Controlling and utilizing optical forces at the nanoscale with plasmonic antennas

Plasmonic dipole antennas are powerful optical devices for many applications since they combine a high field enhancement with outstanding tunability of their resonance frequency. The field enhancement, which is mainly localized inside the nanogap between both arms, is strong enough to generate attractive forces for trapping extremely small objects flowing nearby. Furthermore it dramatically enhances their Raman scattering cross-section generating SERS emission. In this publication, we demonstrate how plasmonic antennas provide unique means for bringing analyte directly into hot-spots by merely controlling the optical force generated by the plasmon resonance. This technique is very suitable for immobilizing objects smaller that the diffraction limit and requires a very little power density. In this work, 20nm gold nanoparticles functionalized with Rhodamine 6G are trapped in the gap of nanoantennas fabricated with e-beam lithography on glass substrate. The entire system is integrated into a microfluidic chip with valves and pumps for driving the analyte. The field enhancement is generated by a near-IR laser (lambda=808nm) that provides the trapping energy. It is focused on the sample through a total internal reflection (TIRF) objective in dark field configuration with a white light source. The scattered light is collected through the same objective and the spectrum of one single antenna spectrum is recorded and analyzed every second. A trapping event is characterized by a sudden red-shift of the antenna resonance. This way, it is possible to detect the trapping of extremely small objects. The SERS signal produced by a trapped analyte can then be studied by switching from the white light source to a second laser for Raman spectroscopy, while keeping the trapping laser on. The trapping and detection limit of this approach will be discussed in detail.