Optical trapping of metal nanoparticles investigates phenomena at the interface of plasmonics and optical micromanipulation. This thesis combines ideas of optical properties of metals originating from solid state physics with force mechanism resulting from optical trapping. We explore the influence of the particle plasmon
resonance of gold and silver nanospheres on their trapping properties. We aspire
to predict the force mechanisms of resonant metal particles with sizes in the Mie
regime, beyond the Rayleigh limit.
Optical trapping of metal nanoparticles is still considered difficult, yet it provides
an excellent tool to investigate their plasmonic properties away from any interface
and offers opportunities to investigate interaction processes between light and
nanoparticles. Due to their intrinsic plasmon resonance, metal nanoparticles show
intriguing optical responses upon interaction with laser light. These differ greatly
from the well-known bulk properties of the same material.
A given metal nanoparticle may either be attracted or repelled by laser light,
only depending on the wavelength of the latter. The optical forces acting on the
particle depend directly on its polarisability and scattering cross section. These
parameters vary drastically around the plasmon resonance and thus not only change
the magnitude but also the direction and entire nature of the acting forces. We
distinguish between red-detuned and blue-detuned trapping, that is using a trapping
wavelength shorter or longer than the plasmon resonance of the particle. So far
optical trapping of metal nanoparticles has focussed on a wavelength regime far
from the particle’s resonance in the infrared. We experiment with laser wavelengths
close to the plasmon resonance and expand the knowledge of metal nanoparticle
trapping available to date. Existing theoretical models are put to the test when we
compare these with our real experimental situations
