High-Resolution Large-Ensemble Nanoparticle Trapping with

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High-Resolution Large-Ensemble Nanoparticles Trapping with Multifunctional Thermoplasmonic Nanohole Metasurface Justus C Ndukaife, Yi Xuan, Agbai George Agwu Nnanna, Alexander V. Kildishev, Vladimir M. Shalaev, Steven T. Wereley, and Alexandra Boltasseva ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00318 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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High-Resolution Large-Ensemble Nanoparticles Trapping with Multifunctional Thermoplasmonic Nanohole Metasurface Justus C. Ndukaife,†‡* Yi Xuan,‡ Agbai George Agwu Nnanna,|| Alexander V. Kildishev,‡ Vladimir M. Shalaev,‡ Steven T. Wereley,¶* Alexandra Boltasseva‡* †

Department of Electrical Engineering and Computer Science, and Vanderbilt Institute of

Nanoscale Science and Engineering, Vanderbilt University, Nashville, TN, USA ‡

School of Electrical and Computer Engineering and Birck Nanotechnology Center,

Purdue University, West Lafayette, IN 47907, USA ¶

School of Mechanical Engineering and Birck Nanotechnology Center, Purdue University,

West Lafayette, IN 47907, USA ||

Water Institute, Purdue University Northwest, Hammond, IN 46323, USA

KEYWORDS: thermoplasmonics, nano-optical tweezers, metasurface, nanohole-array, particle trapping, self-assembly

ABSTRACT: 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

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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 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 extra-ordinarily 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, self-assembly and creates a versatile platform for exploring non-equilibrium physics.

The reduced dimensionality of optical metasurfaces and the metasurface-based planar photonics concepts lead to functionalities and applications that are distinctly different from those achievable with bulk three-dimensional metamaterials.1,2 In addition to achieving control over the properties of light using an arrangement of subwavelength resonators at an interface,3–6 metasurfaces for thermal and acoustic fields have been also successfully developed.7,8 Several advanced optical components including lenses,9,10 holograms,11 invisibility carpet cloaks,12 polarimeters13 and optical tweezers14 have been reported. However, the high intrinsic loss15,16 of the constituent plasmonic materials in the visible wavelength spectrum has been considered


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detrimental for the realization of efficient flat plasmonic optical elements.17 In order to bypass the loss in plasmonic systems, significant effort has gone into developing metasurfaces that are based on high refractive index dielectric nanostructures with low loss. 18–29 In contrast to conventional plasmonic systems, the intrinsic loss in metal in our thermoplasmonic metasurface (TPM) platform is used to advantage, providing control of nanoparticle trapping. The platform enables directed many-particle array assembly, providing the flexibility to tune inter-particle spacing in an integrated platform. Our TPM enables shaping the temperature field distribution by harnessing the photothermal response of subwavelength plasmonic nanohole elements. Upon illumination of the metasurface, distinct combinations of thermal and optical landscapes are created to enable many-particle assembly on demand. The ability to achieve robust large-ensemble many-particle trapping is of great importance for directed self-assembly,30,31 plasmonic nano-biosensors in rapid capture and identification of target pathogens techniques, overcoming the diffusion limit,32–35 as well as the control of automonous micro and nanomachines. Previously, it has been reported that many-particle trapping cannot be achieved when nanoantennas are illuminated in an inverted position (see figure 1 b), i.e. when trapping is performed against gravitational force.36 So far, only single particle trapping have been achieved in an inverted configuration.37 In ref36 it was argued that many-particle assembly would not be possible in an inverted configuration because of the repulsive thermophoretic38,39 force in the axial direction. Several works have reported the agglomeration of microscale particles in an upright configuration using thermophoretic and convection forces.40–44 Here, we experimentally demonstrate that it is the inverted design of our platform that allows us to integrate fully-controlled many-particle assembling with tunable interelement spacing as shown in figure 1. In the inverted configuration (see panel (i) in figure 1(b)),


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the laser beam passes through the suspension of particles before reaching for the nanohole array. On the other hand, in the standard configuration (panel (ii) in figure 1(b)), the laser beam propagates through the substrate and only the portion that is transmitted through the nanohole array is absorbed by the nanohole array resonators with very low photothermal conversion. The inverted design is also advantageous because the light source that is used to excite the fluorescent particles as well as the fluorescence light emitted by the particles does not need to pass

through

the

nanohole

array

before

reaching

the

detection

camera.

