Michael D. Summers

Optical levitation in a Bessel light beam

A vertically oriented zero order Bessel light beam is shown to create a one-dimensional array of trapped particles over extended (millimeter) distances. The particles take up equilibrium positions over the entire length of the beam and this is a consequence of the interplay between optical scattering and the self-healing properties of the Bessel beam. This work has analogies to recent studies of optically bound matter and allows for the simple creation of one-dimensional particle chains and their subsequent spectroscopic analysis.

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.

Trapping solid aerosols with optical tweezers: a comparison between gas and liquid phase optical traps.

We demonstrate a method for the optical trapping of solid aerosol particles. Suspension of silica particles in ethanol allows their delivery to the trapping volume using a commercial medical nebulizer. The ethanol quickly evaporates, leaving the solid particles trapped in air. We use the technique to make comparisons between aerosol and colloid tweezing through power spectra analysis of the particles positions fluctuations for identical particles trapped in a water or air suspending medium.

The reconstruction of optical angular momentum after distortion in amplitude, phase and polarization

Propagation through a distorting obstacle may significantly influence the amplitude, phase and polarization state of a light beam. This potentially has consequences for the behaviour of the optical angular momentum of light. We experimentally study how both the spin and orbital angular momentum (OAM) of light behaves upon passage through microscopic optically trapped particles. Particles trapped with Gaussian and, separately, Bessel light beams in two spatially distinct sample chambers are studied with trapped objects in the first chamber acting as distorting obstacles. The Bessel beam can reconstruct its spatial form and this shows reconstruction of both spin and OAM over extended distances.

Fiber based optical trapping of aerosols

We present the use of optical fibers to form a counter-propagating optical trap as a means of manipulating both solid and liquid aerosols. We explore the use of single and multimode fibers to achieve trapping of various particles in air, present the trapping properties of the different fiber types and compare the observed trends to those predicted by theory. Using fibers, we are able to hold suspended particles for extended periods of time and to precisely manipulate them over distances of several hundred microns. We discuss the difficulties and advantages of each fiber configuration and conclude with a demonstration that fiber based trapping offers a good candidate for studying optical binding in air.

Manipulation and characterisation of accumulation and coarse mode aerosol particles using a Bessel beam trap

Micron and sub-micron sized aerosol particles are captured, manipulated and characterised in a Bessel beam optical trap. Bright field microscopy and elastic light scattering measurements are used in combination to interrogate trapped particles and explore the optical landscape of the trap. We conclude that the Bessel trap has a number of advantages over optical tweezers in terms of characterisation of accumulation mode particles, manipulation of particles over macroscopic length scales and effective control of the gas phase. As such, the Bessel trap is a valuable addition to the aerosol optical toolkit.

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.

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.

Controlled Formation of Optically Bound Matter in Evanescent Fields

In this paper, we detail two techniques for standing wave evanescent field optical trapping utilizing total internal reflection at a prism-water interface. Firstly, we describe an actively-locked cavity enhancement technique that generates circulating powers in excess of 10 W over an area of 150 μm x 75 μm on the prism surface using a 400 mW source, as well as providing control over the shape of the underlying transverse cavity mode. Secondly, we have combined an inverted optical tweezers with a counter-propagating evanescent wave trapping experiment, providing a useful platform for exploring light scattering interactions between small ensembles of particles. The resulting structures are compared to our theoretical model based upon Generalised Lorentz-Mie Theory.

Optical guiding of aerosols

Aerosol droplets are guided over mm distances using single beam optical traps. The micron-sized particles are confined in two dimensions and guided along the direction of beam propagation. Both Gaussian and Bessel beam geometries are compared for water, ethanol and dodecane droplets. The observed trapping of multiple droplets in 1-D arrays will also be discussed.