Plasmonic optical tweezers can form characteristic micropatterns
Optical trapping techniques have been extensively investigated as noninvasive and versatile manipulation tools. With a conventional optical tweezer using a focused laser beam, micrometer-sized particles such as artificial beads and living cells can be stably trapped and manipulated at a focal point.1,2 However, the techniques still have disadvantages for stable trapping of smaller nanoparticles. Specifically, a high laser intensity (≥MW/cm2) is needed to overcome the Brownian motion of nanoparticles, and the diffraction limit of incident light prevents the spatial resolution of the trapping from being finer than several hundreds of nanometers.
We can potentially overcome these disadvantages by combining optical trapping with metallic nanostructures to enhance the electromagnetic field of the incident light with localized surface plasmons (LSPs).3 An LSP, a kind of polariton, is a cooperative oscillation of free electrons in a metal. When plasmonic nanostructures are irradiated with resonant light, the electromagnetic field of the light is strongly localized in the nanostructured area, whose size is much smaller than the light's diffraction limit.4Consequently, plasmonic structures generate much larger trapping forces using incident light of lower intensity than that used in conventional optical tweezers.
Along with Tatsuya Shoji, Mariko Toshimitsu, and my other coworkers, I report such LSP-based optical trapping of organic nanoparticle systems, which formed interesting micropatterns.5,6 In one study, we investigated LSP-based optical trapping of dye-doped polystyrene nanospheres (500nm diameter) by means of fluorescence microspectroscopy and microscopic real-time video observation.5 We saw that the nanospheres were optically trapped to be closely packed on the plasmonic substrate, and finally formed a 2D hexagonal microassembly at the LSP excitation area (see Figure 1). Thus, by using LSPs, we successfully trapped nanoparticles stably at much lower laser intensity (∼kW/cm2) than conventional optical tweezers require. The microhexagon was a kind of colloidal crystal, with the hexagonal pattern generated by the LSP-based radiation pressure and self-organization.
The polystyrene nanospheres are ‘hard’ polymer beads. We also tested LSP-based trapping of soft matter, namely, flexible polymer molecules in the form of microgels. Again we found intriguing micropatterns formed.6 We used poly(N-isopropylacrylamide) microgels (diameter ∼250nm) labeled with a fluorescent probe, on a nanostructured gold substrate. This polymer is thermoresponsive, changing its volume in response to the solution temperature. When the temperature increases to ∼10K above room temperature, the polymer microgel dehydrates and shrinks to 50nm.
In this work, the fluorescence intensity from the focal point of the sample polymer increased in accordance with LSP excitation, indicating an increase of the polymer concentration caused by LSP-based optical trapping. Upon LSP excitation, the polymer microgels produced a characteristic ring pattern of larger diameter than the LSP-excited area. Furthermore, by optimizing the irradiation condition, we successfully formed a characteristic multiple-ring pattern (see Figure 2).
We are considering the mechanism underlying the characteristic ring pattern formation. In general, laser irradiation that is resonant with LSP excitation in the nanostructured gold surface generates heat as well as an electromagnetic field enhancement. Therefore, upon LSP excitation, the laser produces not only the LSP-enhanced trapping force but also other competing forces: thermal convection and thermophoresis, the latter being a force that pushes small particles in a temperature gradient. We determined that in our experiment, the local temperature rise around the gold surface was ∼10K at 1.0kW/cm2 irradiation, resulting in a huge temperature gradient (0.5K/μm). In such a nonequilibrium situation, the thermophoresis force is no longer negligible.
The thermophoresis works as a repulsive force that pushes a small particle from a hotter region to a colder region, competing with the attractive trapping force. On the other hand, thermal convection assists LSP optical trapping by a mixing effect. It supplies the polymer molecules toward the LSP excitation area from outside of that area. That is, the LSP-enhanced trapping force and these thermal effects simultaneously play significant roles in the trapping. According to our theoretical analysis, both the LSP-based radiation force and the thermophoresis force on a microgel nanosphere are on the order of a few femtonewtons. These thermal forces also had significant effects in our experiments trapping polystyrene nanospheres.5
In summary, we have demonstrated LSP-based optical trapping of both hard and soft polymer objects. We observed characteristic micropattern formations (the 2D closely packed polystyrene hexagon and the ring of polymer microgels) on the nanostructured plasmonic substrate. We found that photothermal effects (thermal convection and thermophoresis) simultaneously induced by laser irradiation may play an important role in the trapping and micropattern formation. For desirable optical trapping, it is very important to precisely control these three competing forces, and such studies are currently in progress in our laboratory.
Yasuyuki Tsuboi received his PhD from Osaka University under the supervision of H. Masuhara in 1995. He started his academic career at the Kyoto Institute in 1996. Currently he is an associate professor in the Graduate School of Science at Hokkaido University, investigating plasmonics and optical trapping.