The synergy of optical and thermal landscapes in our TPM provides features that are drastically different from those observed in our previous article on Hybrid Electrothermoplasmonic Nanotweezer45 where trapping of only one or two nanoparticles at the plasmonic hotspots was demonstrated. The difference arises because the collective heating effect of the nanohole array produces a strong buoyancy driven convection flow, which was absent in the Hybrid Electrothermoplasmonic Nanotweezer, where buoyancy-driven convection is suppressed.

RESULTS AND DISCUSSIONS Large-ensemble trapping of nanoparticles with nanohole array against gravity Figure 1 depicts our multifunctional TPM platform. The plasmonic resonators are comprised of complementary nanoantennas, circular nanoholes perforated in a 125 nm thick gold film. The holes are illuminated by a linearly polarized laser beam with a wavelength of 1064 nm.


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Figure 1. Thermoplasmonic metasurface (TPM) platform. (a) Trapping forces present in the TPM platform when the laser illumination is ON, and AC electric field is either ON or OFF. The electrothermoplasmonic (ETP) flow arises from the combined action of the induced thermal gradient and applied AC electric field. Many-particle trapping is accomplished by illuminating the part of the metasurface containing the closely spaced array of nanoholes (b) Depiction of the inverted and standard configurations. Blue arrows depict the direction of the incident laser illumination.


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The experiments were performed using 220 nm diameter nanoholes with lattice separations of 1100 nm, 800 nm and 600 nm. The array of gold nanoholes was illuminated with a 15 mW laser beam focused to a diffraction-limited spot of 2 µm as illustrated in figure 1a. The tracer particles employed are fluorescent 1 um and 200 nm diameter polystyrene beads that were suspended in a water solution with a low ionic conductivity of 0.4 S/m. The 1 um size bead was chosen because it is large enough that the particle-particle separation distance in the assembly can be resolved, while the 200 nm was the smallest size we imaged and trapped in this device platform. The permittivity of the water solution is taken as 80 F/m.46 There are several crucial physical phenomena that enable the trapping of nanoparticles in our integrated device, which makes it possible to trap the nanoparticles against gravitational forces. The illumination of the nanohole array gives rise to the excitation of in-plane Bloch mode SPPs47–49 and localized surface plasmon (LSP) resonance of the individual nanoholes as depicted in figure 2. These two phenomena result in concentrating the electromagnetic field near the rims of the nanoholes and in the region between the nanoholes. This in turn increases the total power dissipation density within the structured plasmonic film and hence the photothermal conversion efficiency of the nanohole array. Designing the nanohole array to support the excitation of the surface plasmon waves in the vicinity of the nanohole array (figure 2) enables an improved photothermal conversion efficiency and hence higher temperature rise when the nanohole array is illuminated. As we found, the temperature gradient induced in the fluid by the illuminated plasmonic nanohole array results in a gradient in the density of the fluid, which in turn gives rise to thermoplasmonic convection.50


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Figure 2. (a) Local electric field distribution in the vicinity of the thermoplasmonic nanohole metasurface (top view). The dipolar localized surface plasmon field and the Bloch surface plasmon that results in field coupling between the nanoholes are depicted in the figure. The induced temperature gradient also results in a gradient in the fluid’s electrical conductivity and permittivity. When an AC electric field (80,000 V/m, 60 KHz) is applied to the fluid under this condition of local inhomogeneity, a long-range electrothermoplasmonic flow

45

that is


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superimposed on the thermoplasmonic convection flow is additionally induced in the fluid. In addition to the electrothermoplasmonic flow, there is also an AC electro-osmotic flow that is induced when only the AC electric field is applied. The AC electro-osmotic flow arises because when the AC field is applied, the nanoholes induce a local perturbation of the AC electric field lines resulting in a tangential component of the AC electric field. This tangential component of the AC electric field drives the charges in the electrical double layer to induce localized AC electro-osmotic flows.

