This work was supported&#160;by the ICT R&#38;D program of MSIP/IITP (R0190-15-2040, Development of a contents configuration management system and a simulator for 3D printing using smart materials).

Caution: Please refer to all relevant material safety regulations before use. Several of the chemicals used in microchip fabrication are acutely toxic and carcinogenic. Please use all appropriate safety practices when performing the photolithography and etching processes, including the use of engineering controls (fume hood, hot plate, and aligner) and personal protective equipment (safety glasses, gloves, lab coat, full-length pants, and closed-toe shoes).
1. Fabrication of the PDMS Microchannel
<ol>
	<li>Fabrication of the microchannel mold by the photolithograph process
	<ol>
		<li>Completely remove foreign substances on the 4-inch Si wafer surface with piranha cleaning (Figure 1a). Mix sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) at a ratio of 3:1 to make the piranha solution in the dish. Mix by gradually adding small amounts of the strong acid (H2O2) to the weak acid (H2SO4); reversing this order may cause an explosion because of the highly reactive strong acid.</li>
		<li>Immerse the wafer in the piranha solution for 10 min. Subsequently, immerse the wafer in deionized (DI) water for 3 min to remove the remaining piranha solution. Rinse the wafer with flowing DI water for 10 s. Repeat the rinsing procedure 3 times and dry with N2 gas to remove the remaining DI.</li>
		<li>Place the wafer on a hot plate for 20 min at 180 &#176;C to further dehydrate the wafer.</li>
		<li>Pour 5 mL of the negative photoresist on top of the wafer and spin coat for 45 s at 1,500 rpm (Figure 1b); after spin coating, a photoresist bead is created at the wafer edge because of the relatively high viscosity of the photoresist.</li>
		<li>Balance the photoresist-coated wafer by planarization on a leveling stand for 5 h.</li>
		<li>Place the photoresist-coated wafer on a hot plate for 12 min at 65 &#176;C, 35 min at 95 &#176;C, and 12 min at 65 &#176;C (soft baking).</li>
		<li>Fix the film mask on the mask holder and the soft-baked wafer on the substrate stage of the aligner. Expose to ultraviolet (UV) light for 43 s at 650 mJ/cm2 to solidify the photoresist.</li>
		<li>Place the wafer on the hot plate for 5 min at 65 &#176;C, 15 min at 95 &#176;C, and 5 min at 65 &#176;C (post-exposure baking).</li>
		<li>Immerse the wafer in the photoresist developer for 30 min to remove unsolidified photoresist.</li>
		<li>Rinse the wafer with isopropyl alcohol (IPA) and dry with N2 gas to remove the remaining IPA.</li>
	</ol>
	</li>
	<li>Manufacture of the PDMS microchannel
	<ol>
		<li>Treat the surface of the wafer and the photoresist mold for 1 min at a power of 200 W using an atmospheric plasma machine25; the gas flows of CH4 and He should be 6 and 30 sccm, respectively. Perform this hydrophobic treatment to easily detach the polydimethylsiloxane (PDMS) microchannel from the surface of the wafer and photoresist mold (Figure 1c).</li>
		<li>Prepare the PDMS solution by mixing the PDMS base and curing agent at a ratio of 10:1. Stir the mixture for 2 min.</li>
		<li>Place the wafer inside a Petri dish (150 mm x 15 mm) and add 100 mL of the PDMS solution. Remove the bubbles that were created from stirring using a desiccator.</li>
		<li>Place the Petri dish in the oven for 2 h at 80 &#176;C to solidify the PDMS solution (Figure 1d and h).</li>
		<li>Cut along the contours of the PDMS microchannel with a razor blade and detach it from the wafer; the fabricated PDMS microchannel should have the following dimensions: 13 mm long, 300 &#181;m wide, and 150 &#181;m high (Figures 1e, f, and i). 
		NOTE: Two types of holes are produced by a micropuncture to insert the single-mode fiber (SMF) cable and the tubes (inlet and outlet) on the PDMS microchannel (Figure 1g). The SMF cable is used to emit the laser beam to the nanohole milled on the gold plate. The tube is used to insert/extract the particle solution to/from the PDMS microchannel.</li>
