Philip H. Jones

Optical trapping and manipulation of nanostructures

Optical trapping and manipulation of micrometre-sized particles was first reported in 1970. Since then, it has been successfully implemented in two size ranges: the subnanometre scale, where light-matter mechanical coupling enables cooling of atoms, ions and molecules, and the micrometre scale, where the momentum transfer resulting from light scattering allows manipulation of microscopic objects such as cells. But it has been difficult to apply these techniques to the intermediate - nanoscale - range that includes structures such as quantum dots, nanowires, nanotubes, graphene and two-dimensional crystals, all of crucial importance for nanomaterials-based applications. Recently, however, several new approaches have been developed and demonstrated for trapping plasmonic nanoparticles, semiconductor nanowires and carbon nanostructures. Here we review the state-of-the-art in optical trapping at the nanoscale, with an emphasis on some of the most promising advances, such as controlled manipulation and assembly of individual and multiple nanostructures, force measurement with femtonewton resolution, and biosensors.

Brownian Motion of Graphene

Brownian motion is a manifestation of the fluctuation-dissipation theorem of statistical mechanics. It regulates systems in physics, biology, chemistry, and finance. We use graphene as prototype material to unravel the consequences of the fluctuation-dissipation theorem in two dimensions, by studying the Brownian motion of optically trapped graphene flakes. These orient orthogonal to the light polarization, due to the optical constants anisotropy. We explain the flake dynamics in the optical trap and measure force and torque constants from the correlation functions of the tracking signals, as well as comparing experiments with a full electromagnetic theory of optical trapping. The understanding of optical trapping of two-dimensional nanostructures gained through our Brownian motion analysis paves the way to light-controlled manipulation and all-optical sorting of biological membranes and anisotropic macromolecules.

FemtoNewton force sensing with optically trapped nanotubes

We extract the distribution of both center-of-mass and angular fluctuations from three-dimensional tracking of optically trapped nanotubes. We measure the optical force and torque constants from autocorrelation and cross-correlation of the tracking signals. This allows us to isolate the angular Brownian motion. We demonstrate that nanotubes enable nanometer spatial and femtonewton force resolution in photonic force microscopy, the smallest to date. This has wide implications in nanotechnology, biotechnology, nanofluidics, and material science.

Rotation Detection in Light-Driven Nanorotors

We analyze the rotational dynamics of light driven nanorotors, made of nanotube bundles and gold nanorods aggregates, with nonsymmetric shapes, trapped in optical tweezers. We identify two different regimes depending on dimensions and optical properties of the nanostructures. These correspond to alignment with either the laser propagation axis or the dominant polarization direction, or rotational motions caused by either unbalanced radiation pressure or polarization torque. By analyzing the motion correlations of the trapped nanostructures, we measure with high accuracy both the optical trapping parameters and the rotation frequency induced by the radiation pressure. Our results pave the way to improved all-optical detection, control over rotating nanomachines, and rotation detection in nano-optomechanics.

Trapping and manipulation of microscopic bubbles with a scanning optical tweezer

The authors have demonstrated three-dimensional trapping of ultrasound contrast agent microbubbles using a circularly scanning optical tweezers to confine the microbubble in a time-averaged optical potential. They have measured the maximum transverse drag force that may be applied to the trapped microbubble before it escapes and found that this decreases significantly at small trap radii. They explain this in terms of the relative volumes of the microbubble and the trap and anticipate that this feature will be important in experiments involving the insonation of optically trapped microbubbles.

Optical trapping of nanotubes with cylindrical vector beams

We use laser beams with radial and azimuthal polarization to optically trap carbon nanotubes. We measure force constants and trap parameters as a function of power showing improved axial trapping efficiency with respect to linearly polarized beams. The analysis of the thermal fluctuations highlights a significant change in the optical trapping potential when using cylindrical vector beams. This enables the use of polarization states to shape optical traps according to the particle geometry, as well as paving the way to nanoprobe-based photonic force microscopy with increased performance compared to a standard linearly polarized configuration.

Focusing of high order cylindrical vector beams

We present the results of calculations of focusing high order cylindrical vector beams in the limit of high numerical aperture. We derive a form of the vectorial diffraction integrals for arbitrary radial and azimuthal mode indices and evaluate these numerically for a number of different modes. We identify combinations of mode indices and lens filling factors that produce focal volume shapes that may be of interest for a number of applications such as optical trapping, two-photon lithography or optical super-resolution. Finally we evaluate the effect of spherical aberration on the focusing.

Sagnac interferometer method for synthesis of fractional polarization vortices.

We present a method for producing laser beams of nonuniform polarization where the polarization direction rotates on a trajectory about the beam propagation direction. Our method uses a Sagnac interferometer that incorporates a spatial light modulator to combine beams that possess oppositely charged phase vortices in order to achieve the desired polarization vortex. We demonstrate the utility of our method by producing polarization vortices characterized by a fractional index, and we compare the results with calculations of the expected fields.

Rectifying Fluctuations in an Optical Lattice

We have realized a Brownian motor by using cold atoms in a dissipative optical lattice as a model system. In our experiment the optical potential is spatially symmetric and the time symmetry of the system is broken by applying appropriate zero-mean ac forces. We identify a regime of rectification of forces and a regime of rectification of fluctuations, the latter corresponding to the realization of a Brownian motor.

Resonant activation in a nonadiabatically driven optical lattice

We demonstrate the phenomenon of resonant activation in a nonadiabatically driven dissipative optical lattice with broken time symmetry. The resonant activation results in a resonance as a function of the driving frequency in the current of atoms through the periodic potential. We demonstrate that the resonance is produced by the interplay between deterministic driving and fluctuations, and we also show that by changing the frequency of the driving it is possible to control the direction of the diffusion.