Cavity-Enhanced Evanescent Waves Trap Microparticles

Hank Hogan

Like surfers riding an ocean swell, micron-size particles respond to light and can be trapped and arranged in regular patterns. However, it has been difficult to produce a strong enough optical interaction to achieve this micromanipulation over large areas. Now scientists at the University of St. Andrews in the UK have demonstrated a potential solution, using a dielectric resonator and a prism coupler to create Fabry-Perot-like cavity modes that increase the force exerted by evanescent waves.

Colloidal particles form an array of linear chains in response to counterpropagating evanescent waves. The spacing between the arrays is about 8 μm. A resonant cavity was used to boost the optical interaction and thereby create the trap. The micromanipulation technique could be suitable for cell sorting and other applications. Courtesy of Peter J. Reece, University of St. Andrews.

“The enhancement we see is a factor of 10,” said Peter J. Reece, a postdoctoral researcher at the university, “though by playing with the properties of the waveguide, this enhancement could potentially be much greater.”

Specifically, he suggested that an optimized design could boost the enhancement a thousandfold or more. With that, the technique would be suitable for such applications as cell sorting, driving lab-on-a-chip devices without the need for microfluidic control.

The scheme makes use of evanescent waves that can trap and order particles but that typically are weak. In the new approach, this drawback is overcome by creating a resonant cavity, with the finesse of the resonator determining the degree of enhancement.

The scientists formed the cavity in their demonstration using a prism with a two-layer dielectric coating: a 1000-nm-thick coupling layer of SiO₂ with a refractive index of 1.45 and a 127.5-nm-thick cavity layer of ZrO₂. They placed a sample chamber, consisting of a 100-μm-thick nylon spacer layer and a glass coverslip, atop the structure and filled it with a solution that was in direct contact with the coating.

Yb-doped fiber laser

To generate evanescent waves, they used an ytterbium-doped fiber laser operating at 1064 nm. The angle of incidence of the beam to the internally reflective surface of the prism was approximately 60°, the critical angle calculated to induce resonance. They filled the solution with monodispersed 5-μm-diameter polystyrene spheres and found that the force exerted at the critical angle was 10 times that exerted without the resonator.

Using two counterpropagating waves, the researchers trapped 500-nm-diameter particles and found that they formed a linear array. According to Reece, the exact arrangement was unexpected.

He said that the scientists are investigating this self-organization phenomenon. They also are building and testing new devices with optimized waveguides. When this work is done, he suggested, cavity enhanced evanescent wave micromanipulation could be used for large-scale migration and ordering of microscopic particles.

Applied Physics Letters, May 29, 2006, 221116.