Name: optical trapping of nanoparticles
Uploaded: 2013-01-15T13:00:00
Duration: 13 min 39 s
Description: Watch this Scientific Journal Video about Optical Trapping of Nanoparticles at JoVE.com

We acknowledge Thorlabs for sponsoring this publication and funding from the Natural Science and Engineering Research Council (NSERC) of Canada Discovery Grant. We thank Bryce Cyr and Douglas Rennehan for production assistance in the making of this video article.

The principle of the SIBA trapping technique is illustrated in Figure 1. Figure 2 is a schematic of the experimental setup.
1. Optical Trapping Setup
For this section of the procedure, refer to the optical trapping kit manual9 or the optical force measurement module manual10 for details on setting up the kits. Note that an avalanche photodiode (APD) is used instead of a quadrant position detector. For screws not included in the optical trapping kit, use the ones in the cap screw and hardware kit (Thorlabs, HW-KIT2). Eye protection should be worn at all times when the laser is on. Make sure the beam is contained within a safe area and reflective accessories, such as jewelry, should be avoided. Also, electrostatic discharge protection is advisable when handling laser diodes.
<ol>
	<li>Set up the optical tweezer kit (Thorlabs, OTKB/M) and the force measurement module (Thorlabs, OTKBFM) as per their respective manuals. A silicon-based avalanche photodiode (APD) (Thorlabs, APD110A) is used instead of the force measurement module's (Thorlabs, OTKBFM) quadrant position detector.</li>
	<li>Connect the APD to an oscilloscope (Tektronics, TDS1012) via a coaxial cable.</li>
	<li>Add a half-wave plate (Thorlabs, AHWP05M-980) inside the beam expander. The half-wave plate is fastened between two lens tubes (Thorlabs, SM1L03).</li>
</ol>
2. Nanofabrication
<ol>
	<li>Cut a gold-coated test slide (EMF Corp, Cr/Au) into four identical pieces. As an alternative to the commercially available slides, we have also used a 100 nm thick Au film with a 2 nm Ti adhesion layer deposited by e-beam deposition onto a 1 inch square glass slide at an elevated substrate temperature of 200 &deg;C for at least 1 hr. This produces a smooth polycrystalline film.</li>
	<li>Create a bitmap image of the double-nanohole structure as the input to the focused ion beam (FIB) system is a bitmap. The image consists of two solid circles, 160 nm in diameter with a center to center distance of 190 nm. This template creates a tip separation of approximately 15 nm. Between the circles, an optional thin line can be placed to remove any residue metal in between the tips. Figure 3a shows an example bitmap image.</li>
	<li>Fabricate the double-nanohole structure using a FIB (Hitachi, FB-2100) milling system. Convert the bitmap in step 2.2 into a FIB milling pattern (the dark area in the bitmap gets milled by the FIB). Use an ion accelerating voltage of 40 kV, a beam limiting aperture of 15 &mu;m diameter under 60 K times magnification. Mill eighty passes for each double-nanohole with a 5 &mu;sec dose time on each pass. Figure 3b shows a typical resulting structure. Repeat as needed. Multiple nanoholes should be made as to allow for errors.</li>
	<li>Add registration markers, either using the FIB and/or by hand to indicate the approximate location of the double-nanohole(s).</li>
	<li>Optionally, take a SEM image of the holes to accurately evaluate structure quality and tip separation.</li>
</ol>
3. Microfluidic Chamber
A process flow diagram for fabricating the microfluidic chamber is shown in Figure 4.
<ol>
	<li>Pour 10 g of polydimethylsiloxane (PDMS) base (Dow Corning Canada, Sylgard 184 Silicone Elastomer Base) and an additional 1 g of curing agent (Dow Corning Canada, Sylgard 184 Silicone Elastomer Curing Agent) into a disposable cup. Mix for a few minutes.</li>
	<li>Evacuate mixture in vacuum chamber until all bubbles are gone.</li>
	<li>Pour 1.5 g of PDMS into a 9 cm diameter Petri dish. Spin coat PDMS on the bottom of Petri dish at 950 rpm for 65 sec. Figure 4b shows the result. The thickness is not critical as long as it is under 80 &mu;m; the gold film is within the bottom microscope objective's working distance.</li>