		<li>Puncture 1.5-mm inlet and outlet holes at each end of the PDMS microchannel. Puncture a 0.3-mm SMF cable hole at the center of the PDMS microchannel.</li>
	</ol>
	</li>
</ol>
2. Etching Process of the Gold Plate
<ol>
	<li>Prepare a commercially available gold plate with the dimensions of 25 x 6.25 mm2 (Figure 2a).</li>
	<li>Remove any foreign substances on the gold plate with the following cleaning procedures. Clean in the following order by immersing in acetone, methanol, and DI water for 5 min each.</li>
	<li>Rinse the gold plate 3 times with DI water for 10 s and dry the plate with N2 gas to remove the remaining DI water.</li>
	<li>Place the gold plate on a hot plate for 20 min at 180 &#176;C to completely remove any remaining moisture.</li>
	<li>Pour 0.5 mL of hexamethyldisilazane (HMDS) on the gold plate and spin coat for 40 s at 3,000 rpm.</li>
	<li>Pour 0.5 mL of positive photoresist on top of the spin-coated HMDS and spin coat for 40 s at 3,000 rpm (Figure 2b).</li>
	<li>Place the photoresist-coated gold plate on the hot plate for 90 s at 110 &#176;C (soft baking).</li>
	<li>Fix the film mask on the glass wafer and place the soft-baked gold plate on the substrate stage. Expose to UV light for 4.5 s at 64 mJ/cm2 to dissolve the photoresist.</li>
	<li>Immerse the gold plate in the photoresist developer for 1 min to remove the dissolved photoresist (Figure 2c). Rinse the gold plate with DI water and dry with N2 gas.</li>
	<li>Immerse the gold plate in the Au etchant for 45 s at an etching rate of 28 &#197;/s to remove the exposed Au (Figure 2d). Rinse the gold plate with DI water and dry with N2 gas.</li>
	<li>Immerse the gold plate in the Ti etchant for 5 s at an etching rate of 25 &#197;/s to remove the exposed Ti (Figure 2e). Rinse the gold plate with DI water and dry with N2 gas.</li>
	<li>Remove the remaining photoresist on the gold plate by immersing it in acetone, methanol, and DI water for 3 min each (Figure 2f); immerse the plate in the written order.</li>
	<li>Rinse the gold plate 3 times with DI water for 10 s. Dry with N2 gas to remove the DI water.</li>
	<li>Place the gold plate on the hot plate for 3 min at 120 &#176;C to completely remove the moisture; the produced gold block should be 400 x 150 &#181;m2 (Figure 2h).</li>
	<li>Mill a 400-nm nanohole using a focused ion beam (FIB) at the center of the gold block that was fabricated after etching (Figures 2g and i). Create a 370-nm circle pattern to focus on the gold block with an ion accelerating voltage of 30 kV at 28 pA for 3 s.</li>
</ol>
3. Assembly of the Microchip
<ol>
	<li>Treat the two surfaces of the PDMS microchannel and gold plate for 1 min with O2 plasma to attach them together with a plasma system at a power of 80 W and a pressure of 825 mTorr25. 
	NOTE: It is notably difficult to attach them with precision because the gold block and PDMS microchannel are on the micrometer level. Hence, use an aligner with a camera and a manual stage.</li>
	<li>Fix the glass wafer that is used to attach the film mask to the mask holder of the aligner (Figure 3a).</li>
	<li>Attach the O2-plasma-treated PDMS microchannel to the glass wafer; because the PDMS is hydrophilic, it will easily attach to the glass wafer without any adhesion solution. Fix the gold plate on the substrate stage of the aligner (Figure 3a).</li>
	<li>Locate the centers of the SMF cable hole and gold block, which are aligned on the same axis, using the camera on the aligner. Lift the manual stage to combine the two parts (Figures 3b and c).</li>
</ol>
4. Improvement of the Microchip Side Surface Roughness by PDMS Coating
