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Particles can be guided by the evanescent electric field of a light wave at an interface of two optical media (Kawata and Tani 1996) because the evanescent wave field exerts a force parallel to the interface. As demonstrated by Grujic K et al (2005), this phenomenon can be used to reliably sort particles (polystyrene latex, 6 um diameter) by simply shifting the maximum of the optical power (at 1066 nm) from one to another branch of a Y-waveguide.
On the other hand, Garcés-Chávez V et al (2005) used two counter-propagating evanscent waves to construct an optical trap and (by blocking one of the waves) a particle transporter that can simultaneously handle ~1000 particles. The guiding field node lines are generated by imaging a Ronchi ruling (500 line pairs per inch) at a prism surface at an incidence angle assuring TIR at that surface. A 600 mW laser beam at 1070 nm traps and moves 5 µm polystyrene latex particles along the ruling lines. In the transporter case, polystyrene particles move with a velocity on the order of 1 µm/s and human red blood cells with ~1/7th of that velocity. Most particles maintain a distance between each other when trapped or guided in this manner.
Grujic and Hellesř (2007) showed that the evanescent field of waves counter-propagating in a linear waveguide can easily trap and auto-assemble long chains of ~7 µm particles from the suspension overlying the waveguide. Both single particles and chains could be moved by adjusting the balance of light power propagating in either direction, much as in the experiment of Garcés-Chávez V et al (2005). The velocity of short, bi-sphere chains was greater than that of single particles by a factor of ~1.15. This was eexplained by Grujic and Hellesř as being a consequence of hydrodynamic coupling of the particles in a chain.
When the node lines of the electromagnetic field are separated by a distance on the order of the wavelength of light and the particles are roughly of the same size, the arrangement of particles is effected also by interaction between the particles through multiple light scattering. This effect, termed "optical binding" by Burns MM et al (1989), has been recently used by Mellor and Bain (2005) to arrange sub-wavelength sized polystyrene microspheres into checkerboard-patterned arrays. These arrays were found to behave as a single object in respect of, for example, Brownian motion.
| CITATION: Jonasz M. 2006. Particle trapping/guiding by evanescent light waves (www.tpdsci.com/Tpc/PtTpmEvWav.php). In: Top. Part. Disp. Sci. (www.tpdsci.com). |
HISTORY: Published: 13-Jan-2006 Modified: 24-May-2007 Peer-reviewed: PENDING |
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