Plasmonics with a Twist

Licentiatavhandling, 2014

This thesis focuses on the exploration of interactions between circularly polarized light and plasmonic nanostructures, and nanoparticles. Polarization is an integral part of light and has a wealth of applications in the modern world. Circularly polarized light in particular interacts in a specific way with the foundation of life itself: the amino-acids, among which 23 are chiral. These amino-acids exhibit an optical activity. However this phenomenon is weak due to the weak coupling of molecules to an external light field. Plasmonic particles, on the other hand, have strong interactions with an external light field. They could therefore be perfect candidates in manipulating light polarization and interactions with chiral molecules. Two paths were taken in the use of circularly polarized light: one was to transfer spin angular momentum from the circularly polarized light to get the highest spinning frequency possible of a trapped particle, the other was to fabricate structures with high circular dichroism in the visible. Nanoparticles in the form of gold spheres were trapped in aqueous solution by a 2D optical trap as described in Appended Paper I. By using a circularly polarized light beam for the trapping, it is possible to set these particles into a rotational motion. The reason for this rotation is the transfer of the spin angular momentum of the incident photons to the absorbing particle. Frequencies of several kHz, the highest reported in literature for particles in water, were recorded during the experiments. The results are well explained by classic electromagnetism theory and hydrodynamic theory. The maximum spinning frequency is reached at equilibrium between the external torque induced by the transfer of spin angular momentum, and of the frictional torque originating from the viscosity of the surrounding medium. It is worth noting that this viscosity is reduced in the immediate vicinity of the particle by the particle heating due to the use of high laser power. The transfer of spin angular momentum is put in evidence by an increase of the rotational frequency as the laser power is increased. However at high power, both temperature and friction increase, which leads to an increase of the influence of the stochastic torque on the particle motion. In Appended Paper II, right- and left-circularly polarized light were used as means of characterization of the circular dichroism (CD) of nanofabricated metal structures. The CD is a measure of the chiral optical response of a material. The structures showing the strongest CD in the visible to near-infrared wavelength range were closely packed silver tetramers that consist of four disks of increasing heights. They were realized based on both the hole-mask colloidal lithography technique and angular metallic evaporation. It provided us with a simple fabrication technique for obtaining a uniform coverage of large areas. The finite element method (Comsol) and the coupled-dipole approximation were used to investigate the origin of this strong CD. It originates in strong near-field interactions between the particles caused by the small interparticle distances, combined with the 3D nature of the arrangement of the four nanodisks. The gaps present within each structure could be of interest in future molecular analysis applications.