Justus C. Ndukaife

Local Heating with Lithographically Fabricated Plasmonic Titanium Nitride Nanoparticles

Titanium nitride is considered a promising alternative plasmonic material and is known to exhibit localized surface plasmon resonances within the near-infrared biological transparency window. Here, local heating efficiencies of disk-shaped nanoparticles made of titanium nitride and gold are compared in the visible and near-infrared regions numerically and experimentally with samples fabricated using e-beam lithography. Results show that plasmonic titanium nitride nanodisks are efficient local heat sources and outperform gold nanodisks in the biological transparency window, dispensing the need for complex particle geometries.

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.

Plasmonics—turning loss into gain

The optical losses usually associated with plasmonic materials could be used in applications The light-induced electronic excitations that occur at the surface of metals—plasmons—provide the extraordinary ability to confine electromagnetic energy to the subwavelength scale. Such extreme optical confinement can enhance the light-matter interaction and enable miniaturized optical and optoelectronic devices. However, this confinement requires that plasmonic materials possess free carriers, which unavoidably results in light being lost or absorbed in the system (1). This optical loss has hampered the realization of device designs with ultracompact, on-chip optical components and nanometer-scale resolution imaging. Because of the detrimental effects of plasmonic losses, several avenues are being explored to mitigate the high absorption, such as using gain to compensate for the losses, and synthesizing alternative low-loss plasmonic materials (2). Rather than continuing to pursue low-loss plasmonics approaches, we draw attention to the benefit of losses by high-lighting recent groundbreaking discoveries that were enabled by intrinsic losses in plasmonic systems.

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.

On-demand rapid transport and stable trapping of nanoparticles of nanoparticles by a hybrid electrothermoplasmonic nanotweezer(Conference Presentation)

In plasmon nano-optical tweezers, plasmonic nanoantennas are illuminated to generate highly localized and enhanced electromagnetic field in the vicinity of the nanoantenna. The highly localized and enhanced electromagnetic field creates much stronger optical gradient forces and tighter potential wells for confining particles than in conventional optical tweezers, thus providing a means to trap nanoscale objects and molecules. This approach have been successfully applied for trapping small particles such as protein molecules. However a long standing problem in this field is how to rapidly load the potential well without relying on Brownian diffusion. Conventional design rely on Brownian diffusion to load the trap, which is very slow and could take several minutes to hours depending on the concentration of the nanoscale objects. Furthermore since the plasmonic trapping sites are pre-patterned on a substrate, current plasmonic nanotweezers suffer from the problem of lack of dynamic control over the particles in the trap. Recently we have addressed these challenges by introducing a novel design paradigm known as the Hybrid Electrothermoplasmonic Nanotweezer (HENT)1, where the intrinsic photo-induced heating of the plasmonic nanoantenna is combined with an applied AC electric field to induce a large scale microfluidic flow on-demand. The microfluidic flow enables rapid delivery of suspended nanoparticles to an illuminated plasmonic nanoantenna where they are trapped within a few seconds. In this talk I will discuss the working principle of HENT, as well as HENT-based nanotweezers utilizing alternative plasmonic materials.

Plasmon-assisted optoelectrofluidics

By harnessing the photo-induced heating of a single plasmonic nanostructure and AC E-field in our research at the interface between plasmonics and optofluidics we demonstrate on-demand fluid flow control with unparalleled micron per second-scale velocities.

Electrothermoplasmonic flow for plasmon-assisted optical trapping (Presentation Recording)

Plasmonic nanostructures, which support highly localized and enhanced electromagnetic field are now actively researched as a means for efficient trapping of nanoscale objects, not addressable by conventional diffraction-limited optical tweezers. An issue of critical concern is how to efficiently transport and deliver the suspended particles to the illuminated plasmonic nanostructure. There are primarily two main approaches that researchers employ for trapping of particles with plasmonic nanostructure(s) on a substrate. The first approach involves illuminating arrays of closely-spaced plasmonic nanostructures. However resonant illumination of the nanostructures results in collective heating and this produces strong fluid convection that exerts drag forces on the particles. Elucidating the roles of these heating-induced forces and optical gradient forces arising from plasmonic field enhancement have so far remained elusive. The other scheme involves illuminating a single plasmonic nanostructure. However, due to the absence of thermoplasmonic convection in this case, the dynamics of the suspended particle to be trapped becomes dictated by Brownian motion- an inherently slow process. We will discuss a new fluid flow mechanism, which we have termed electrothermoplasmonic (ETP) flow to resolve this dilemma. ETP flow harnesses intrinsic plasmonic heating combined with AC electric field to generate on-demand fluid and particle transport, which means that particles could be rapidly transported for trapping in sub-wavelength plasmonic hotspots only when desired, and without any competition between heating-induced forces and optical gradient forces. These new capabilities certainly provide new directions for research in the field of plasmon-assisted optical trapping, which will be discussed.