51–53

To ascertain the velocity profile of the induced AC electroosmotic

flow, we performed the numerical simulation detailed below. We begin by first solving for the AC electric field distribution to obtain the tangential component. This is accomplished by solving the Laplace equation given by: = 0 (1) ∇ ϕ where ϕ(t) is the electrostatic potential, and the electric field = −∇ϕ (2) The nanohole array distort the AC electric field to establish a tangential component that would act on the diffuse charges in the electrical double layer. The velocity of the AC electroosmotic flow is determined by solving the incompressible NavierStokes equation given by:54 (( ) ∙ ∇)( ) + ∇ − ∇ ( ) = (3) The forcing term F is set to zero and Helmholtz-Smoluchowski slip velocity is applied at the boundaries of the regions surrounding the nanoholes. The slip velocity is given by:55


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〈 〉 =

〈 〉 (4)

Where is the tangential component of the AC electric field obtained from equation (2), and is the zeta potential of the medium. The equations are solved numerically with the finite element method using Comsol Multiphysics software package. The velocity profile of the AC electroosmotic flow is depicted in figure 3.

Figure 3. Velocity profile of the induced AC electroosmotic flow formed by the distortion of the AC electric field by the nanoholes. The induced local AC electroosmotic flow is short range and can only act on particles that are close to the electrode surface. The electrothermoplasmonic flow induced when both the laser induced heating and AC electric field are applied on the other hand is long range, extending over


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several hundreds of microns. We experimentally measured the global electrothermoplasmonic vortex using micro-particle image velocimetry. The interplay of these flows provides an opportunity to achieve nanoparticle control. The electrothermoplasmonic flow act on the individual nanoparticles of radius a, exerting a drag force, F = 6πηaν, where η and ν respectively denote the fluid viscosity and velocity as depicted in figure 1a. The particles carried in the electrothermoplasmonic flow are brought close to the nanohole array, where they are influenced by the flow field of the AC electro-osmotic flow and optical gradient trapping force, which acts to assemble them together. Under the condition when both the AC electric field and laser illumination are applied, the forces that enable trapping in the lateral direction include the drag forces from the electrothermoplasmonic flow Fetp-r, local AC electro-osmotic flow and the radial component of the optical gradient force Fgrad-r. These forces overcome the lateral component of the opposing thermophoretic force Fth-r, and the dipole repulsion force between the polarized nanoparticles in the lateral direction. The thermophoretic force in this system is repulsive and pushes the particles away from the hot nanohole array. In the presence of surfactant solution, it is possible to induce an attractive thermophoretic force that would attract particles towards the hot area.56–58 In the vertical direction, the axial component of the electrothermoplasmonic flow acts to pull the nanoparticles away from the surface of the nanohole array and must be overcome by the plasmonic gradient force and the particle surface interaction force collectively represented by Ftrap-z for trapping to occur. These multiple forces enable trapping of the nanoparticles when both the AC field and laser illumination are applied. If the AC field is turned OFF, while the laser illumination is still ON, the thermoplasmonic convection drag force kicks-in, which along with the optical gradient forces act on the particles to enable agglomeration of the nanoparticles (figure 4a). Figure 4a shows the experimentally measured radial velocity vector plots of the


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electrothermoplasmonic flow induced when both the laser excitation of the plasmonic nanohole array and AC field are applied. The experimental measurement of the radial velocity vectors was performed

