Optical trapping at the nanoscale !
The optical trapping of Au nanoparticles with dimensions as small as 10 nm in the gap of plasmonic dipole antennas is demonstrated. Single nanoparticle trapping events are recorded in real time by monitoring the Rayleigh scattering spectra of individual plasmonic antennas. Numerical simulations are also performed to interpret the experimental results, indicating the possibility to trap nanoparticles only a few nanometers in size. This work unveils the potential associated with the integration of plasmonic trapping with localized surface plasmon resonance based sensing techniques, in order to deliver analyte to specific, highly sensitive regions (“hot spots”).
The demonstration in the 70's that light can produce a force on microscopic objects has driven a huge research effort to understand and master these optical forces. Today, optical tweezers – which rely on this principle – are routinely used to manipulate cells in a non-invasive way.
Trapping objects smaller than a micrometer remains however a great challenge, because the radiation pressure tends to prevent catching very small objects. In a recent experiment, we have demonstrated that it is however possible to trap particles as small as 10nm in the near-field of a plasmonic antenna.
The setup is shown in the previous figure: Gold colloidal nanoparticles in a solution are trapped in the very strong near-field generated in the gap of a plasmonic antenna. When the trapping laser is switched on, the gradient force produced on the nanoparticles is such that they become trapped between the arms of the antenna. In turn, this changes the optical response of the antenna and produces a frequency shift that can be easily measured, even for very small nanoparticles.
The possibility of measuring the trapping event by monitoring the resonance wavelength of the antenna provides a very powerful mean of detecting extremely small particles, that would not be easily observable otherwise. This figure shows the plasmonic trapping of a 20 nm Au particle in the 25nm gap of a gold antenna with arm length of 80nm. The left is a waterfall plot for 500 normalized Rayleigh scattering spectra of the antenna collected continuously; the trapping laser was on during the measurement. The right shows the resonance peak wavelength of the spectra as a function of time. The inset in reveals a typical Rayleigh scattering spectrum (measured along the white dashed line in the left panel). Note that a notch filter is used to suppress the laser wavelength used for trapping around λ=800nm. After some oscillations, the particle reaches a stable trapping position, leading to a stable resonance wavelength for the antenna.
Demonstrating optical trapping at the nanoscale opens a huge field of applications in nanotechnology, sensing and life sciences.