	<li>Gently place 3-5 #1.5 coverslips (Fisher Scientific, 12-541-B) onto the PDMS such that they do not overlap and evacuate for 30 min as shown in Figure 4c.</li>
	<li>If coverslips moved and stacked one on top of each other during evacuation, gently move them off each other. Caution must be taken as to keep PDMS under coverslips thin and uniform.</li>
	<li>If manipulation of coverslips was required, evacuate Petri dish again for 30 min.</li>
	<li>Remove Petri dish from vacuum chamber and cook on hot plate for 20 min at 85 &deg;C.</li>
	<li>Using a razor blade, cut out one of the cover slips then gently pry up the slide using fine tip tweezers. A thin layer of PDMS will stay on the coverslip as PDMS is more adhesive to the glass cover slip than the PMMA Petri dish as in Figure 4e.</li>
	<li>Cut out a 3 x 3 mm window in the PDMS with a razor blade as in Figure 4f. This window will form the chamber where the nanoparticle solution will be kept.</li>
</ol>
4. Sample Preparation
<ol>
	<li>Fabricate a microscope slide with a &frac34;" diameter hole at the center using acrylic. This can be accomplished with a laser cutter. Other materials can be used as well. The gold sample will be placed inside the hole.</li>
	<li>Tape the circumference of the hole with double sided tape. Use a razor blade to cut excess tape.</li>
	<li>Place microscope slide on to the coverslip, PDMS face up.</li>
	<li>Dilute the polystyrene nanosphere solution (Thermo Scientific, 3020A) from 1 % w/v to 0.05 % w/v using deionized water. A micropipette can be used.</li>
	<li>Add a few drops of solution in the PDMS window. Add a drop onto the gold sample where the nanoholes are located.</li>
	<li>Place gold sample on top of coverslips such that the nanoholes are inside the PDMS window. Make sure bubbles are not present inside chamber. Press gold sample against coverslip and dab any excess solution.</li>
	<li>If using an oil immersion objective (as is the case here, but not necessary), add a drop of immersion oil on the opposite side of the coverslip, underneath the PDMS window. Take note of the location of the nanoholes.</li>
	<li>Insert microscope slide into slide holder, oil facing down, and then lower slide holder until immersion oil makes contact with the microscope objective.</li>
	<li>Roughly align slide stage such that indicator marks are underneath the objective.</li>
	<li>Follow indicator lines leading up to the nanoholes. Position slide such that indicator marks and other open areas are cleared from the screen center. Excessive light transmission can damage the APD.</li>
	<li>Turn on laser. As the dichroic mirror is not perfect, a spot near the center of the screen from the laser beam should appear.</li>
	<li>Using the piezo stage control software, further refine the alignment on all three axes.</li>
</ol>
5. Data Acquisition
<ol>
	<li>With the help of the indicator marks, position the spot close to a known nanohole location. The nanoholes will be too small to be resolved and should appear only as spots.</li>
	<li>The light transmission through the sample is indicated by the signal level on the oscilloscope. Further align the sample as to maximize light transmission. Be careful of indicator marks and visible and non-visible scratches as light transmission will be high in these areas. Nanoholes will show sudden jumps in light transmission while scratches exhibit more gradual changes.</li>
	<li>Using the waveplate, adjust the light polarization for highest light transmission as the double-nanohole structure is polarized.</li>
	<li>To minimize noise, build an RC filter with the breadboard, 200 K&Omega; resistor and 100 pF capacitor and connect it after the APD via a coaxial cable. These values can be adjusted for best performance, considering the bandwidth of data acquisition required.</li>
	<li>Connect oscilloscope and data acquisition module (Omega, USB-4711A) to the RC filter with coaxial cables and T adapter.</li>
	<li>Sample the APD's voltage using the data acquisition module for the desired time. Acquisition times are typically in the hundreds of seconds. In this case, a custom software package was used for data acquisition. The voltage is sampled at 2,000 times per second.</li>
	<li>Using Matlab, filter the acquired data using a Savitzky-Golay filter and plot it versus time on a graph.</li>
</ol>

