**5. Thermoplasmonic nanohole metasurface**

In this section, we discuss thermoplasmonic nanotweezer based on nanohole metasurface [50], which enables high-throughput large-ensemble nanoparticle assembly in a lab-on-a-chip platform. As mentioned in previous sections, optical metasurfaces achieve control over the properties of light. Besides the optical response, thermoplasmonic metasurfaces allow to engineer the thermal response at micro and nanoscale. Upon illumination of metasurface, the combination of optical and thermal effects enables robust large-ensemble many-particle trapping.

The nanohole metasurface comprises of an array of subwavelength nanoholes in a 125 nm thick gold film and illuminated with a laser of 1064 nm wavelength. This nanohole metasurface serves as a plasmonic resonator supporting both the localized surface plasmon resonance (LSPR) and Bloch mode surface plasmon polariton (SPP). Due to the plasmon response, electromagnetic field is confined near the rims of the nanohole and the region in between nanohole, as depicted in **Figure 2**. This enhances the photothermal conversion efficiency and hence higher temperature rise under illumination of the structured plasmonic film. As predicted, when an external AC field is applied onto the nanohole metasurface, a long-range electrothermoplasmonic (ETP) flow is generated inside the fluid, bringing particles from long distance towards the hot spot. Simultaneously, because of the existence of nanohole array, the AC electric field in the channel is no longer uniform but tangential components are created, which induces AC electroosmotic flow. The numerical simulation results of electroosmotic flow are depicted in the **Figure 3**.

For the experimental demonstration, the gold metasurface is fabricated using template stripping method on a silicon wafer. Using standard nanofabrication procedures, an array of nanoholes are fabricated on a silicon wafer, which is the same dimension and scale as the nanohole array on gold film. A 125 nm gold film is then deposited onto this silicon template using an electron beam evaporation process. Finally, a UV-sensitive curing-agent was uniformly spread onto the gold film and covered by an ITO-coated glass substrate with the same size as the silicon template.

After UV illumination is applied to harden the curing-agent, the gold film layer is stripped off and transferred to a glass substrate by gently inserting a blade in between the gold film and silicon template due to the weak attachment between gold and silicon. The silicon template can be cleaned by soaking in hydrogen peroxide and sulfuric acid solution in the ratio 3:1 to remove the organics on the surface and any remaining gold flakes on the template. Furthermore, the residual gold remaining on the template can be removed thoroughly by a gold etchant. The

*Illustration of microfluidic chip coupling with nanohole plasmonic metasurface. Adapted with permission from*

*Velocity profile of the induced AC electro-osmosis flow from numerical simulation. Adapted with permission*

*from Ref. [50]. Copyright (2018) American Chemical Society.*

*Nanomanipulation with Designer Thermoplasmonic Metasurface*

*DOI: http://dx.doi.org/10.5772/intechopen.91880*

To make the gold nanohole metasurface into a microfluidic chip, another ITOcoated glass coverslip is placed above the nanohole array sample, with a 120 μm dielectric spacer in between. For the trapping experiments, a dilute solution of 1 μm PS beads was injected into the channel. AC electric field is applied in between the ITO-coated glass cover slip and gold film. A 1064 nm laser illumination is focused on the nanohole array. The mechanism of particle trapping in the thermoplasmonic nanohole metasurface channel is depicted in **Figure 4**. The strong photo-induced heating in combination with an applied AC electric field creates the ETP flow that enables long-range capture and transport of particles towards the laser position. The

template can be reused multiple times.

*Ref. [50]. Copyright (2018) American Chemical Society.*

**Figure 3.**

**Figure 4.**

**105**

#### **Figure 2.**

*Electric field distribution of one of the single nanoholes by numerical simulation. Adapted with permission from Ref. [50]. Copyright (2018) American Chemical Society.*

#### **Figure 3.**

So far, we have briefly introduced the main mechanisms which could occur in plasmonic nanotweezers. The physical phenomena could be harnessed to introduce new capabilities in plasmonic nanotweezers. A recent article [49] has proposed the use of DEP force to promote particle transport towards plasmonic resonators.