NOTE: The gold plate with fixed dimensions of 400 x 150 &#181;m2 is relatively more difficult to cut out than the PDMS material. Therefore, to detach the PDMS microchannel from the wafer, a razor blade is used to cut out a larger piece than the gold plate. After combining the two parts, the excess parts of the PDMS relative to the gold plate must then be cut so that the inside of the channel can be observed from the side using a microscope (Figure 4a). However, the cut surface, which is used as a window, has a high surface roughness and consequently produces cloudy images of the particles that flow in the channel (Figure 4b). Coating with the PDMS solution is performed again to resolve this problem.
<ol>
	<li>Prepare the PDMS solution by mixing the PDMS base and curing agent at a 10:1 ratio and stir for 2 min.</li>
	<li>Pour 2 mL of the PDMS solution into the Petri dish and perform the spin coating for 30 s at 1,000 rpm (Figure 4c).</li>
	<li>Place the microchip surface that is going to be located on the microscope on the Petri dish (Figure 4d). Place the Petri dish in the oven for 1 h at 80 &#176;C to solidify the PDMS solution.</li>
	<li>Cut the border of the microchip and PDMS using a razor blade and subsequently detach it from the Petri dish (Figures 4e, f).</li>
</ol>
5. Laser Coupling to Insert the SMF Cable to the Microchip
NOTE: For the plasmonic tweezer system, an optical fiber incident laser with a 1,064-nm wavelength is used. The SMF cable is used because the diameter of the incident laser (5 mm) is too immense to emit the laser beam at the nanohole milled on the gold block (400 x 150 &#181;m2) in the microchip. The cladding diameter of the SMF cable is 125 &#181;m. Thus, the incident laser and SMF cable must be coupled.
<ol>
	<li>Connect a 40X objective lens to the microscope objective mount on the SMF coupler. Fix the SMF cable on the fiber clamp of the SMF coupler. Align the incident laser beam to fill in the back aperture of the objective lens.</li>
	<li>Focus the laser beam to the core of the SMF cable by adjusting the three-axis manual stage equipped on the SMF coupler.</li>
	<li>Insert the opposite end of the SMF cable into the SMF cable hole of the microchip. Measure the laser power prior to the insertion at the edge of the fiber cable, because the fixed fiber cable at the microchip cannot be detached.</li>
	<li>Seal the SMF cable hole using epoxy glue to block the leakage of the flowing particle solution from the gap between the SMF cable hole (300 &#181;m) and the cladding of the SMF cable (125 &#181;m); the end of the inserted fiber cable should not enter the microchannel to avoid the fluid flow. Manually align the fiber cable using visual feedback so that it is perpendicular to the gold block that hosts the nanohole.</li>
</ol>
6. Plasmonic Trapping of Single Fluorescent Polystyrene Particle in the Microchip
<ol>
	<li>Attach the syringe, which is filled with the particle solution, to a syringe micropump. Place the cover glass on the sample stage of the fluorescent microscope. Connect tubes to the inlet/outlet holes of the microchip. Place the PDMS-coated microchip surface on top of the cover glass.</li>
	<li>Position the microchip orthogonally to the 60X water immersion objective lens by observing the inside of the channel with the camera installed on the fluorescent microscope. Use transparent tape to fix the microchip in place. Connect the inlet tube of the microchip with the syringe needle.</li>
	<li>Insert the particle solution to the microchip by controlling the micropump at 20 &#181;m/s. At this moment, confirm that the fluorescent particle can be observed well in the channel when the fluorescent lamp is turned on.</li>
	<li>Wait until the particle solution exits from the outlet of the microchip. Set the speed to 3.4 &#181;m/s.</li>
	<li>Turn the laser source device so that it emits the laser into the nanohole; the fluorescent particle will be trapped at the rim of the nanohole.</li>
	<li>Ramp the fluid speed in increments of 0.4 &#181;m/s by controlling the micropump until the trapped particle escapes. Measure the fluid speed when the trapped particles escape. Obtain the maximal trapping force for each laser intensity using this measured fluid speed.</li>
</ol>