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Production and Free Access to this article is sponsored by Thorlabs.

The current setup has effective trapping abilities owing to the structure of the nanohole. This nanohole traps ~10 nm-scale dielectric particles at low optical intensities. Other novel optical traps include optical dipole antennas11, whispering-gallery-mode optical resonators12,13 and waveguides14; however, they typically operate in the perturbative regime, which is limited by the inverse fourth order scaling of the required optical power versus particle size, unlike the SIBA and double-nanohole trap. Alternative aperture shapes have also been presented for trapping, such as a rectangular plasmonic nanopore15. Other favorable qualities shown by the double-nanohole trap include particle size-selective behavior7, a single trapping location (to limit multi-particle trapping) and ease of fabrication16. As an alternative to using a FIB, double-nanoholes can be fabricated by using a colloidal lithography6.
Trapping of biological materials of large polarizability and size has included bacteria3, living cells17,2,18, the tobacco mosaic virus3 and manipulation and stretching of DNA strands tethered at the ends with large dielectric particles19; however, direct trapping of smaller biological samples without tethering remains challenging. This trapping configuration is capable of trapping small dielectric particles at lower light intensities than conventional light tweezers and the circular nanohole, allowing small biological particles to be held for long periods of time without damage or tethering. Also, the trapping events exhibit a high signal-to-noise ratio allowing this setup to work as a sensitive sensor and detect the smallest biological particles, such as viruses and proteins. In fact, 20 nm polystyrene spheres have a refractive index of 1.59 which is comparable to the smallest biological particles such as viruses. This method could become a reliable and mature technique for immobilization and manipulation of nanoparticles, including biological particles.
Applications of this technique include integration into a microfluidic environment. Instead of a single microfluidic chamber, a channel would be used to dynamically control the environment, ideal for refractive index sensing. Such a setup would be set in a single microfluidic chip leading to a more stable and robust setup and faster analysis of solutes. Another option is development of a fluorescence detection scheme for characterization of single fluorescent-tagged viruses, semiconductor quantum dots and green fluorescent proteins. This setup also has potential for modification into a biosensor for a single virus or protein, allowing very small samples to be analyzed. Drug discovery21 and disease and infection detection22 would benefit from a single protein detector. Raman spectroscopy can be incorporated for detection of Raman signals of particles and single binding events. The double-nanohole structure allows strong local field enhancements at the tips, appropriate for tip-enhanced Raman spectroscopy23. A highly specific label-free method of material characterization would also be possible using Raman spectroscopy24.