using

a

technique

known

as

micro-particle

image

velocimetry.59

The

thermoplasmonic convection flow induced when only laser illumination of the plasmonic nanoholes is present is also superimposed on the same plot. Temperature gradients along the surface of the colloidal particles may also drive additional interfacial hydrodynamic flows by thermo-osmosis.60 However the electrothermoplasmonic flow induced in the presence of AC electric field and photo-induced thermal gradient generates a flow field with a larger radius of action, extending to over 100 um in comparison to thermo-osmotic flows reported in ref 60. The sequence of agglomeration of multiple nanoparticles by the background thermoplasmonic convection flow, when only laser illumination is applied is depicted in figure 4b (and supplementary video_1). This spontaneous agglomeration of the nanoparticles in an inverted configuration against gravity when the laser illumination is applied does not occur in the region of the TPM with a single nanohole or the unpatterned part. It is only achievable in the section of the TPM with plasmonic nanohole array. The particles can be trapped at a much faster rate when both the laser illumination and AC field are applied due to the fast electrothermoplasmonic flow. The trapping effect still occurs for different nanohole array periodicities. We also performed experiments with holes having a lattice constant of 600 nm and 800 nm (while keeping the diameter of 220 nm the same), and we observed that the 200 nm diameter beads were trapped for all lattice constants as depicted in figure 1 of the supporting information.


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Figure 4. Large-ensemble trapping (a) Experimentally measured radial velocity distribution of the electrothermoplasmonic flow induced by laser–induced heating of the nanohole array (having a periodicity of 1100 nm) and an applied AC electric field. The thermoplasmonic convection flow

induced

by

laser

heating

of

the

nanohole

array

is

superimposed.

The


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electrothermoplasmonic flow has a large magnitude and spatial extent in comparison with the buoyancy-driven convection flow. (b) Sequence of transport and trapping of 200 nm diameter polystyrene beads on the surface of the plasmonic nanohole array when only laser illumination is applied. The polystyrene beads have a higher density than the fluid (water). This spontaneous agglomeration does not occur when either the planar film section or a single nanohole is illuminated. Tuning inter-element spacing Another attractive feature of the developed TPM platform operating in the large ensemble trapping regime is the ability to dynamically tune the inter-particle spacing in the assembly. This feature becomes evident in figure 4c and supplementary video_2, where we show that when the AC field is ON, the particles become polarized and a dipole repulsion force is induced between the particles leading to an increased inter-particle spacing between the agglomerated particles. If the AC field is OFF while the laser illumination is ON, the dipole repulsion force disappears, and the lateral component of the thermoplasmonic convection flow acts to ensure a more tightly packed cluster. The process is reversible without any hysteresis. Thus using our TPM platform, we can dynamically switch the inter-element affinities to create different inter-element spacing. The ability to tune the inter-element spacing by switching off the AC field was not observed in our studies (ACS Nano article)

61

using an array of closely spaced plasmonic nanodots.

Furthermore, in ref 61 the assembled particles are expelled from the surface of the nanopillars when the AC field is turned OFF, while the laser is still ON. This is because the significant repulsive thermophoretic force expelled the particles from the trap.


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Figure 5. Controlling the particle assembly. (a) If the AC electric field is applied, the induced dipole repulsion force increases the interparticle spacing between the particles in the assembly. A compact assembly is achieved by turning OFF the AC field, while the laser is ON and the process is reversible. (b) Depiction of the primary forces that dictate the spacing between the particles in the assembly when both laser illumination and AC field is applied. The AC field polarizes the particles and establish a dipole-dipole force between the particles that is repulsive in a perpendicular electric field. The drag force from the electothermoplasmonic flow and convection act to push the particles together. Blue and red horizontal arrows depict the lateral


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components of drag force from buoyancy-driven convection and electrothermoplasmonic flow. (c) The spacing between the particles depends on the frequency of the applied AC field. The spacing reduces as the AC frequency is increased. The AC frequencies tested are below the charge relaxation frequency. The particle size used for these experiments was 1 um polystyrene beads. We also investigated the effect of the applied AC frequency and amplitude. The particleparticle spacing also depends on the frequency of the applied AC field. This spacing is primarily dictated by the balance between the dipole-dipole interaction force and the drag force from the electrothermoplasmonic flow. The dipole-dipole interaction force between neighboring polarized particles is given by 62,53

Where a is the particle radius, α is the particle polarizability, r is the center-to-center spacing between particles, and θ is the angle between the particles with respect to the direction of the electric field (E). In our device configuration, the applied AC electric field is aligned orthogonal to the line connecting particle centers ( = 90). Hence, the repulsion (radial) force between the

particles is

. The dipole-dipole repulsive interaction force is also

frequency dependent due to the square of polarizability term

that depends on the AC field

frequency. This force opposes the drag force from the electrothermoplasmonic flow and convection, which acts to push the particles together (figure 5b). At AC frequencies below the charge relaxation frequencies, the electrothermoplasmonic flow is frequency-independent. The


