Optical trapping and spectroscopic characterisation of ionic liquid solutions.

This paper presents a study of the optical trapping of aerosols containing ionic liquid (IL). Droplets comprised of aqueous solutions of the IL ethylammonium nitrate (EAN) are demonstrated to be readily trapped by optical tweezers and are characterized spectroscopically, by analysing the morphology dependent resonances present within backscattered light originating from a broadband light emitting diode. The response of the droplets to conditions of varying relative humidity has also been investigated, as an important first step in measuring the uptake of gases by ILs. Finally, a comparison between broadband Mie scattering and cavity enhanced Raman spectroscopies is provided.

Single aerosol trapping with an annular beam: improved particle localisation.

In this paper we explore the trapping of aerosol droplets using an annular beam, formed by blocking the central portion of a Gaussian beam, and quantify the improvements over conventional Gaussian beam traps. Recent work on the modelling of single aerosol dynamics within an optical tweezer trap [Burnham et al., Journal of the Optical Society of America B, 2011, 28, 2856-2864] has indicated that the use of annular beams can allow smaller droplets to be trapped, which we experimentally verify. We also demonstrate that annular beams allow droplets to be trapped at higher powers, and with reduced axial displacement with increasing power, than Gaussian beams. We confirm these results, due to a reduction in the axial scattering forces, using this theoretical model. Finally back focal plane interferometry is used to determine the axial and lateral trap stiffnesses for a series of droplets, showing a significant increase in the axial : lateral trap stiffness ratio from 0.79 ± 0.04 to 1.15 ± 0.04 when an annular beam is used.

Directed assembly of optically bound matter.

We present a study of optically bound matter formation in a counter-propagating evanescent field, exploiting total internal reflection on a prism surface. Small ensembles of silica microspheres are assembled in a controlled manner using optical tweezers. The structures and dynamics of the resulting optically bound chains are interpreted using a simulation implementing generalized Lorentz-Mie theory. In particular, we observe enhancement of the scattering force along the propagation direction of the optically bound colloidal chains leading to a microscopic analogue of a driven pendulum which, at least superficially, resembles Newton's cradle.

Direct observation of grain rotation-induced grain coalescence in two-dimensional colloidal crystals.

A spatially modulated laser is used to produce multiple localized thermal gradients in a colloidal sample placed above a gold surface. We use an optical microscope to observe real time dynamics of the resulting two-dimensional colloidal crystal grains and find that grain rotation-induced grain coalescence (GRIGC) occurs with the rotation of both grains before coalescence. Control over the grain size shows that the time scale for grain boundary annealing in our system is in good agreement with theoretical expressions formulated for nanocrystal growth.

Optical manipulation of airborne particles: techniques and applications.

In the following paper, we discuss new methods to trap and manipulate airborne liquid aerosol droplets. We discuss the single gradient force trapping of water aerosols in the 2-14 micron diameter range using both 532 nm and 1064 nm light, as well as the holographic optical trapping of arrays of aerosols. Using this holographic technique, we are able to show controlled aerosol coagulation. We also discuss two techniques based on the radiation pressure trapping of aerosols, namely the dual beam fibre trap and the controlled guiding of aerosols using Bessel beams. We conclude with a discussion of new topics for study based upon these techniques and some possible applications.