<table border="1">
	<tbody>
		<tr>
			<td>Name</td>
			<td>Manufacturer</td>
			<td>Serial Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Immersion Oil</td>
			<td>Cargille Labs</td>
			<td>16484</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Sylgard 184 Silicone Elastomer Kit</td>
			<td>Dow Corning Canada</td>
			<td>&nbsp;</td>
			<td>Quantity: 1 
			Contains both PDMS base and curing agent</td>
		</tr>
		<tr>
			<td>Gold Coated Test Slides</td>
			<td>EMF Corp</td>
			<td>Cr/Au</td>
			<td>Quantity: 1 
			A Ti adhesion layer can be used as well</td>
		</tr>
		<tr>
			<td>No 1.5 Coverslips</td>
			<td>Fisher Scientific</td>
			<td>12-541-B</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Focused-Ion Beam System</td>
			<td>Hitachi</td>
			<td>FB-2100</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Portable Data Acquisition Module</td>
			<td>Omega Engineering</td>
			<td>USB-4711A</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Linear Stage</td>
			<td>Parker</td>
			<td>4034M</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Laser Diode Head and Controller</td>
			<td>Sacher Lasertechnik Group</td>
			<td>TEC 120</td>
			<td>Quantity: 1 
			Manual Tunable Littrow Laser System</td>
		</tr>
		<tr>
			<td>Digital Oscilloscope</td>
			<td>Tektronics</td>
			<td>TDS1012</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>20 nm Nanosphere Size Standards</td>
			<td>Thermo Scientific</td>
			<td>3020A</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>1" Lens Mount</td>
			<td>Thorlabs</td>
			<td>LMR1</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>0.3" Lens Tube</td>
			<td>Thorlabs</td>
			<td><a href="http://www.thorlabs.com/thorProduct.cfm?partNumber=SM1L03" target="_blank">SM1L03</a></td>
			<td>Quantity: 2</td>
		</tr>
		<tr>
			<td>Absorptive ND 4.0 Filter</td>
			<td>Thorlabs</td>
			<td>NE40A</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Aluminum Breadboard</td>
			<td>Thorlabs</td>
			<td>MB1824</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Avalanche Photodiode</td>
			<td>Thorlabs</td>
			<td>APD110A</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Digital Optical Power Meter</td>
			<td>Thorlabs</td>
			<td>PM100</td>
			<td>Quantity: 1 
			Obsolete, others will do</td>
		</tr>
		<tr>
			<td>Force Measurement Module</td>
			<td>Thorlabs</td>
			<td>OTKBFM</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Kinematic Mirror Mount</td>
			<td>Thorlabs</td>
			<td>KM200-E03</td>
			<td>Quantity: 1 
			With Near IR Laser Quality Mirror</td>
		</tr>
		<tr>
			<td>Laser Diode Constant Current Driver</td>
			<td>Thorlabs</td>
			<td>LD1255R</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>LD1255 Optical Table Mounting Plate</td>
			<td>Thorlabs</td>
			<td>LD1255P</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Mounted Achromatic Half-Wave Plate</td>
			<td>Thorlabs</td>
			<td>AHWP05M-980</td>
			<td>Quantity: 1 
			690 - 1200 nm</td>
		</tr>
		<tr>
			<td>Optical Tweezer Kit</td>
			<td>Thorlabs</td>
			<td>OTKB/M</td>
			<td>Quantity: 1 
			Metric or Imperial</td>
		</tr>
		<tr>
			<td>Post Holder Base</td>
			<td>Thorlabs</td>
			<td>BA2</td>
			<td>Quantity: 2</td>
		</tr>
		<tr>
			<td>Power Supply</td>
			<td>Thorlabs</td>
			<td>PS-12DC-US</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Power Supply Cable</td>
			<td>Thorlabs</td>
			<td>LD1255-CAB</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Right Angle Plate</td>
			<td>Thorlabs</td>
			<td>AP90</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Right Angle Post Clamp</td>
			<td>Thorlabs</td>
			<td>RA90</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Stainless Steel Optical Post</td>
			<td>Thorlabs</td>
			<td>TR3</td>
			<td>Quantity: 1</td>
		</tr>
		<tr>
			<td>Table Clamp</td>
			<td>Thorlabs</td>
			<td>CL1</td>
			<td>Quantity: 2 
			Obsolete, others will do</td>
		</tr>
		<tr>
			<td>Thermal Sensor</td>
			<td>Thorlabs</td>
			<td>PM210</td>
			<td>Quantity: 1 
			For digital optical power meter</td>
		</tr>
		<tr>
			<td>100 pF Capacitor</td>
			<td></td>
			<td></td>
			<td>Quantity: 1 
			Any brand, not critical</td>
		</tr>
		<tr>
			<td>200 KOhm Resistor</td>
			<td></td>
			<td></td>
			<td>Quantity: 1 
			Any brand, not critical</td>
		</tr>
		<tr>
			<td>Acrylic Sheet</td>
			<td></td>
			<td></td>
			<td>Quantity: 3" x 1" 
			Any brand, not critical</td>
		</tr>
		<tr>
			<td>Assortment of coaxial cables, wires and connectors</td>
			<td></td>
			<td></td>
			<td>As needed</td>
		</tr>
		<tr>
			<td>Breadboard</td>
			<td></td>
			<td></td>
			<td>Quantity: 1 
			Any brand, not critical</td>
		</tr>
		<tr>
			<td>Concave Lens</td>
			<td></td>
			<td></td>
			<td>Quantity: 1 
			Any brand, not critical</td>
		</tr>
		<tr>
			<td>Diamond Cutter</td>
			<td></td>
			<td></td>
			<td>Quantity: 1 
			Any brand, not critical</td>
		</tr>
		<tr>
			<td>Double Sided Tape</td>
			<td></td>
			<td></td>
			<td>Any brand, not critical</td>
		</tr>
		<tr>
			<td>Razor Blade</td>
			<td></td>
			<td></td>
			<td>Quantity: 1 
			Any brand, not critical</td>
		</tr>
		<tr>
			<td>Tweezers</td>
			<td></td>
			<td></td>
			<td>Quantity: 1 
			Any brand, fine tipped</td>
		</tr>
	</tbody>
</table>

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.

