Microfabricated zone plate for optical trapping

The integration of optical tweezers into microfluidic chips would open up ways to sort and manipulate particles and carry out biophysical force measurements. But the short working distance, size, and cost of microscopic lenses hinder their integration into microfluidic chips. Kenneth Crozier and colleagues at Harvard University have now fabricated a Fresnel zone plate capable of producing well-calibrated optical traps in microfluidic systems.

A zone plate—sometimes called a Fresnel zone plate—focuses light by diffraction. It consists of a series of radially symmetrical rings, known as Fresnel zones, that alternate between opaque and transparent. When light hits the plate, it diffracts around the opaque zones. The zones are spaced in such a way that the diffracted light constructively interferes at the desired focus to create an image.

Crozier and colleagues fabricated their zone plate on a glass slide with rings of gold as the opaque regions. They formed a liquid sample cell by sandwiching a layer of water between the zone plate and a coverslip. Fluorescent 2 µm diam beads were trapped in the sample cell at the zone plate focus. By comparing the trapping properties of the zone plate to those of traditional optical tweezers, Crozier and colleagues determined that zone-plate optical tweezers had a stiffness similar to that of conventional optical tweezers.

The investigators point out that unlike traditional optical tweezers, once a particle is trapped by the zone plate, the sample cell can be translated tens of micrometers without the particles moving out of position with respect to the sample cell. (Appl. Phys. Lett. 2008, DOI 10.1063/1.2837538)