Optical and Optothermal route towards dynamic nanomanipulation in fluids
Manipulation of materials with light has led to significant breakthroughs in biology, material science and soft condensed matter physics. Conventional optical tweezer, which is effectively a tightly focused laser beam, can trap and transport microscale particles suspended in any fluid. The main feature that makes this technology unique is its ability to remotely manipulate colloidal objects as desired, which allows independently controlled manipulation of individual specimens along chosen trajectories within a fluidic volume. This powerful technology is only limited by diffraction limited focusing that necessitates high optical intensities to trap nanoscale objects. To overcome this limitation, it is possible to incorporate plasmonics in tweezing applications such as to have strongly localised and enhanced electromagnetic intensity in the near-field. Plasmonic tweezers are undoubtedly much more efficient than optical tweezers in terms of optical power requirement and significantly relax the constraints imposed by diffraction-limit. Unfortunately, they often rely on the slow diffusion of colloids and therefore limited by speed, which restricts this otherwise versatile manipulation technology. Additionally, trapping happens at specific locations of a two-dimensional substrate that needs to be nano-patterned. Therefore, unlike traditional laser tweezers, these devices cannot be operated dynamically throughout the bulk fluidic volume. In this dissertation, we propose and demonstrate possible solutions to address some of the major problems of plasmonic-based nanomanipulation. As we show here, by integrating plasmonic nanostructures with magnetic helical micropropellers, plasmonic tweezers can be remotely manoeuvred within the bulk fluid while trapping single as well as multiple particles (cargo). The working range of these mobile nanotweezers (MNTs) match with state of the art plasmonic tweezers, and in addition allow selective pick-up, transport, release and positioning of sub-micron objects with unprecedented speed and accuracy. It is also possible to drive the MNTs in three dimensions as well as stamp them temporarily onto the chamber surface where they may be used as regular static plasmonic traps. Besides, the MNTs can be used in standard microfluidic chambers to load pre-existing traps and applicable to a variety of materials, including bacteria and fluorescent nanodiamonds. Later, we have also explored the plasmon induced heating effect in MNTs due to resonant absorption of the incident illumination and have considered different strategies for their effective thermal management. In an alternate strategy, plasmonic nanodisks promising new approach toward allfabricated over dielectric microrods provide a optical active nanomanipulation. These hybrid structures can be manoeuvred by conventional optical tweezers and simultaneously generate strongly confined optical nearfields i n their vicinity, functioning as nearfield traps themselves for colloids as small as 40 nm. These “Active Colloidal Tweezers” (ACTs) can be used to transport nanoscale cargoes even in ionic solutions at optical intensities lower than the damage threshold of living micro organisms, and additionally, allow parallel and independently controlled manipulation of different types of colloids, including fluorescent nanodiamonds and magnetic nanoparticles. Finally, we present the first experimental demonstration of thermoplasmonic manipulation , where the trapping and heating regions have been physically separated within the microfluidic volume. This was achieved by engineering the convective flow to drive colloids close to the trapping zone. As a result, effective d elivery of colloidal cargoes to load the standard static plasmonic traps is possible with optothermal forces alone; while reducing the risk of thermal damage of the trapped specimen significantly. The crucial role of thermal effects through convection and thermophoresis for developing a micrometre per second fluid flow is confirmed experimentally, as well as through numerical simulations.