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The authors have nothing to disclose.

The SMF cable was inserted in the SMF cable hole on the microchip, as shown in the rectangular dot of Figure 6a. Because the SMF cable hole is larger than the cable diameter, epoxy glue was used to seal the gap to block the leakage of the flowing particle solution. Before the application of epoxy glue, the gold block and cable edge should be coaxially aligned by hand using a microscope. Although it is ideal for the inserted cable edge and the nanohole to be coaxially aligned, a slight misalignment can be...

The ability to manipulate microscale objects is an indispensable feature for many micro/nano experiments. Direct contact manipulations can damage the manipulated objects. Releasing the previously held objects is also challenging because of stiction problems. To overcome these issues, several indirect methods using fluidic1, electric2, magnetic3, or photonic forces4,5,6,7,8 have been proposed. Plasmonic tweezers that use photonic forces are based on the physics of extraordinary field enhancement several orders larger than the incident intensity9. This extremely strong field enhancement enables the trapping of extremely small nanoparticles. For example, it has been shown to immobilize and manipulate nanoscale objects, such as polystyrene particles7,10,11,12,13,14, polymer chains15, proteins16, quantum dots17, and DNA molecules8,18. Without plasmonic tweezers, it is difficult to trap nanoparticles because they quickly disappear before they are effectively examined or because they are damaged due to the high intensity of the laser.
Many plasmonic studies have used various nanoscale gold structures. We can categorize the gold structures as protruding nanodisk types12,13,14,15,19,20,21 or suppressed nanohole types7,8,10,11,22,23. In terms of imaging convenience, the nanodisk types are more suitable than the nanohole types because, for the latter, the gold substrates can obstruct the observation view. Moreover, the plasmonic trapping occurs near the plasmonic structure and makes observation even more challenging. To the best of our knowledge, plasmonic trapping on nanohole types was only verified using indirect scattering signals. However, no successful direct observations, such as microscopic images, have been reported. Few studies have described the position of trapped particles. One such result was presented by Wang et al. They created a gold pillar on a gold substrate and observed the particle motion using a fluorescent microscope24. However, this is only effective for monitoring lateral movements not in the direction parallel to the beam axis.
In this paper, we introduce new fluidic microchip design and fabrication procedures. Using this chip, we demonstrate the monitoring of plasmonically trapped particles, both in directions parallel and orthogonal to the plasmonic nanostructure. Furthermore, we measure the maximal force of the immobilized particle by increasing the fluid velocity to find the tipping velocity in the microchip. This study is unique because most studies on plasmonic tweezers cannot quantitatively show the maximal trapping forces used in their experimental setups.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Negative photoresist&#160;</td>
			<td>MicroChem</td>
			<td>SU-8 2075</td>
		</tr>
		<tr>
			<td>Developer</td>
			<td>MicroChem</td>
			<td>SU-8 Developer</td>
		</tr>
		<tr>
			<td>Positive photoresist&#160;</td>
			<td>Merck Ltd.</td>
			<td>AZ GXR-601</td>
		</tr>
		<tr>
			<td>AZ Photoresist Developers</td>
			<td>Merck Ltd.</td>
			<td>AZ 300 MIF</td>
		</tr>
		<tr>
			<td>HMDS</td>
			<td>Merck Ltd.</td>
			<td>AZ Adhesion Promoter</td>
		</tr>
		<tr>
			<td>Aligner</td>
			<td>Midas System</td>
			<td>MDA 400M</td>
		</tr>
		<tr>
			<td>Atmospheric plasma machine&#160;</td>
			<td>Atmospheric Process 
			Plasma Co.</td>
			<td>IDP-1000</td>
		</tr>
		<tr>
			<td>Polydimethylsiloxane (PDMS)</td>
			<td>Dow Corning</td>
			<td>Sylgard 184 A/B</td>
		</tr>
		<tr>
			<td>Gold coated test slides</td>
			<td>EMF Co.</td>
			<td>TA124(Ti/Au)</td>
		</tr>
		<tr>
			<td>Au etchant&#160;</td>
			<td>Transene Inc.</td>
			<td>TFA</td>
		</tr>
		<tr>
			<td>Ti etchant&#160;</td>
			<td>Transene Inc.</td>
			<td>TFT</td>
		</tr>
		<tr>
			<td>40X objective lens&#160;</td>
			<td>Edmund Optics</td>
			<td>40X DIN</td>
		</tr>
		<tr>
			<td>60X water immersion 
			objective lens&#160;</td>
			<td>Olympus</td>
			<td>LUMPLFLN 60XW</td>
		</tr>
		<tr>
			<td>Optical fiber incident laser&#160;</td>
			<td>IPG Photonic</td>
			<td>YLR 10</td>
		</tr>
		<tr>
			<td>SMF coupler</td>
			<td>Thorlabs</td>
			<td>MBT612D/M</td>
		</tr>
		<tr>
			<td>Syringe micropump</td>
			<td>Harvard</td>
			<td>PC2 70-4501</td>
		</tr>
		<tr>
			<td>Fluorescent microscope&#160;</td>
			<td>Olympus</td>
			<td>IX-51</td>
		</tr>
		<tr>
			<td>Plasma system</td>
			<td>Femto Science Inc</td>
			<td>CUTE-MPR</td>
		</tr>
	</tbody>
</table>

plasmonic trapping and release of nanoparticles in a monitoring environment

Plasmonic tweezers use surface plasmon polaritons to confine polarizable nanoscale objects. Among the various designs of plasmonic tweezers, only a few can observe immobilized particles. Moreover, a limited number of studies have experimentally measured the exertable forces on the particles. The designs can be classified as the protruding nanodisk type or the suppressed nanohole type. For the latter, microscopic observation is extremely challenging. In this paper, a new plasmonic tweezer system is introduced to monitor particles, both in directions parallel and orthogonal to the symmetric axis of a plasmonic nanohole structure. This feature enables us to observe the movement of each particle near the rim of the nanohole. Furthermore, we can quantitatively estimate the maximal trapping forces using a new fluidic channel.