In this section, we discuss thermoplasmonic nanotweezer based on nanohole metasurface [50], which enables high-throughput large-ensemble nanoparticle assembly in a lab-on-a-chip platform. As mentioned in previous sections, optical metasurfaces achieve control over the properties of light. Besides the optical response, thermoplasmonic metasurfaces allow to engineer the thermal response at micro and nanoscale. Upon illumination of metasurface, the combination of optical

The nanohole metasurface comprises of an array of subwavelength nanoholes in a 125 nm thick gold film and illuminated with a laser of 1064 nm wavelength. This nanohole metasurface serves as a plasmonic resonator supporting both the localized surface plasmon resonance (LSPR) and Bloch mode surface plasmon polariton (SPP). Due to the plasmon response, electromagnetic field is confined near the rims of the nanohole and the region in between nanohole, as depicted in **Figure 2**. This enhances the photothermal conversion efficiency and hence higher temperature rise under illumination of the structured plasmonic film. As predicted, when an external AC field is applied onto the nanohole metasurface, a long-range electrothermoplasmonic (ETP) flow is generated inside the fluid, bringing particles from long distance towards the hot spot. Simultaneously, because of the existence of nanohole array, the AC electric field in the channel is no longer uniform but tangential components are created, which induces AC electroosmotic flow. The numerical

and thermal effects enables robust large-ensemble many-particle trapping.

simulation results of electroosmotic flow are depicted in the **Figure 3**.

**Figure 2.**

**104**

*Ref. [50]. Copyright (2018) American Chemical Society.*

For the experimental demonstration, the gold metasurface is fabricated using template stripping method on a silicon wafer. Using standard nanofabrication procedures, an array of nanoholes are fabricated on a silicon wafer, which is the same dimension and scale as the nanohole array on gold film. A 125 nm gold film is then deposited onto this silicon template using an electron beam evaporation process. Finally, a UV-sensitive curing-agent was uniformly spread onto the gold film and covered by an ITO-coated glass substrate with the same size as the silicon template.

*Electric field distribution of one of the single nanoholes by numerical simulation. Adapted with permission from*

**5. Thermoplasmonic nanohole metasurface**

*Nanoplasmonics*

*Velocity profile of the induced AC electro-osmosis flow from numerical simulation. Adapted with permission from Ref. [50]. Copyright (2018) American Chemical Society.*

#### **Figure 4.**

*Illustration of microfluidic chip coupling with nanohole plasmonic metasurface. Adapted with permission from Ref. [50]. Copyright (2018) American Chemical Society.*

After UV illumination is applied to harden the curing-agent, the gold film layer is stripped off and transferred to a glass substrate by gently inserting a blade in between the gold film and silicon template due to the weak attachment between gold and silicon. The silicon template can be cleaned by soaking in hydrogen peroxide and sulfuric acid solution in the ratio 3:1 to remove the organics on the surface and any remaining gold flakes on the template. Furthermore, the residual gold remaining on the template can be removed thoroughly by a gold etchant. The template can be reused multiple times.

To make the gold nanohole metasurface into a microfluidic chip, another ITOcoated glass coverslip is placed above the nanohole array sample, with a 120 μm dielectric spacer in between. For the trapping experiments, a dilute solution of 1 μm PS beads was injected into the channel. AC electric field is applied in between the ITO-coated glass cover slip and gold film. A 1064 nm laser illumination is focused on the nanohole array. The mechanism of particle trapping in the thermoplasmonic nanohole metasurface channel is depicted in **Figure 4**. The strong photo-induced heating in combination with an applied AC electric field creates the ETP flow that enables long-range capture and transport of particles towards the laser position. The particles brought close to the nanohole array are trapped by the optical gradient force. In the lateral direction, the particle-particle separation distance is tuned by the dipole-dipole repulsion force between the particles [51, 52].

Under the microscope, when both laser illumination and AC electric field is turned on, particles are seen moving very directionally towards the nanohole metasurface. By selectively turning on and off AC electric field, particle-particle spacing can be controlled dynamically, due to the polarization of the particles induced by AC electric field. Briefly, particles are polarized by the AC field and a dipole-dipole repulsive force creates an in-plane interparticle separation between them. The ETP flow is measured experimentally using microparticle image velocimetry and its radial velocity is shown in **Figure 5(a)**.

