Microfabricated Optical Tweezers

The incorporation of optical tweezers into microfluidic chips would provide exciting new functionalities for these systems. These include particle sorting, particle manipulation, measurement of fluid properties, and biophysical force measurements. This motivates the development of microfabricated structures for optical trapping that are suitable for integration into microfluidic chips. Surface plasmon nanostructures generate optical near-fields that could be employed for the trapping of nanoparticles. For trapping microparticles, however, other structures are suitable.

We have recently demonstrated that microfabricated Fresnel zone plates can be used for trapping micro particles [1]. Fresnel zone plates are relatively simple to fabricate, making them suitable for integration into microfluidic systems. A photograph of a zone plate we fabricated is shown as Fig. 1a, and consists of concentric gold rings that block odd Fresnel zones. The zone plate is incorporated into the set-up of Fig. 1b to perform trapping. Light from a laser is collimated and illuminates the zone plate, which focuses it to a spot. Beads are trapped by the focused spot, and additional optics is used to image these beads onto a CCD. A CCD image of a trapped bead is shown as Fig. 1c. The zone plate has performance comparable to conventional optical tweezers, when its diffraction efficiency is taken into account. This work was published in Applied Physics Letters in February 2008 [1].

Fig. 1 a). Photo of microfabricated Fresnel zone plate. Light regions: gold. Dark regions: glass. b). Experimental set-up for trapping beads with zone plate. c). CCD image of trapped bead.

REFERENCE

[1] E. Schonbrun, C. Rinzler, and K.B. Crozier, “Microfabricated Water Immersion Zone Plate Optical Tweezer,” Applied Physics Letters, vol. 92 , 071112 (2008)