The fabrication process of the PDMS microchannel and nanohole gold plate is shown in Figures 1 and 2. The method to combine the two parts and the actual microchip is shown in Figure 3. The PDMS was cut to reveal the inside of the channel from the side of the microchip. However, it was difficult to observe the particles flowing in the channel because of the surface roughness of the cutting plane. Therefore, we introduced the PDMS coating m...

Watch this Scientific Journal Video about Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment at JoVE.com

Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment

A microchip fabrication process that incorporates plasmonic tweezers is presented here. The microchip enables the imaging of a trapped particle to measure maximal trapping forces.

Plasmonic tweezers use surface plasmon polaritons to confine polarizable nanoscale objects. Among the various designs of plasmonic tweezers, only a few ...

plasmonic-trapping-release-nanoparticles-monitoring

Research

JoVE Journal

Engineering

7.5K Views.  Korea Basic Science Institute (KBSI). A microchip fabrication process that incorporates plasmonic tweezers is presented here. The microchip enables the imaging of a trapped particle to measure maximal trapping forces.

Video: Plasmonic Trapping and Release of Nanoparticles in a Monitoring Environment

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.
The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.
To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 &mu;m thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm2, which can be increased up to 17-18 mA/cm2 (~25% higher) after application of a simple diffuse back reflector made of a white paint.
To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23&deg;C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF2, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF2, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm2, up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.
Introduction
Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) 1 and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) 2. For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector 3, silicon plasma texturing 4 or high refractive index nanoparticle reflector 5 have been suggested.
Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect 6. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment 6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested 8. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles 9. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance 10.
The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF2 or SiO2, from the rear reflector to avoid mechanical and chemical damage 7. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric 6. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.

Polycrystalline silicon thin-film solar cells on glass are fabricated by deposition of boron and phosphorous doped silicon layers followed by crystallisation, defect passivation and metallisation. Plasmonic light-trapping is introduced by forming Ag nanoparticles on the silicon cell surface capped with a diffused reflector resulting in ~45% photocurrent enhancement.

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To ...

Optical Trapping of Nanoparticles

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping and manipulating small biological particles. Ashkin and co-workers first demonstrated optical tweezers using a single focused beam1. The single beam trap can be described accurately using the perturbative gradient force formulation in the case of small Rayleigh regime particles1. In the perturbative regime, the optical power required for trapping a particle scales as the inverse fourth power of the particle size. High optical powers can damage dielectric particles and cause heating. For instance, trapped latex spheres of 109 nm in diameter were destroyed by a 15 mW beam in 25 sec1, which has serious implications for biological matter2,3.
A self-induced back-action (SIBA) optical trapping was proposed to trap 50 nm polystyrene spheres in the non-perturbative regime4. In a non-perturbative regime, even a small particle with little permittivity contrast to the background can influence significantly the ambient electromagnetic field and induce a large optical force. As a particle enters an illuminated aperture, light transmission increases dramatically because of dielectric loading. If the particle attempts to leave the aperture, decreased transmission causes a change in momentum outwards from the hole and, by Newton's Third Law, results in a force on the particle inwards into the hole, trapping the particle. The light transmission can be monitored; hence, the trap can become a sensor. The SIBA trapping technique can be further improved by using a double-nanohole structure.
The double-nanohole structure has been shown to give a strong local field enhancement5,6. Between the two sharp tips of the double-nanohole, a small particle can cause a large change in optical transmission, thereby inducing a large optical force. As a result, smaller nanoparticles can be trapped, such as 12 nm silicate spheres7 and 3.4 nm hydrodynamic radius bovine serum albumin proteins8. In this work, the experimental configuration used for nanoparticle trapping is outlined. First, we detail the assembly of the trapping setup which is based on a Thorlabs Optical Tweezer Kit. Next, we explain the nanofabrication procedure of the double-nanohole in a metal film, the fabrication of the microfluidic chamber and the sample preparation. Finally, we detail the data acquisition procedure and provide typical results for trapping 20 nm polystyrene nanospheres.

The following setup approach details low power optical trapping of dielectric nanoparticles using a double-nanohole in metal film.

Optical trapping is a technique for immobilizing and manipulating small objects in a gentle way using light, and it has been widely applied in trapping ...

