Optical manipulation and heating of gold nanoparticles near interfaces

Licentiate thesis, 2018

By focusing laser light to small volumes, its momentum can be used to trap and manipulate objects in the size range from cells down to single atoms. Devices using this effect are called optical tweezers, and have found use in measuring and applying minuscule forces and torques, contributed to deepening our knowledge of molecular motors, unraveling the mechanics of cells and DNA, and better understand statistical mechanics and hydrodynamic interactions at the nanoscale. In short, the optical tweezer is a crucial component in our aspiration to understand and unlock the potential of nano-scaled objects.

One class of nano-elements worth devoting special attention to are nanoparticles supporting plasmonic resonances. These present strongly enhanced light-matter interactions and may find use in as diverse fields as high-density data storage, single molecule detection, and personalized medicine. One potential use of plasmonic nanorods is as rotary nanomotors. These are capable of reaching record rotation frequencies of several tens of kilohertz when optically trapped in water against a glass surface. This thesis focuses on studying vital questions related to such rotary nanomotors, which are interesting to resolve from both a fundamental and from an applied point of view.

It is well-known that metallic nanoparticles are efficiently heated by light. This will give rise to several photothermal effects affecting the nanoparticle and its surrounding. How these influence the performance of the nanomotor is evaluated. Through spectroscopic measurements, morphological changes induced by atomic migration is observed. Moreover, the elevated thermal environment around the nanoparticle is probed using two separate techniques, and temperatures above $200^\circ$C are routinely reached, but could be kept as low as a few degrees above ambient under the right circumstances.

The gold nanoparticle is trapped at a small, but hitherto unknown, distance from a glass interface. The vicinity to a surface can affect several of a nanoparticles properties, including its diffusion and thermal environment, and knowing this distance is hence critical for any claims about the nanomotors' performance. Therefore, total internal reflection microscopy is performed on the trapped nanoparticles and it is found that they are confined less than 100 nm from the surface. The distance can be controlled by altering the radiation pressure, or Coulomb repulsion.

In summary, the work performed in this thesis present important building blocks towards a complete understanding of this highly promising rotary motor system.