Surface Plasmon Radiation Forces
Optical forces have become a powerful tool in physical, chemical, and especially biological experiments where an accurate and non-invasive manipulation of small objects from the nanometer to the micrometer scale is required. Within the current general trend for miniaturization, the implementation of the so-called "optical tweezers" (OT) at a surface remains today a key aspiration which would open a huge potential for future reconfigurable lab-on-a-chip devices entirely operated with light. The transition from 3D to 2D optical manipulation is made possible by exploiting evanescent fields bound at interfaces. In particular, surface plasmons (SP) bound at metal surfaces are expected to lead to key breakthroughs. SP are surface modes associated to a resonant oscillation of the surface charges of the metal with the incident electromagnetic field. They result in a multifold increase of the surface field strength and potentially allow for scaling trapping volumes down to the nanometer scale.
We provided the first experimental demonstration of the plasmon optical forces. The enhanced force field experienced by a single micrometer bead exposed to the SP at a flat metal/dielectric interface has been for the first time measured using a Photonic Force Microscope. The total force magnitude was found to be 40 times stronger compared to otherwise, without feeding the SP. The simplicity and flexibility of SP-based optical manipulation pave the way toward a new generation of analytical devices where the analyte would be manipulated, arranged and analyzed at the chip surface for low cost and parallel bio-analysis.