Construction of a High Resolution Microscope with Conventional and Holographic Optical Trapping Capabilities

High resolution microscope systems with optical traps allow for precise manipulation of various refractive objects, such as dielectric beads 1 or cellular organelles 2,3, as well as for high spatial and temporal resolution readout of their position relative to the center of the trap. The system described herein has one such "traditional" trap operating at 980 nm. It additionally provides a second optical trapping system that uses a commercially available holographic package to simultaneously create and manipulate complex trapping patterns in the field of view of the microscope 4,5 at a wavelength of 1,064 nm. The combination of the two systems allows for the manipulation of multiple refractive objects at the same time while simultaneously conducting high speed and high resolution measurements of motion and force production at nanometer and piconewton scale.

The system described herein employs a traditional optical trap as well as an independent holographic optical trapping line, capable of creating and manipulating multiple traps. This allows for the creation of complex geometric arrangements of refractive particles while also permitting simultaneous high-speed, high-resolution measurements of the activity of biological enzymes.

High resolution microscope systems with optical traps allow for precise manipulation of various refractive objects, such as dielectric beads 1 or cellular ...

Design, Fabrication, and Experimental Characterization of Plasmonic Photoconductive Terahertz Emitters

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on photoconduction, which has been one of the most commonly used techniques for terahertz generation 1-8. Terahertz generation in a photoconductive emitter is achieved by pumping an ultrafast photoconductor with a pulsed or heterodyned laser illumination. The induced photocurrent, which follows the envelope of the pump laser, is routed to a terahertz radiating antenna connected to the photoconductor contact electrodes to generate terahertz radiation. Although the quantum efficiency of a photoconductive emitter can theoretically reach 100%, the relatively long transport path lengths of photo-generated carriers to the contact electrodes of conventional photoconductors have severely limited their quantum efficiency. Additionally, the carrier screening effect and thermal breakdown strictly limit the maximum output power of conventional photoconductive terahertz sources. To address the quantum efficiency limitations of conventional photoconductive terahertz emitters, we have developed a new photoconductive emitter concept which incorporates a plasmonic contact electrode configuration to offer high quantum-efficiency and ultrafast operation simultaneously. By using nano-scale plasmonic contact electrodes, we significantly reduce the average photo-generated carrier transport path to photoconductor contact electrodes compared to conventional photoconductors 9. Our method also allows increasing photoconductor active area without a considerable increase in the capacitive loading to the antenna, boosting the maximum terahertz radiation power by preventing the carrier screening effect and thermal breakdown at high optical pump powers. By incorporating plasmonic contact electrodes, we demonstrate enhancing the optical-to-terahertz power conversion efficiency of a conventional photoconductive terahertz emitter by a factor of 50 10.

We describe methods for the design, fabrication, and experimental characterization of plasmonic photoconductive emitters, which offer two orders of magnitude higher terahertz power levels compared to conventional photoconductive emitters.

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on ...

Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information support1,2,3. In this context, metal nanowires are especially desirable for realizing dense routing networks4. A prerequisite to operate such shared nanowire-based platform relies on our ability to electrically contact individual metal nanowires and efficiently excite surface plasmon polaritons5 in this information support. In this article, we describe a protocol to bring electrical terminals to chemically-synthesized silver nanowires6 randomly distributed on a glass substrate7. The positions of the nanowire ends with respect to predefined landmarks are precisely located using standard optical transmission microscopy before encapsulation in an electron-sensitive resist. Trenches representing the electrode layout are subsequently designed by electron-beam lithography. Metal electrodes are then fabricated by thermally evaporating a Cr/Au layer followed by a chemical lift-off. The contacted silver nanowires are finally transferred to a leakage radiation microscope for surface plasmon excitation and characterization8,9. Surface plasmons are launched in the nanowires by focusing a near infrared laser beam on a diffraction-limited spot overlapping one nanowire extremity5,9. For sufficiently large nanowires, the surface plasmon mode leaks into the glass substrate9,10. This leakage radiation is readily detected, imaged, and analyzed in the different conjugate planes in leakage radiation microscopy9,11. The electrical terminals do not affect the plasmon propagation. However, a current-induced morphological deterioration of the nanowire drastically degrades the flow of surface plasmons. The combination of surface plasmon leakage radiation microscopy with a simultaneous analysis of the nanowire electrical transport characteristics reveals the intrinsic limitations of such plasmonic circuitry.

Silver nanowires can simultaneously transport electrons and optical information in the form of surface plasmons. A procedure is described here to realize such a shared circuitry and the limitations at propagating both information carriers are evaluated.

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information ...

