Abstract
We consider the problem of sorting, by size, spherical particles of order 100 nm radius. The scheme we analyze consists of a heterogeneous stream of spherical particles flowing at an oblique angle across an optical Gaussian mode standing wave. Sorting is achieved by the combined spatial and size dependencies of the optical force. Particles of all sizes enter the flow at a point, but exit at different locations depending on size. Exiting particles may be detected optically or separated for further processing. The scheme has the advantages of accommodating a high throughput, producing a continuous stream of continuously dispersed particles, and exhibiting excellent size resolution. We performed detailed Monte Carlo simulations of particle trajectories through the optical field under the influence of convective air flow. We also developed a method for deriving effective velocities and diffusion constants from the Fokker-Planck equation that can generate equivalent results much more quickly. With an optical wavelength of 1064 nm, polystyrene particles with radii in the neighborhood of 275 nm, for which the optical force vanishes, may be sorted with a resolution below 1 nm.
Highlights
Following demonstrations by Ashkin and co-workers [1, 2], optical forces have been widely used to manipulate small particles with dimensions in the range of nanometers to micrometers [3, 4, 5]
We show optical sorting of aerosol particles by simulating conditions readily implemented in an actual device
A FabryPerot cavity of modest finesse (≈ 1000) and small numerical aperture (≈ 10−3) can be used to establish a standing wave with intensities great enough to achieve the same magnitude of forces commonly used in optical tweezers
Summary
Following demonstrations by Ashkin and co-workers [1, 2], optical forces have been widely used to manipulate small particles with dimensions in the range of nanometers to micrometers [3, 4, 5]. Strong intensity gradients can be produced by near-field structures [7, 8, 9, 10, 11], interference fringes [12, 13, 14, 15, 16, 17], and optical fibers [18, 19] Many of these techniques utilize high NA optics. We propose and analyze a scheme for optically sorting airborne nanoparticles It has the advantages of (1) utilizing a simple, low NA optical system with high particle throughput, (2) producing a continuous stream of continuously dispersed particles, and (3) exhibiting a size resolution better than 1 nm. For example, the drag force is nearly two orders of magnitude larger, while Brownian motion is nearly one order of magnitude smaller The former can be accommodated by adjusting controllable parameters.
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