Controlled transfer of transverse optical angular momentum to optically trapped birefringent particles

We report on the observation and measurement of the transfer of transverse angular momentum to birefringent particles several wavelengths in size. A trapped birefringent particle is much larger than the nano-particles systems for which transverse angular momentum was previously investigated. The larger birefringent particle interacts more strongly with both the trapping beam and fluid surrounding it. This technique could be used to transfer transverse angular momentum for studies of diverse micro-systems. Thus, it can be used for investigation of the dynamics of complex fluids in 3D as well as for shear on cell mono-layers. The trapping of such a particle with highly focused light is complex and can lead to the emergence of effects such as spin--orbit coupling. We estimate the transfer of spin angular momentum using Stokes measurements. We outline the physics behind the construction of the beam used to control the particles, perform quantitative measurement of transverse spin angular momentum transfer, as well as demonstrate the generation of fluid flow around multiple rotation axes.

Momentum of an electromagnetic wave in dielectric media

Almost a hundred years ago, two different expressions were proposed for the energy--momentum tensor of an electromagnetic wave in a dielectric. Minkowskis tensor predicted an increase in the linear momentum of the wave on entering a dielectric medium, whereas Abrahams tensor predicted its decrease. Theoretical arguments were advanced in favour of both sides, and experiments proved incapable of distinguishing between the two. Yet more forms were proposed, each with their advocates who considered the form that they were proposing to be the one true tensor. This paper reviews the debate and its eventual conclusion: that no electromagnetic wave energy--momentum tensor is complete on its own. When the appropriate accompanying energy--momentum tensor for the material medium is also considered, experimental predictions of all the various proposed tensors will always be the same, and the preferred form is therefore effectively a matter of personal choice.

T-matrix calculation via discrete-dipole approximation, point matching and exploiting symmetry

We present a method of incorporating the discrete dipole approximation (DDA) method with the point matching method to formulate the T-matrix for modeling arbitrarily shaped micro-sized objects. The emph{T}-matrix elements are calculated using point matching between fields calculated using vector spherical wave functions and DDA. When applied to microrotors, their discrete rotational and mirror symmetries can be exploited to reduce memory usage and calculation time by orders of magnitude; a number of optimization methods can be employed based on the knowledge of the relationship between the azimuthal mode and phase at each discrete rotational point, and mode redundancy from Floquets theorem. A reduced-mode T-matrix can also be calculated if the illumination conditions are known.

Fabrication of micro-structures for optically driven micromachines using two-photon photopolymerization of UV curing resins

Two-photon photopolymerization of UV curing resins is an attractive method for the fabrication of microscopic transparent objects with size in the tens of micrometers range. We have been using this method to produce three-dimensional structures for optical micromanipulation, in an optical system based on a femtosecond laser. By carefully adjusting the laser power and the exposure time we were able to create micro-objects with well-defined 3D features and with resolution below the diffraction limit of light. We discuss the performance and capabilities of a microfabrication system, with some examples of its products.

Collecting single molecules with conventional optical tweezers

The size of particles which can be trapped in optical tweezers ranges from tens of nanometres to tens of micrometres. This size regime also includes large single molecules. Here we present experiments demonstrating that optical tweezers can be used to collect polyethylene oxide (PEO) molecules suspended in water. The molecules that accumulate in the focal volume do not aggregate and therefore represent a region of increased molecule concentration, which can be controlled by the trapping potential. We also present a model which relates the change in concentration to the trapping potential. Since many protein molecules have molecular weights for which this method is applicable the effect may be useful in assisting nucleation of protein crystals.