Photonic crystal cavities for resonant evanescent field trapping of single bacteria

In monitoring the quality of drinking water with respect to the presence of hazardous bacteria there is a strong need for on-line sensors that allow quick identification of bacterium species at low cost. In this respect, the combination of photonics and microfluidics is promising for lab-on-a-chip sensing of these contaminants. Photonic crystal slabs have proven to form a versatile platform for controlling the flow of light and creating resonant cavities on a wavelength scale. The goal of our research is to use photonic crystal cavities for optical trapping of microorganisms in water, exploiting the enhanced evanescent field of the cavity mode. We optimize the H0, H1 and L3 cavities for optical trapping of bacteria in water, by reducing out-of-plane losses and taking into account the trapping-induced resonance shift and the in-plane coupling with photonic crystal waveguides. The cavities are fabricated on silicon-on-insulator material, using e-beam lithography and dry etching. A fluidic channel is created on top of the photonic crystal using dry film resist techniques. Transmission measurements show clear resonances for the cavities in water. In the present state of our research, we demonstrate optical trapping of 1 mu m diameter polystyrene beads for the three cavities, with estimated trapping forces on the order of 0.7 pN.