Modeling of optical traps for aerosols
Spotlight summary: To appreciate the importance of aerosols, one need only look at the sky. These systems of micrometer-sized liquid droplets in air are a major part of the atmosphere—important in processes from acid rain to global warming. However, aerosols are notoriously difficult to work with in the laboratory, particularly when studied at the single-droplet level.
Burnham and McGloin have brought the power of mathematical modelling to bear on this problem, and in their paper they present results that offer theoretical explanations for a number of peculiar effects observed when optically manipulating micrometer-sized droplets.
A laser beam focussed through a microscope can hold and manipulate small, transparent objects without mechanical contact, using the light’s momentum to apply forces to the particle. Trapped micrometer-sized glass spheres in liquid media have been used to make incredibly sensitive force measurements in these “optical tweezers,” but extending the technique to work in air involves significant challenges.
First, the problem of producing a sharp focal spot in water has been extensively optimized by commercial microscope producers, but the same problem in air has received less attention. Second, the higher refractive-index contrast between water and air (as opposed to glass and water) makes light scattering and resonance effects much more pronounced. Third and last, air is three orders of magnitude less dense and also much less viscous than water. This last effect means we can no longer neglect a trapped object’s mass and must pay attention to gravity and mechanical oscillations.
This latest Journal of the Optical Society of America B paper builds on previous work modelling optical traps in liquids to include the additional aberrations on a beam in a typical aerosol-trapping experiment. Their improved theoretical model, also accounting for the droplet’s weight, allows them to explore the parameter space of aerosol trapping with different particle sizes and refractive indices. They find that the higher refractive-index contrast when trapping in air means that particles can often be trapped only with the assistance of gravity, termed “quasi optical tweezers,” which agrees with many laser-power- and particle-size-dependent effects seen in experiments. The model also predicts resonance effects strongly dependent on droplet radius, which may explain some of the puzzling axial oscillations observed in the laboratory.
Finally, Burnham and McGloin go on to explore the possibility of trapping a much wider range of particles by blocking out the central part of the trapping beam. This annular beam has no on-axis light, so the scattering force that often pushes particles out of the trap is reduced. Theoretically, this technique makes stable trapping possible over a much wider range of droplet radii—often without relying on gravity.
While this paper is a theoretical one, McGloin’s team is also experienced experimentalists, so there is no doubt the insights presented therein will be applied in the laboratory. With an improved model of the trapping mechanisms, many of the difficulties in aerosol trapping will not only be understood, but overcome. We can look forward, then, to a powerful new tool for studying aerosols—and the consequent advances in our understanding of the delicate atmosphere in which we live.
Technical Division: Optoelectronics
ToC Category: Optical Trapping and Tweezing
|OCIS Codes:||(010.1110) Atmospheric and oceanic optics : Aerosols|
|(350.4855) Other areas of optics : Optical tweezers or optical manipulation|
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