Thermoplasmonic Effects in Microfluidic Systems

Licentiate thesis, 2019

The field of plasmonics has enabled a plethora of scientific achievements over the past few decades. Such advances have been the result of the increased light-matter interaction that occurs over nanoscale dimensions facilitated by the localized plasmonic resonances supported by noble metal nanoparticles. These plasmonic resonances enable increased coupling of light between the near-field (where interesting phenomena occur) and the far field (where the signal can be easily detected). In this way a plasmonic structure acts as a localized transducer in microfluidic environments that are beneficial in many biological studies. The field of plasmonics has been hindered by the enhanced optical absorption that occurs within the metallic nanoparticles due to their finite electrical conductivity at optical frequencies. Absorption can lead to significant temperature increases which has thus far been considered a detrimental side effect of these plasmonic resonances.
Thermal effects in plasmonic systems have recently become an area of intense study in their own right. Thermoplasmonics involves studying the influence that a plasmonically supported localized temperature increase has on the surrounding environment. These thermal effects are an interesting field of study both for their implications in traditional optical-plasmonic-sensing schemes, as well as due to new and exciting thermally based applications. In this thesis I explore two such cases. First, the detrimental impact of thermoplasmonic effects in plasmonic-optical-tweezers. And second, the formation of small thermoplasmonic nanobubbles that may be utilized for active microfluidic manipulation.
In plasmonic-optical-trapping the electric field enhancement supported by a plasmonic antenna amplifies the optical gradient force thereby enabling incredibly small particles to be “held” by the antenna. However, the same phenomenon that enhanced the optical forces also increases the localized heating. Here I will show how the localized heating essentially negates a portion of the optical force amplification by increasing the Brownian motion of the trapped particle. Even more impactful is the formation of a thermal barrier that prohibits diffusive transport of analytes to the antenna thereby significantly decreasing the interaction probability. This thermal barrier was found to extend several microns from the antenna – far beyond the range of optical interactions.
One potentially advantageous utilization of thermoplasmonics is in bubble formation. The presence of a thermally generated bubble can induce incredibly fast fluid currents in the surrounding environment. One such utilization is towards mixing multiple fluid streams in microfluidic environments. However, most studies to date have focused on thermoplasmonic microbubbles which tend to be quasi-stable, therefore prohibiting high throughput applications. Here I will demonstrate how isolated plasmonic structures can form thermoplasmonic nanobubbles which are quickly decaying and therefore advantageous as active manipulation elements. Such bubbles were able to be modulated at frequencies up to a few kilohertz in water, several orders of magnitude greater than previously demonstrated.