rotational motion has been generated in spherical particles by maneuvering the laser spots of holographic optical tweezers. However, since the spherical particles, which are required to minimise drag are perfectly isotropic, a controllable torque cannot be applied with it. It remains free to spin about any axis even after moving the tweezers beams. It is here that we trap birefringent particles of about 3 $\mu$m diameter in two tweezers beams and then change the depth of one of the beam foci controllably to generate a pitch rotational torque-wrench and avoid the free spinning of the particle. We also detect the rotation with newly developed pitch motion detection technique and apply controlled torques on the particle.

Microchemomechanical devices using DNA hybridization

The programmability of DNA oligonucleotides has led to sophisticated DNA nanotechnology and considerable research on DNA nanomachines powered by DNA hybridization. Here, we investigate an extension of this technology to the micrometer-colloidal scale, in which observations and measurements can be made in real time/space using optical microscopy and holographic optical tweezers. We use semirigid DNA origami structures, hinges with mechanical advantage, self-assembled into a nine-hinge, accordion-like chemomechanical device, with one end anchored to a substrate and a colloidal bead attached to the other end. Pulling the bead converts the mechanical energy into chemical energy stored by unzipping the DNA that bridges the hinge. Releasing the bead returns this energy in rapid (>20 μm/s) motion of the bead. Force-extension curves yield energy storage/retrieval in these devices that is very high. We also demonstrate remote activation and sensing—pulling the bead enables binding at a distant site. This work opens the door to easily designed and constructed micromechanical devices that bridge the molecular and colloidal/cellular scales.

Positioning Accuracy in Holographic Optical Traps

Spatial light modulators (SLMs) have been widely used to achieve dynamic control of optical traps. Often, holographic optical tweezers have been presumed to provide nanometer or sub-nanometer positioning accuracy. It is known that some features concerning the digitalized structure of SLMs cause a loss in steering efficiency of the optical trap, but their effect on trap positioning accuracy has been scarcely analyzed. On the one hand, the SLM look-up-table, which we found to depend on laser power, produces positioning deviations when the trap is moved at the micron scale. On the other hand, phase quantization, which makes linear phase gratings become phase staircase profiles, leads to unexpected local errors in the steering angle. We have tracked optically trapped microspheres with sub-nanometer accuracy to study the effects on trap positioning, which can be as high as 2 nm in certain cases. We have also implemented a correction strategy that enabled the reduction of errors down to 0.3 nm.

Generation of Hybrid Optical Trap Array by Holographic Optical Tweezers

Enabled by multiple optical traps, holographic optical tweezers can manipulate multiple particles in parallel flexibly. Spatial light modulators are widely used in holographic optical tweezers, in which Gaussian point (GP) trap arrays or special mode optical trap arrays including optical vortex (OV) arrays, perfect vortex (PV) arrays, and Airy beam arrays, etc., can be generated by addressing various phase holograms. However, the optical traps in these arrays are almost all of the same type. Here, we propose a new method for generating a hybrid optical trap array (HOTA), where optical traps such as GPs, OVs, PVs, and Airy beams in the focal plane are combined arbitrarily. Also, the axial position and peak intensity of each them can be adjusted independently. The energy efficiency of this method is theoretically studied, while different micro-manipulations on multiple particles have been realized with the support of HOTA experimentally. The proposed method expands holographic optical tweezers’ capabilities and provides a new possibility of multi-functional optical micro-manipulation.

Optical tweezers: theory and practice

The possibility for the manipulation of many different samples using only the light from a laser beam opened the way to a variety of experiments. The technique, known as Optical Tweezers, is nowadays employed in a multitude of applications demonstrating its relevance. Since the pioneering work of Arthur Ashkin, where he used a single strongly focused laser beam, ever more complex experimental set-ups are required in order to perform novel and challenging experiments. Here we provide a comprehensive review of the theoretical background and experimental techniques. We start by giving an overview of the theory of optical forces: first, we consider optical forces in approximated regimes when the particles are much larger (ray optics) or much smaller (dipole approximation) than the light wavelength; then, we discuss the full electromagnetic theory of optical forces with a focus on T-matrix methods. Then, we describe the important aspect of Brownian motion in optical traps and its implementation in optical tweezers simulations. Finally, we provide a general description of typical experimental setups of optical tweezers and calibration techniques with particular emphasis on holographic optical tweezers.

Single-Cell Elasticity Measurement with an Optically Actuated Microrobot

A cell elasticity measurement method is introduced that uses polymer microtools actuated by holographic optical tweezers. The microtools were prepared with two-photon polymerization. Their shape enables the approach of the cells in any lateral direction. In the presented case, endothelial cells grown on vertical polymer walls were probed by the tools in a lateral direction. The use of specially shaped microtools prevents the target cells from photodamage that may arise during optical trapping. The position of the tools was recorded simply with video microscopy and analyzed with image processing methods. We critically compare the resulting Young’s modulus values to those in the literature obtained by other methods. The application of optical tweezers extends the force range available for cell indentations measurements down to the fN regime. Our approach demonstrates a feasible alternative to the usual vertical indentation experiments.