Plasmon assisted optothermal control and manipulation of colloids and fluids

Abstract

Plasmo-fluidics is an emerging field which integrates plasmonics and microfluidics at nano/microscale. Using plasmons, we can confine light to regions beyond the diffraction limit of light. This helps to create strong electric fields in small volumes and also generate heat in the metallic nanostructures because of Joule heating. A fluidic environment on the other hand provides flexibility in designing reconfigurable devices which can be difficult to create in a solid state device. Herein, we will discuss how light-matter interaction and heat generation in a fluidic environment can be utilized to manipulate and control other dielectric/metallic nanostructures. It can give rise to several thermodynamic processes in the fluid such as thermophoresis, convection, bubble formation and Marangoni flows. Firstly, we will discuss surface plasmon polaritons assisted assembly of colloids. Typically, colloids undergo diffusive Brownian motion in a fluid. Herein, we modulate the dynamics of the colloids by locally heating metallic nanostructures, such as gold microplate and silver nanowire. The excitation of surface plasmons generates heat in these structures which is subsequently released into the fluid. The resulting temperature gradient created in the system drives the colloids from a random Brownian motion to a directed motion towards the metallic structure. This motion is attributed to thermophoretic migration of colloids. Using this process, we create a long range assembly of colloids whose dimension can be controlled by modulating several parameters such as polarization of the incoming beam, size of the colloids as well as the surface/volume ratio of the heating structure. We also show directional pulling and transport of a single colloid from one location to another by exciting surface plasmon polaritons which are confined quasi-one dimensionally. Next, we focus our attention towards microbubble formation and consequent manipulation of nanostructures. While the above discussed methods work on the basis of thermophoresis, the microbubble based manipulation relies on temperature gradient dependence of the surface tension on the surface of the bubble. The generation of a microbubble creates strong marangoni flows which helps to move the particles at high speeds, which is usually not possible if we rely only on the convection currents induced by heating a nanostructure alone. Finally we will conclude by discussing some challenges in the field and future direction of the work.