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charge relaxation frequency is given by !"# = $(2&' = 90 +,-, where $ = 0.4 01/0 and ' = 80 4/0 are respectively the electric conductivity and permittivity of the fluid. Hence changing the AC frequency will change the dipole-repulsion force and hence the particle-particle separation as depicted in figure 5c. The particle-particle separation is higher at lower frequencies and decreases as the AC frequency increases. At any given AC frequency, once the AC field is turned OFF, while the laser illumination is still ON, the particle-particle separation reduces further, due to the radial thermoplasmonic convection that pushes the particles closer together. This spacing control provides an additional knob to expand the complexity of structures that can be fabricated via directed self-assembly. The particle-particle separation is independent of AC field magnitude. This is because both the dipole-dipole interaction (repulsive) force and the electrothermoplasmonic flow (attractive) force scale as the square of the applied AC field magnitude. Hence changes in the AC field strength are followed by the equal changes in the two counteracting radial forces of electrothermoplasmonic drag and dipole-dipole repulsion interaction such that their influence cancels out. This ability to tune the inter-element spacing can be used to tune the far-field diffractive scattering of particles to produce different spectral responses. Furthermore, it can also be utilized to assemble colloidal microlenses to make photonic nanojets to refocus an optical beam at multiple points for use in imaging, optical trapping and microscopy.63–65 The ability to control the spacing between the colloidal microlenses would allow to control the distribution of the resulting photonic nanojets.


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Extra-ordinary Electrothermoplasmonic Flow Another important application of our approach is the concept of extraordinary electrothermoplasmonic flow. Early work by Ebessen and co-workers47 showed that enhanced optical transmission through an array of subwavelength nanoholes can be achieved due to the excitation of Bloch mode surface plasmon polaritons. By similar analogy to extraordinary optical transmission, we show that nano-hole array enables extra-ordinary electrothermoplasmonic flow with transport velocity that well-exceeds what can be achieved using an un-patterned thin film or by a single nanohole. The extra-ordinary electrothermoplasmonic flow is mediated by strong absorption induced by the localized surface resonance of the nanoholes and Bloch surface plasmon polaritons excited by the nanohole array giving rise to a high photothermal efficiency. By comparing the magnitude of the generated electrothermoplasmonic flow radial velocity for the planar thin film, single nanohole and nanohole array regions, it is evident that the highest velocity was achieved in the region of the TPM with nanohole array. The maximum velocity of 35 µm/s achieved with the nanohole array is about three times higher than the velocity generated on the planar thin film surface as shown in figure 6. Furthermore, with only laser illumination applied, a significant thermoplasmonic convection with maximum radial velocity of 2.5 µm/s was achieved with the nanohole array, while no measurable velocity was obtained when the planar film or single nanohole was illuminated. The electrothermoplasmonic flow velocity field generated when the nanohole array is illuminated and the AC field applied also has a large radius of action with a velocity of roughly 20 µm/s at a distance that is 100 µm away as depicted in figure 6. On the other hand, when only laser illumination is applied, the thermoplasmonic convection flow generated by the same nanohole array under similar illumination condition is


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only approximately 1 µm/s at a distance of 60 µm from the illuminated spot. Thus, applying an AC electric field and laser illumination provides a higher efficiency for the rate of particle capture in the trap, even at low particle concentrations. Our work shows that plasmonic nanoholes can serve as subwavelength thermoplasmonic elements for enhancing and shaping the temperature field distribution on a planar surface, which is compatible with planar nanofabrication technologies. The ability to shape and enhance the microfluidic flow field using thermoplasmonic metasurface will be beneficial towards the design of integrated plasmo-nanofluidic chips for miniaturized chemical and biological analysis systems.

