Plasmon-based optical trapping

The implementation of Lab-on-a-chip (LOC) devices requires the challenging miniaturization of laboratory analysis techniques onto the small surface of a chip. While microfluidics is expected to be one of the main engines in LOCs, researchers are very active in developing functionalities. A promising strategy is to use light, both for sensing and for manipulation of biological samples. optical forces have been broadly used to manipUlate biological specimens using the so-called optical tweezers (OT). Despite their great success, OT are diffraction limited and difficult to integrate. In order to overcome these limitations, it has been suggested to use evanescent fields (Kawata et al. 1992, Kawata et al. 1996) to facilitate the integration of optical forces onto coplanar geometries as motors towards future optically driven on-a-chip devices (Reece et al. (2006). Beyond conventional evanescent fields, surface plasmons (SP) supported at metal/dielectric interfaces could benefit to evanescent optical manipulation in two ways: (i) The enhancement of SP fields is expected to decrease the incident laser intensity required for trapping. (ii) The ability of SP fields to be confined down to the subwavelength regime opens new perspectives for optical trapping of objects in the nanometer scale. In this thesis we describe two new optical manipulation schemes, based on surface plasmons integrated on a chip. The first one uses micrometric gold discs supporting surface plasmon polaritons (SPP) resonances, designed bye-beam nanolithography onto a glass substrate (Righini et al. 2007). We show that a suitable engineering of plasmon fields near the structures enables trapping colloidal beads and micrometric sized cells in water at specific locations with two orders of magnitude weaker incident laser intensity as compared to conventional optical tweezers. Experiments are supported by a series of numerical simulations based on the theory of the Green dyadic. We also show that, beyond their low power requirement, SPP optical tweezers offers new perspectives in optical manipulation including parallel trapping with a single beam and controllable selectivity on the object polarizability (Righini et al. 2008a). However, SPP-tweezers based on isolated gold pads fail in trapping any object smaller than one micrometer. This limitation can be overcome exploiting the electromagnetic coupling between adjacent metal nanostructures supporting localized resonant modes. The second proposed scheme uses an array of plasmonic gap antennas (Righini et al. 2009). The latest advances in nano-optics are exploited to design sub-A optical potential wells at the vicinity of the antennas capable of efficiently trapping low-contrast refractive index objects at moderate incident laser intensity. We first systematically assess the ability of gap antennas to trap Rayleigh (200 nm) PS beads in water and we support our observation by numerical simulations. Thereafter, we demonstrate the applicability of our method to parallel trapping of living Escherichia coli bacteria. Remarkably, we observe a systematic alignment of the trapped bacteria along the antenna axis. Besides, a study of growth and division rate of trapped bacteria indicates significantly lower levels of invasiveness. Finally, in order to demonstrate the compatibility of our techniques with LOC devices, we report the design and realization of an optical trapping platform based on the integration of SPP-tweezers on a waveguide, operated by simple fiber-coupling. The system consists of a dielectric channel optical waveguide decorated with an arra of old micro- ads. Throu h a suitable en ineerin of the waveguide mode, light is coupled to the surface plasmon resonance of the gold pads that act as individual plasmonic traps. The workability of this platform is demonstrated by parallel trapping of both micrometer size polystyrene beads and yeast cells at predetermined locations on the chip (Wong et al. 2010).