Yoshihiko Arita

Laser-induced rotation and cooling of a trapped microgyroscope in vacuum.

Quantum state preparation of mesoscopic objects is a powerful playground for the elucidation of many physical principles. The field of cavity optomechanics aims to create these states through laser cooling and by minimizing state decoherence. Here we demonstrate simultaneous optical trapping and rotation of a birefringent microparticle in vacuum using a circularly polarized trapping laser beam—a microgyroscope. We show stable rotation rates up to 5 MHz. Coupling between the rotational and translational degrees of freedom of the trapped microgyroscope leads to the observation of positional stabilization in effect cooling the particle to 40 K. We attribute this cooling to the interaction between the gyroscopic directional stabilization and the optical trapping field.

Dynamics of microparticles trapped in a perfect vortex beam

We trap and rotate particles using a perfect vortex beam with integer or fractional topological charges. A linear relationship is observed between the rotation speed and orbital angular momentum content of the beam.

Optical injection of mammalian cells using a microfluidic platform

The use of a focused laser beam to create a sub-micron hole in the plasma membrane of a cell (photoporation), for the selective introduction of membrane impermeable substances (optical injection) including nucleic acids (optical transfection), is a powerful technique most commonly applied to treat single cells. However, particularly for femtosecond photoporation, these studies have been limited to low throughput, small-scale studies, because they require sequential dosing of individual cells. Herein, we describe a microfluidic photoporation system for increased throughput and automated optical injection of cells. Hydrodynamic focusing is employed to direct a flow of single-file cells through a focused femtosecond laser beam for photoporation. Upon traversing the beam, a number of transient pores potentially open across the extracellular membrane, which allows the uptake of the surrounding fluid media into the cytoplasm, also containing the chosen injection agent. The process is entirely automated and a rate of 1 cell/sec could readily be obtained, enabling several thousand cells to be injected per hour using this system. The efficiency of optically injecting propidium iodide into HEK293 mammalian cells was found to be 42 ± 8%, or 28 ± 4% taking into account the requirement of post-injection viability, as tested using Calcein AM. This work now opens the way for combining photoporation with microfluidic analyses, sorting, purification or on-chip cell culture studies.

Picoliter Rheology of Gaseous Media Using a Rotating Optically Trapped Birefringent Microparticle

An optically trapped birefringent microparticle is rotated by a circularly polarized beam in a confined gaseous medium. By recording the terminal rotation velocity and the change in polarization of the incident trapping beam, we determine the viscosity by probing a picoliter volume of air, carbon dioxide, and argon in the vicinity of the microparticle. We also characterize the optical force acting on a trapped particle in air using the generalized Lorenz-Mie theory taking into account the aberrations present. This opens up a new potential application of optical tweezers for the accurate measurement of gas viscosity in confined geometries.

Enhancement of optical forces using slow light in a photonic crystal waveguide

The paradigm of slow light in photonic crystal waveguides has already led to startling advances in nonlinear interactions and optical switching. Importantly, as slow light implies a highly reduced group velocity, this also leads to an original route for the enhancement of optical forces by appropriate tuning of the waveguide properties. Here, we demonstrate the use of slow light to enhance the guiding of submicrometer dielectric particles on a photonic crystal waveguide. Studies are based on a range of particle sizes, and we observe a four-fold enhancement in guiding velocity simply by changing the wavelength of the exciting laser within the slow light region. The particle velocity is therefore seen to be dependent upon the group velocity of light in the waveguide in agreement with force simulations. Finally, the enhancement of the lateral trap stiffness transverse to the waveguide axis further confirms the benefit of slow light for particle manipulation.

Rotation of two trapped microparticles in vacuum: observation of optically mediated parametric resonances

We demonstrate trapping and rotation of two mesoscopic particles in vacuum using a spatial-light-modulator-based approach to trap more than one particle, induce controlled rotation of individual particles, and mediate interparticle separation. By trapping and rotating two vaterite particles, we observe intensity modulation of the scattered light at the sum and difference frequencies with respect to the individual rotation rates. This first demonstration of optical interference between two microparticles in vacuum leads to a platform to potentially explore optical binding and quantum friction effects.

Rotational Dynamics and Heating of Trapped Nanovaterite Particles

We synthesize, optically trap, and rotate individual nanovaterite crystals with a mean particle radius of 423 nm. Rotation rates of up to 4.9 kHz in heavy water are recorded. Laser-induced heating due to residual absorption of the nanovaterite particle results in the superlinear behavior of the rotation rate as a function of trap power. A finite element method based on the Navier-Stokes model for the system allows us to determine the residual optical absorption coefficient for a trapped nanovaterite particle. This is further confirmed by the theoretical model. Our data show that the translational Stokes drag force and rotational Stokes drag torque need to be modified with appropriate correction factors to account for the power dissipated by the nanoparticle.

Optical trapping with a perfect vortex beam

Vortex beams with different topological charge usually have different profiles and radii of peak intensity. This introduces a degree of complexity the fair study of the nature of optical OAM (orbital angular momentum). To avoid this, we introduced a new approach by creating a perfect vortex beam using an annular illuminating beam with a fixed intensity profile on an SLM that imposes a chosen topological charge. The radial intensity profile of such an experimentally created perfect vortex beam is independent to any given integer value of its topological charge. The well-defined OAM density in such a perfect vortex beam is probed by trapping microscope particles. The rotation rate of a trapped necklace of particles is measured for both integer and non-integer topological charge. Experimental results agree with the theoretical prediction. With the flexibility of our approach, local OAM density can be corrected in situ to overcome the problem of trapping the particle in the intensity hotspots. The correction of local OAM density in the perfect vortex beam therefore enables a single trapped particle to move along the vortex ring at a constant angular velocity that is independent of the azimuthal position. Due to its particular nature, the perfect vortex beam may be applied to other studies in optical trapping of particles, atoms or quantum gases.

Imaging the cellular response to transient shear stress using stroboscopic digital holography.

We use stroboscopic quantitative phase microscopy to study cell deformation and the response to cavitation bubbles and transient shear stress resulting from laser-induced breakdown of an optically trapped nanoparticle. A bi-directional transient displacement of cytoplasm is observed during expansion and collapse of the cavitation bubble. In some cases, cell deformation is only observable at the microsecond time scale without any permanent change in cell shape or optical thickness. On a time scale of seconds, the cellular response to shear stress and cytoplasm deformation typically leads to retraction of the cellular edge most exposed to the flow, rounding of the cell body and, in some cases, loss of cellular dry mass. These results give a new insight into the cellular response to cavitation induced shear stress and related plasma membrane permeabilization. This study also demonstrates that laser-induced breakdown of a nanoparticle offers localized cavitation, which interacts with a single cell but without causing cell lysis.

Dynamics of a levitated microparticle in vacuum trapped by a perfect vortex beam: Three-dimensional motion around a complex optical potential

We trap a single silica microparticle in a complex three-dimensional optical potential with orbital angular momentum in vacuum. The potential is formed by the generation of a “perfect vortex” in vacuum which, upon propagation, evolves to a Bessel light field. The optical gradient and scattering forces interplay with the inertial and gravitational forces acting on the trapped particle to produce a rich variety of orbital motions with respect to the propagation axis. As a result, the particle undergoes a complex trajectory, part of which is rotational motion in the plane of the “perfect vortex.” As the particle explores the whole three-dimensional volume and is not solely restricted to one anchor point, we are able to determine the three-dimensional optical potential in situ by tracking the particle. This represents the first demonstration of trapping a microparticle within a complex three-dimensional optical potential in vacuum. This may open up new perspectives in levitated optomechanics with particle dynamics on complex trajectories.