A typical acquisition trace is shown in Figure 5a. A trapping event is characteristically sudden, with a sharp edge, showing a clear switch between two transmission power levels. As particles are subject to Brownian motion, the trapping events will occur randomly. For 20 nm particles, transmission changes from trapping were typically around 5-10 % and trapping times, around 10-300 sec. The typical time to achieve a trapping event for the power and concentration outlined above is on the order of a minute. Due to steric hindrance it is uncommon to see multiple particle trapping simultaneously, although once a particle is released, it is typically followed by a subsequent trapping event. Depending on the quality of the results, there may be some increase in signal noise in the trapped state. This noise increase comes from the Brownian motion of the trapped particle. Without the trapped particle, this noise source is not present.
Some artifacts may show up in the results which are not indicative of trapping events. Results showing drifting, slow changes in the transmission over a period of minutes as shown in Figure 5b, should be discarded. Other artifacts may also be present such as inconsistent transmission changes, excessive noise or no trapping at all. For example, bubbles can cause discontinuous intensity jumps if care is not taken to ensure that the chamber is bubble-free. These bubbles will respond differently to the trapping events in terms of dynamic behavior and intensity change, and so they are readily identifiable. Such symptoms could be caused by a poor double-nanohole structure, contaminants or mechanical vibrations. A quiet, low-activity setting is highly recommended to place this setup. Also, allowing laser and stage to settle a few minutes after aligning can help as well.
<img alt="Figure 1" src="/files/ftp_upload/4424/4424fig1.jpg" /> 
Figure 1. Subwavelength aperture optical transmission: a) Without particle; b) Increased transmission due to dielectric particle; c) If particle attempts to leave, the decrease in light momentum (&Delta;T) will cause a force (F) on the particle as to pull it back into the hole; d) Red-shifting of the transmission curve caused by the particle, leading to the change in transmission &Delta;T.
<img alt="Figure 2" src="/files/ftp_upload/4424/4424fig2.jpg" /> 
Figure 2. a) Overall schematic of trapping setup, enlargement of red circle is shown in b); b) Enlargement showing the double-nanohole, and the nanospheres inside the PDMS chamber; c) SEM image of the double-nanohole structure. Acronyms used: LD = laser diode; ODF = optical density filter; HWP = half-wave plate; BE = beam expander; MR = mirror; MO = microscope objective; OI MO = oil immersion microscope objective; DH = double-nanohole; APD = avalanche photodetector.
<img alt="Figure 3" src="/files/ftp_upload/4424/4424fig3.jpg" /> 
Figure 3. a) Example bitmap figure used in the FIB fabrication; b) A SEM image of a double-nanohole.
<img alt="Figure 4" src="/files/ftp_upload/4424/4424fig4.jpg" /> 
Figure 4. Process diagram for fabricating the microfluidic chamber.
<img alt="Figure 5" src="/files/ftp_upload/4424/4424fig5.jpg" /> 
Figure 5. (a) Typical acquisition of trapping events with 20 nm polystyrene spheres. (b) A poor acquisition showing acute drifting.

Watch this Scientific Journal Video about Optical Trapping of Nanoparticles at JoVE.com

Optical Trapping of Nanoparticles

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 ...

Research

JoVE Journal

Engineering

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 ...

Construction and Testing of Coin Cells of Lithium Ion Batteries

Rechargeable lithium ion batteries have wide applications in electronics, where customers always demand more capacity and longer lifetime. Lithium ion batteries have also been considered to be used in electric and hybrid vehicles1 or even electrical grid stabilization systems2. All these applications simulate a dramatic increase in the research and development of battery materials3-7, including new materials3,8, doping9, nanostructuring10-13, coatings or surface modifications14-17 and novel binders18. Consequently, an increasing number of physicists, chemists and materials scientists have recently ventured into this area. Coin cells are widely used in research laboratories to test new battery materials; even for the research and development that target large-scale and high-power applications, small coin cells are often used to test the capacities and rate capabilities of new materials in the initial stage. 
 In 2010, we started a National Science Foundation (NSF) sponsored research project to investigate the surface adsorption and disordering in battery materials (grant no. DMR-1006515). In the initial stage of this project, we have struggled to learn the techniques of assembling and testing coin cells, which cannot be achieved without numerous help of other researchers in other universities (through frequent calls, email exchanges and two site visits). Thus, we feel that it is beneficial to document, by both text and video, a protocol of assembling and testing a coin cell, which will help other new researchers in this field. This effort represents the "Broader Impact" activities of our NSF project, and it will also help to educate and inspire students.
In this video article, we document a protocol to assemble a CR2032 coin cell with a LiCoO2 working electrode, a Li counter electrode, and (the mostly commonly used) polyvinylidene fluoride (PVDF) binder. To ensure new learners to readily repeat the protocol, we keep the protocol as specific and explicit as we can. However, it is important to note that in specific research and development work, many parameters adopted here can be varied. First, one can make coin cells of different sizes and test the working electrode against a counter electrode other than Li. Second, the amounts of C black and binder added into the working electrodes are often varied to suit the particular purpose of research; for example, large amounts of C black or even inert powder were added to the working electrode to test the "intrinsic" performance of cathode materials14. Third, better binders (other than PVDF) have also developed and used18. Finally, other types of electrolytes (instead of LiPF6) can also be used; in fact, certain high-voltage electrode materials will require the uses of special electrolytes7.