Figure 6. Extraordinary electrothermoplasmonic flow. The nanohole array enables an enhanced electrothermoplasmonic flow that is higher than the velocity induced when the planar film or a single nanohole is excited. With the nanohole array, a maximum thermoplasmonic convection velocity of 2.5 µm/s was experimentally measured when only the laser illumination was applied


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and the AC was turned off. For a single nanohole or a planar film, no thermoplasmonic convection flow was observed under similar illumination condition. CONCLUSION We experimentally demonstrate a nanoparticle trapping approach by utilizing a thermoplasmonic nanohole metasurface for high-throughput, many-particle assembly in a lab-on-a-chip platform. We show that contrary to previous reports, the assembly of the nanoparticles can be performed against the gravitational force in an inverted configuration. Furthermore, we report an extraordinarily enhanced electrothermoplasmonic flow in the region of the developed nanohole TPM, thus proposing the means to mold and amplify the flow field in plasmofluidic devices. The thermoplasmonic metasurface platform presented in this work is a universal trapping system that enables the immediate implementation of numerous exciting applications across multiple research frontiers. The use of plasmonic nanohole array for large-ensemble trapping of nanoparticles offers a number of advantages. First of all, because of the highly sensitive nature of the Bloch mode surface plasmons, our platform may be used to develop nanobiosensors that can perform the multiple functions of analyte capture, delivery and detection. Furthermore, the ability to rapidly deliver target analytes adds a vital capability to rapidly modulate the molecular environment, a key requirement for molecular analysis. To enhance their emission efficiency, quantum emitters could be also trapped and coupled to the nanohole antennas. To sum up, designer thermoplasmonic metasurfaces could be used to enable improved functionalities in labon-a-chip devices to impact several fields including quantum photonics, single molecule analysis and self-assembly. Our work shows that the intrinsic loss in plasmonic systems is not always detrimental but could be efficiently harnessed to work in synergy with plasmonic field


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enhancement to realize device designs for applications in nanophotonics, nanomanufacturing, life science, and quantum photonics. METHODS The plasmonic nanohole array was fabricated via template stripping process. The process begins coating a clean silicon substrate with 600 nm layer of ZEP 520A photoresist. The resist is lithographically defined using electron beam lithography and subsequently developed for 2 minutes and 30 seconds using ZED N50. The defined resist structures were used as a mask to etch the silicon substrate to create nanoholes in the wafer reactive ion etching. After the silicon etch, the resist was removed using ZDMAC, and the sample was rinsed with acetone and IPA. The depth of the grooves in the silicon substrate was about 400 nm. Subsequently gold film of 125 nm thickness was deposited on the silicon substrate. One drop of transparent UV curable epoxy was placed on the sample and another clean substrate of ITO-coated glass was placed on the sample subsequently. Finally, the cascaded sample was exposed to a UV light source for 20 minutes. After the curing process, a blade was used to separate the two substrates thereby transferring the nanohole pattern to the ITO-coated glass substrate. The silicon template can be reused by rinsing in a 1:3 ratio of sulphuric acid and hydrogen peroxide.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. The following files are available free of charge. Supporting information for the manuscript


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Supplementary video_1: showing large ensemble trapping of 200 nm polystyrene beads on the plasmonic nanohole array surface. Supplementary video_2: showing the ability to tune the particle-particle separation distance by switching the AC field ON and OFF. AUTHOR INFORMATION Corresponding Authors Correspondence: [email protected], [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources NSF MRSEC DMR-1120923, AFOSR under grant 29017320-51649-D, and DARPA/DSO Extreme Optics and Imaging (EXTREME) Program Award HR00111720032, Vanderbilt University Start-up Fund, and Purdue Water Institute. ACKNOWLEDGMENT The authors acknowledge support from NSF MRSEC DMR-1120923 and AFOSR under grant 29017320-51649-D. JCN also acknowledges support from Purdue Water Institute and Vanderbilt University. AVK acknowledge support from DARPA/DSO Extreme Optics and Imaging (EXTREME) Program, Award HR00111720032. The authors also thank Y. Choi for help with nanofabrication.


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(65) Li, Y.; Xin, H.; Liu, X.; Zhang, Y.; Lei, H.; Li, B. Trapping and Detection of Nanoparticles and Cells Using a Parallel Photonic Nanojet Array. ACS Nano, 2016, 10, 58005808


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