Maurizio Righini

Nano-optical Trapping of Rayleigh Particles and Escherichia coli Bacteria with Resonant Optical Antennas

Immobilizing individual living microorganisms at designated positions in space is important to study their metabolism and to initiate an in situ scrutiny of the complexity of life at the nanoscale. While optical tweezers enable the trapping of large cells at the focus of a laser beam, they face difficulties in maintaining them steady and can become invasive and produce substantial damage that prevents preserving the organisms intact for sufficient time to be studied. Here we demonstrate a novel optical trapping scheme that allows us to hold living Escherichia coli bacteria for several hours using moderate light intensities. We pattern metallic nanoantennas on a glass substrate to produce strong light intensity gradients responsible for the trapping mechanism. Several individual bacteria are trapped simultaneously with their orientation fixed by the asymmetry of the antennas. This unprecedented immobilization of bacteria opens an avenue toward observing nanoscopic processes associated with cell metabolism, as well as the response of individual live microorganisms to external stimuli, much in the same way as pluricellular organisms are studied in biology.

Light-induced manipulation with surface plasmons

We review recent advances achieved in the field of surface plasmon-based optical manipulations. We first discuss enhanced optical forces at the surface of a flat metal film and their use for self-organizing a large number of micro-objects. We then show how a suitable engineering of plasmon fields near micro-gold pads enables trapping at a specific location with much weaker laser intensity compared to conventional optical tweezers. This part is illustrated by a series of numerical simulations based on the theory of the Green dyadic. Finally, we show that, beyond their low power requirement, this new generation of integrated optical tweezers offers new perspectives in optical manipulation including parallel trapping with a single beam and controllable selectivity on the object polarizability.

On-a-chip surface plasmon tweezers

We report on an integrated optical trapping platform operated by simple fiber coupling. The system consists of a dielectric channel optical waveguide decorated with an array of gold micro-pads. Through a suitable engineering of the waveguide mode, we achieve light coupling to the surface plasmon resonance of the gold pads that act as individual plasmonic traps. We demonstrate parallel trapping of both micrometer size polystyrene beads and yeast cells at predetermined locations on the chip with only 20 mW total incident laser power.

Parallel and selective trapping in a patterned plasmonic landscape

The implementation of optical tweezers at a surface opens a huge potential towards the elaboration of future lab-on-a-chip devices entirely operated with light. The transition from conventional three-dimensional (3D) tweezers to 2D is made possible by exploiting evanescent fields bound at interfaces. In particular, surface plasmons (SP) at metal/dielectric interfaces are expected to be excellent candidates to relax the requirements on incident power and to achieve subwavelength trapping volumes. Here, we report on novel 2D SP-based optical tweezers formed by finite gold areas fabricated at a glass surface. We demonstrate that SP enable stable trapping of single dielectric beads under unfocused illumination with considerably reduced laser intensity compared with conventional optical tweezers. We show that the method can be extended to parallel trapping over any predefined pattern. Finally, we demonstrate how SP tweezers can be designed to selectively trap one type of particles out of a mixture, acting as an efficient optical sieve.

Multiple trapping in a patterned plasmonic landscape

We report on a novel optical manipulation method based on a transparent surface patterned with metal microstructures supporting surface plasmon (SP). Such SP-tweezers enable from a single light beam to arrange according to any predefined pattern many micro-objects on a surface.

The strength of surface plasmons

We review recent advances achieved in the field of integrated optical manipulations based on the control of surface plasmons. In particular, we describe how intense and confined optical force fields can be precisely engineered in the vicinity of transparent surfaces patterned with plasmonic metal structures.Recent progresses in the design of such devices is presented together with the latest experiments that have demonstrated efficient trapping of micro-objects under reduced laser intensity compared with conventional optical tweezers. Finally, we review other proposals where the use of localized surface plasmons in coupled metal nanostructures opens new perspectives in scaling down the trapping volumes well below the diffraction limit for the manipulation of single nano-objects.

Plasmon-based optical manipulation

The current work provides a first experimental demonstration of a novel in-plane manipulation method based on the action of SP forces rather than conventional photonic forces, both at homogeneous and patterned metal surfaces.