Mass Transport via Thermoplasmonics

Doctoral thesis, 2020

When a metallic nanoparticle is illuminated with light under resonant conditions, the free electron gas oscillates in such a way that substantial amplification of the local electric field amplitude is achieved – this is known as a plasmonic resonance. This resonance enhances both the optical scattering as well as absorption. In many applications, the enhanced scattering can facilitate efficient coupling between the near-field and the far-field, which enables optical interrogation of nanoscale volumes. Simultaneously, however, the enhanced absorption results in localized heating and substantial temperature gradients. The resulting temperature profile can drive other thermal processes, some beneficial others detrimental. Thermoplasmonics is the study of these plasmonically enhanced thermal processes.

Elevated temperatures increase the Brownian motion of small particles. Moreover, if large temperature gradients are present, then a process known as thermophoresis is likely to occur. Thermophoresis tends to cause a local depletion of Brownian particles around a hot region. From the context of “conventional” plasmonic applications (like molecular sensing), these thermally driven mass transport mechanisms are adverse side effects since they reduce the interaction rate between the plasmonic system and the analyte. An investigation of thermal effects in plasmonic optical tweezers showed that the increased Brownian motion essentially negated the optical tweezing effect, resulting in an overall insensitivity between the resonance condition of the antenna and the particle confinement when evaluated in terms of the local temperature increase. Additionally, a significant thermophoretic depletion of analytes occurred, extending tens of microns from the plasmonic structure. This depletion acts in opposition to the plasmonically enhanced optical forces, which are restricted to a region of only a few hundred nanometres.

However, thermoplasmonic effects can also be used for advantageous means. Once example is by driving thermocapillary flows directed towards the plasmonic system, thereby facilitating the efficient accumulation of analytes. One method of employing this effect is to superheat a plasmonic particle to a high enough temperature such that a bubble is nucleated. Once a bubble is formed, thermocapillary effects at the bubble interface drive fluid motion with a flow profile similar to that of a Stokeslet. This fluid flow can be utilized for analyte accumulation near the plasmonic structure. In addition to the thermocapillary induced flow, it was found that even more intense flow speeds were achieved immediately upon nucleation due to the mechanical action of the bubble. This transient peak in flow speed was approximately an order of magnitude faster than the subsequent persistent (thermocapillary) flow. By designing the plasmonic nanoparticle so that the Laplace pressure restricted the ultimate bubble size, these bubbles could be kept small enough to permit high modulation rates and maximize the relative effect of the peak transient flow.