Analyzing the Movement of the Nauplius 'Artemia salina' by Optical Tracking of Plasmonic Nanoparticles

We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A single gold nanoparticle held by an optical tweezer is used as a sensor to quantify the rhythmic motion of a Nauplius larva (Artemia salina) in a water sample. This is achieved by monitoring the time dependent displacement of the trapped nanoparticle as a consequence of the Nauplius activity. A Fourier analysis of the nanoparticle&#39;s position then yields a frequency spectrum that is characteristic to the motion of the observed species. This experiment demonstrates the capability of this method to measure and characterize the activity of small aquatic larvae without the requirement to observe them directly and to gain information about the position of the larvae with respect to the trapped particle. Overall, this approach could give an insight on the vitality of certain species found in an aquatic ecosystem and could expand the range of conventional methods for analyzing water samples.

Analyzing the Movement of the Nauplius 'Artemia salina' by Optical Tracking of Plasmonic Nanoparticles

We use optical tracking of plasmonic nanoparticles to probe and characterize the frequency movements of aquatic organisms.

We demonstrate how optical tweezers may provide a sensitive tool to analyze the fluidic vibrations generated by the movement of small aquatic organisms. A ...

Energy Dispersive X-ray Tomography for 3D Elemental Mapping of Individual Nanoparticles

Energy dispersive X-ray spectroscopy within the scanning transmission electron microscope (STEM) provides accurate elemental analysis with high spatial resolution, and is even capable of providing atomically resolved elemental maps. In this technique, a highly focused electron beam is incident upon a thin sample and the energy of emitted X-rays is measured in order to determine the atomic species of material within the beam path. This elementally sensitive spectroscopy technique can be extended to three dimensional tomographic imaging by acquiring multiple spectrum images with the sample tilted along an axis perpendicular to the electron beam direction.
Elemental distributions within single nanoparticles are often important for determining their optical, catalytic and magnetic properties. Techniques such as X-ray tomography and slice and view energy dispersive X-ray mapping in the scanning electron microscope provide elementally sensitive three dimensional imaging but are typically limited to spatial resolutions of &gt; 20 nm. Atom probe tomography provides near atomic resolution but preparing nanoparticle samples for atom probe analysis is often challenging. Thus, elementally sensitive techniques applied within the scanning transmission electron microscope are uniquely placed to study elemental distributions within nanoparticles of dimensions 10-100 nm.
Here, energy dispersive X-ray (EDX) spectroscopy within the STEM is applied to investigate the distribution of elements in single AgAu nanoparticles. The surface segregation of both Ag and Au, at different nanoparticle compositions, has been observed.

The use of energy dispersive X-ray tomography in the scanning transmission electron microscope to characterize elemental distributions within single nanoparticles in three dimensions is described.

Energy dispersive X-ray spectroscopy within the scanning transmission electron microscope (STEM) provides accurate elemental analysis with high spatial ...

Measurement of Scattering Nonlinearities from a Single Plasmonic Nanoparticle

Plasmonics, which are based on the collective oscillation of electrons due to light excitation, involve strongly enhanced local electric fields and thus have potential applications in nonlinear optics, which requires extraordinary optical intensity. One of the most studied nonlinearities in plasmonics is nonlinear absorption, including saturation and reverse saturation behaviors. Although scattering and absorption in nanoparticles are closely correlated by the Mie theory, there has been no report of nonlinearities in plasmonic scattering until very recently. 
Last year, not only saturation, but also reverse saturation of scattering in an isolated plasmonic particle was demonstrated for the first time. The results showed that saturable scattering exhibits clear wavelength dependence, which seems to be directly linked to the localized surface plasmon resonance (LSPR). Combined with the intensity-dependent measurements, the results suggest the possibility of a common mechanism underlying the nonlinear behaviors of scattering and absorption. These nonlinearities of scattering from a single gold nanosphere (GNS) are widely applicable, including in super-resolution microscopy and optical switches. 
In this paper, it is described in detail how to measure nonlinearity of scattering in a single GNP and how to employ the super-resolution technique to enhance the optical imaging resolution based on saturable scattering. This discovery features the first super-resolution microscopy based on nonlinear scattering, which is a novel non-bleaching contrast method that can achieve a resolution as low as l/8 and will potentially be useful in biomedicine and material studies.

Saturable and reverse saturable scattering were discovered in isolated plasmonic particles and adopted as a novel non-bleaching contrast method in super-resolution microscopy. Here the experimental procedures of detecting and extracting nonlinear scattering are explained in detail, as well as how to enhance resolution with the aid of saturated excitation microscopy.

