High-Resolution Large-Ensemble Nanoparticle Trapping with Multifunctional Thermoplasmonic Nanohole Metasurface.

The intrinsic loss in a plasmonic metasurface is usually considered to be detrimental for device applications. Using plasmonic loss to our advantage, we introduce a thermoplasmonic metasurface that enables high-throughput large-ensemble nanoparticle assembly in a lab-on-a-chip platform. In our work, an array of subwavelength nanoholes in a metal film is used as a plasmonic metasurface that supports the excitation of localized surface plasmon and Bloch surface plasmon polariton waves upon optical illumination and provides a platform for molding both optical and thermal landscapes to achieve a tunable many-particle assembling process. The demonstrated many-particle trapping occurs against gravity in an inverted configuration where the light beam first passes through the nanoparticle suspension before illuminating the thermoplasmonic metasurface, a feat previously thought to be impossible. We also report an extraordinarily enhanced electrothermoplasmonic flow in the region of the thermoplasmonic nanohole metasurface, with comparatively larger transport velocities in comparison to the unpatterned region. This thermoplasmonic metasurface could enable possibilities for myriad applications in molecular analysis, quantum photonics, and self-assembly and creates a versatile platform for exploring nonequilibrium physics.

Long-range and rapid transport of individual nano-objects by a hybrid electrothermoplasmonic nanotweezer.

Plasmon-enhanced optical trapping is being actively studied to provide efficient manipulation of nanometre-sized objects. However, a long-standing issue with previously proposed solutions is how to controllably load the trap on-demand without relying on Brownian diffusion. Here, we show that the photo-induced heating of a nanoantenna in conjunction with an applied a.c. electric field can initiate rapid microscale fluid motion and particle transport with a velocity exceeding 10 µm s(-1), which is over two orders of magnitude faster than previously predicted. Our electrothermoplasmonic device enables on-demand long-range and rapid delivery of single nano-objects to specific plasmonic nanoantennas, where they can be trapped and even locked in place. We also present a physical model that elucidates the role of both heat-induced fluidic motion and plasmonic field enhancement in the plasmon-assisted optical trapping process. Finally, by applying a d.c. field or low-frequency a.c. field (below 10 Hz) while the particle is held in the trap by the gradient force, the trapped nano-objects can be immobilized into plasmonic hotspots, thereby providing the potential for effective low-power nanomanufacturing on-chip.

Photothermal heating enabled by plasmonic nanostructures for electrokinetic manipulation and sorting of particles.

Plasmonic nanostructures support strong electromagnetic field enhancement or optical "hot spots" that are accompanied by local heat generation. This heating effect is generally seen as an obstacle to stable trapping of particles on a plasmonic substrate. In this work, instead of treating the heating effect as a hindrance, we utilized the collective photoinduced heating of the nanostructure array for high-throughput trapping of particles on a plasmonic nanostructured substrate. The photoinduced heating of the nanostructures is combined with an ac electric field of less than 100 kHz, which results in creation of a strong electrothermal microfluidic flow. This flow rapidly transports suspended particles toward the plasmonic substrate, where they are captured by local electric field effects. This work is envisioned to have application in biosensing and surface-enhanced spectroscopies such as SERS.