Plasmonic nanoparticles, typically gold and silver colloids, can be trapped by a highly focused Gaussian beam.
The behavior of the particles in an optical trap, such as the alignment, stability and interaction between particles,
depends on their plasmonic nature, determined by the correlation between the size, shape and material of the
particles, and the wavelength and polarization of the trapping laser. For instance, an elongated nanoparticle
aligns parallel to the polarization of a NIR trapping laser to minimize the optical potential energy. However,
nanowires tend to align perpendicular to the polarization. A dimer of two isotropic nanoparticles in principle
acts similar to a nanorod with its "long axis" (dimer axis) parallel to the laser polarization. These results
are evidenced by dark-field scattering imaging and spectra, and agree well with discrete dipole approximation
simulations of the near-fields around different nanostructures. Elongated nanoparticles, dimers and nanowires all
rotate when the laser polarization is rotated. Irradiated under a circularly polarized laser, trapped objects spin
spontaneously due to the transfer of angular momentum from the incident photons. The interaction between
two gold nanoparticles in a dimer is complex because it involves the optical potential and the DLVO potential.
The latter can be probed to some extent using dark-field scattering spectroscopy.