Plasmonics, which are based on the collective oscillation of electrons due to light excitation, involve strongly enhanced local electric fields and thus ...

Multifunctional Hybrid Fe2O3-Au Nanoparticles for Efficient Plasmonic Heating

One of the most widely used methods for manufacturing colloidal gold nanospherical particles involves the reduction of chloroauric acid (HAuCl4) to neutral gold Au(0) by reducing agents, such as sodium citrate or sodium borohydride. The extension of this method to decorate iron oxide or similar nanoparticles with gold nanoparticles to create multifunctional hybrid Fe2O3-Au nanoparticles is straightforward. This approach yields fairly good control over Au nanoparticle dimensions and loading onto Fe2O3. Additionally, the Au metal size, shape, and loading can easily be tuned by changing experimental parameters (e.g., reactant concentrations, reducing agents, surfactants, etc.). An advantage of this procedure is that the reaction can be done in air or water, and, in principle, is amenable to scaling up. The use of such optically tunable Fe2O3-Au nanoparticles for hyperthermia studies is an attractive option as it capitalizes on plasmonic heating of gold nanoparticles tuned to absorb light strongly in the VIS-NIR region. In addition to its plasmonic effects, nanoscale Au provides a unique surface for interesting chemistries and catalysis. The Fe2O3 material provides additional functionality due to its magnetic property. For example, an external magnetic field could be used to collect and recycle the hybrid Fe2O3-Au nanoparticles after a catalytic experiment, or alternatively, the magnetic Fe2O3 can be used for hyperthermia studies through magnetic heat induction. The photothermal experiment described in this report measures bulk temperature change and nanoparticle solution mass loss as functions of time using infrared thermocouples and a balance, respectively. The ease of sample preparation and the use of readily available equipment are distinct advantages of this technique. A caveat is that these photothermal measurements assess the bulk solution temperature and not the surface of the nanoparticle where the heat is transduced and the temperature is likely to be higher.

Multifunctional Hybrid Fe2O3-Au Nanoparticles for Efficient Plasmonic Heating

We describe the synthesis and properties of multifunctional Fe2O3-Au nanoparticles produced by a wet chemical approach and investigate their photothermal properties using laser irradiation. The composite Fe2O3-Au nanoparticles retain the properties of both materials, creating a multifunctional structure with excellent magnetic and plasmonic properties.

One of the most widely used methods for manufacturing colloidal gold nanospherical particles involves the reduction of chloroauric acid (HAuCl4) to ...

High Temperature Fabrication of Nanostructured Yttria-Stabilized-Zirconia (YSZ) Scaffolds by In Situ Carbon Templating Xerogels

We demonstrate a method for the high temperature fabrication of porous, nanostructured yttria-stabilized-zirconia (YSZ, 8 mol% yttria - 92 mol% zirconia) scaffolds with tunable specific surface areas up to 80 m2&#183;g-1. An aqueous solution of a zirconium salt, yttrium salt, and glucose is mixed with propylene oxide (PO) to form a gel. The gel is dried under ambient conditions to form a xerogel. The xerogel is pressed into pellets and then sintered in an argon atmosphere. During sintering, a YSZ ceramic phase forms and the organic components decompose, leaving behind amorphous carbon. The carbon formed in situ serves as a hard template, preserving a high surface area YSZ nanomorphology at sintering temperature. The carbon is subsequently removed by oxidation in air at low temperature, resulting in a porous, nanostructured YSZ scaffold. The concentration of the carbon template and the final scaffold surface area can be systematically tuned by varying the glucose concentration in the gel synthesis. The carbon template concentration was quantified using thermogravimetric analysis (TGA), the surface area and pore size distribution was determined by physical adsorption measurements, and the morphology was characterized using scanning electron microscopy (SEM). Phase purity and crystallite size was determined using X-ray diffraction (XRD). This fabrication approach provides a novel, flexible platform for realizing unprecedented scaffold surface areas and nanomorphologies for ceramic-based electrochemical energy conversion applications, e.g. solid oxide fuel cell (SOFC) electrodes.

High Temperature Fabrication of Nanostructured Yttria-Stabilized-Zirconia YSZ Scaffolds by In Situ Carbon Templating Xerogels

A protocol for fabricating porous, nanostructured yttria-stabilized-zirconia (YSZ) scaffolds at temperatures between 1,000 &#176;C and 1,400 &#176;C is presented.

We demonstrate a method for the high temperature fabrication of porous, nanostructured yttria-stabilized-zirconia (YSZ, 8 mol% yttria - 92 mol% zirconia) ...