Optical manipulation of plasmonic nanoparticles: Applications in surface chemistry and nano-optics

Optical tweezers are devices that can manipulate nano- and microparticles using a laser. The principle of optical tweezers is to apply a force to an object using the momentum of light. This force is very small, but it is sufficient to move things in the microscopic world. Consequently, optical tweezers can grab, move, and even assemble objects on a much smaller scale, just like human hands.

Optical tweezers have revolutionized the way scientists experiment with small particles in many fields of science, and their utility as a tool for understanding nanoscale processes is continuously expanding. In this dissertation, we discuss the new application possibilities of optical tweezers in two distinct disciplines: surface sciences and nano-optics.

We first employ optical tweezers in surface science by trapping gold nanoparticles at an interface and observing their motion. Due to their strong scattering and absorption, gold nanoparticles are a superb optical probe and a nanoscale heater that can be remotely controlled by light. They can provide heat to the molecules adsorbing on the surrounding surfaces and stimulate further interactions. We compare these findings to those obtained using non-thermal dielectric probes and find that heat-induced effects are indeed present in our experiments. This study demonstrates that gold nanoparticles are highly effective probes that can apply heat locally and examine the resulting interaction in real time. Thermal phenomena exist in many scientific disciplines; hence, the optically trapped gold nanoparticles can serve as a valuable tool for investigating these phenomena at the nanoscale.

We then assemble gold nanoparticles into a complex structure using optical tweezers. Individually, gold nanoparticles possess exceptional optical properties. However, when two or more of them are arranged adjacently, their properties become even more intriguing due to the strong interaction of the near fields, which is one of the most vital areas of nano-optics. Optical tweezers offer a unique method for organizing these particles in a controlled manner, producing a strong near-field coupling. We examine this coupling behavior by numerically simulating and experimentally realizing optical assemblies. This concept of tweezer-organized assemblies can create exciting opportunities for constructing three-dimensional nanoscale architectures beyond the current technologies that have evolved from conventional semiconductor device fabrication processes. Moreover, optical tweezers enable the flexible combination of structures derived from distinct methods, such as colloidal particles and lithographically defined nanostructures.