Experimental images of trapped particles on the surface of the nanohole array are depicted in **Figure 5(b)**. When both the laser illumination and the AC electric field are applied, the particles are trapped with a certain interparticle spacing between them due to the in-plane dipole-dipole repulsion force. When the AC field is turned off, with the laser still on, the AC electric field-induced dipole-dipole repulsion force disappears, and the assembly becomes more compact.

They also show that the ETP flow induced by the array of nanohole is higher than that can be achieved with a single nanohole or an unpatterned gold film. The plasmonic resonance induced by patterned nanohole truly enhances the electrothermal effect. These results obtained from micro-particle image velocimetry analysis are depicted in **Figure 6**.

**6. Conclusion**

**Figure 6.**

*Society.*

**Acknowledgements**

**Conflict of interest**

University.

**107**

We have introduced the importance of plasmonic metasurface for applications in optical trapping, nano-tweezers and tiny particle manipulation, by demonstrating a nanoparticle trapping approach that utilizes a thermoplasmonic nanohole metasurface. The application of laser illumination and a.c. electric field results in new physical effects such as electrothermoplasmonic flow and a.c. electroosmosis that can work in concert with optical gradient forces to enable to new advanced features in micro and nanoparticle manipulation. The recent reports in the literature as articulated in this section shows that the intrinsic loss in plasmonic systems is not always detrimental but could work in synergy with the high electric field enhance-

*The nanohole array enables an enhanced ETP flow that is higher than the velocity induced when the planer film or a single nanohole is excited. Adapted with permission from Ref. [50]. Copyright (2018) American Chemical*

ment to realize advanced lab-on-a-chip devices in nanomanufacturing,

The authors declare no conflict of interest in this book section.

The authors acknowledge support from NSF ECCS-1933109 and Vanderbilt

nanophotonics, life science and quantum optics.

*Nanomanipulation with Designer Thermoplasmonic Metasurface*

*DOI: http://dx.doi.org/10.5772/intechopen.91880*

#### **Figure 5.**

*(a) Experimentally measured radial velocity of the ETP flow (b) sequence of trapping and particle assembling of 200 nm diameter polystyrene beads on the surface of the plasmonic nanohole array when AC field is alternatively turned on and off. Adapted with permission from Ref. [50]. Copyright (2018) American Chemical Society.*

*Nanomanipulation with Designer Thermoplasmonic Metasurface DOI: http://dx.doi.org/10.5772/intechopen.91880*

**Figure 6.**

particles brought close to the nanohole array are trapped by the optical gradient force. In the lateral direction, the particle-particle separation distance is tuned by

Under the microscope, when both laser illumination and AC electric field is turned on, particles are seen moving very directionally towards the nanohole metasurface. By selectively turning on and off AC electric field, particle-particle spacing can be controlled dynamically, due to the polarization of the particles induced by AC electric field. Briefly, particles are polarized by the AC field and a dipole-dipole repulsive force creates an in-plane interparticle separation between them. The ETP flow is measured experimentally using microparticle image

Experimental images of trapped particles on the surface of the nanohole array are depicted in **Figure 5(b)**. When both the laser illumination and the AC electric field are applied, the particles are trapped with a certain interparticle spacing between them due to the in-plane dipole-dipole repulsion force. When the AC field is turned off, with the laser still on, the AC electric field-induced dipole-dipole

They also show that the ETP flow induced by the array of nanohole is higher than that can be achieved with a single nanohole or an unpatterned gold film. The plasmonic resonance induced by patterned nanohole truly enhances the electrothermal effect. These results obtained from micro-particle image velocimetry anal-

*(a) Experimentally measured radial velocity of the ETP flow (b) sequence of trapping and particle assembling of 200 nm diameter polystyrene beads on the surface of the plasmonic nanohole array when AC field is alternatively turned on and off. Adapted with permission from Ref. [50]. Copyright (2018) American*

the dipole-dipole repulsion force between the particles [51, 52].

velocimetry and its radial velocity is shown in **Figure 5(a)**.

ysis are depicted in **Figure 6**.

*Nanoplasmonics*

**Figure 5.**

**106**

*Chemical Society.*

repulsion force disappears, and the assembly becomes more compact.

*The nanohole array enables an enhanced ETP flow that is higher than the velocity induced when the planer film or a single nanohole is excited. Adapted with permission from Ref. [50]. Copyright (2018) American Chemical Society.*