A protocol to construct and test coin cells of lithium ion batteries is described. The specific procedures of making a working electrode, preparing a counter electrode, assembling a cell inside a glovebox and testing the cell are presented.

Rechargeable lithium ion batteries have wide applications in electronics, where customers always demand more capacity and longer lifetime.  Lithium ion ...

Fabricating Metamaterials Using the Fiber Drawing Method

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe their electromagnetic properties to the structure of their constituents, instead of the atoms that compose them. For example, sub-wavelength metal wires can be arranged to possess an effective electric permittivity that is either positive or negative at a given frequency, in contrast to the metals themselves 2. This unprecedented control over the behaviour of light can potentially lead to a number of novel devices, such as invisibility cloaks 3, negative refractive index materials 4, and lenses that resolve objects below the diffraction limit 5. However, metamaterials operating at optical, mid-infrared and terahertz frequencies are conventionally made using nano- and micro-fabrication techniques that are expensive and produce samples that are at most a few centimetres in size 6-7. Here we present a fabrication method to produce hundreds of meters of metal wire metamaterials in fiber form, which exhibit a terahertz plasmonic response 8. We combine the stack-and-draw technique used to produce microstructured polymer optical fiber 9 with the Taylor-wire process 10, using indium wires inside polymethylmethacrylate (PMMA) tubes. PMMA is chosen because it is an easy to handle, drawable dielectric with suitable optical properties in the terahertz region; indium because it has a melting temperature of 156.6 &deg;C which is appropriate for codrawing with PMMA. We include an indium wire of 1 mm diameter and 99.99% purity in a PMMA tube with 1 mm inner diameter (ID) and 12 mm outside diameter (OD) which is sealed at one end. The tube is evacuated and drawn down to an outer diameter of 1.2 mm. The resulting fiber is then cut into smaller pieces, and stacked into a larger PMMA tube. This stack is sealed at one end and fed into a furnace while being rapidly drawn, reducing the diameter of the structure by a factor of 10, and increasing the length by a factor of 100. Such fibers possess features on the micro- and nano- scale, are inherently flexible, mass-producible, and can be woven to exhibit electromagnetic properties that are not found in nature. They represent a promising platform for a number of novel devices from terahertz to optical frequencies, such as invisible fibers, woven negative refractive index cloths, and super-resolving lenses.

Metamaterials at terahertz frequencies offer unique opportunities, but are challenging to fabricate in bulk. We adapt the fabrication procedure for microstructured polymer optical fibers to inexpensively fabricate metamaterials potentially on an industrial scale. We produce polymethylmethacrylate fibers containing ~10 &mu;m diameter indium wires separated by ~100 &mu;m, which exhibit a terahertz plasmonic response.

Metamaterials are man-made composite materials, fabricated by assembling components much smaller than the wavelength at which they operate 1. They owe ...

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 ...

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 &#039;Artemia salina&#039; 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 ...

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

Writing Bragg Gratings in Multicore Fibers

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong suppression at a single wavelength requires that all cores have matching transmission profiles. These gratings cannot be inscribed using the same method as for single-core fibers because the curved surface of the cladding acts as a lens, focusing the incoming UV laser beam and causing variations in exposure between cores. Therefore we use an additional optical element to ensure that the beam shape does not change while passing through the cross-section of the multicore fiber. This consists of a glass capillary tube which has been polished flat on one side, which is then placed over the section of the fiber to be inscribed. The laser beam enters the fiber through the flat surface of the capillary tube and hence maintains its original dimensions. This paper demonstrates the improvements in core-to-core uniformity for a 7-core fiber using this method. The technique can be generalized to larger multicore fibers.

We describe a technique for inscribing identical fiber Bragg gratings into each core of a multicore fiber. This is achieved by introducing an additional surface into the optical path to mitigate lensing by the curved surface of the fiber cladding.

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong ...

Compact Lens-less Digital Holographic Microscope for MEMS Inspection and Characterization

A micro-electro-mechanical-system (MEMS) is a widely used component in many industries, including energy, biotechnology, medical, communications, and automotive. However, effective inspection and characterization metrology systems are needed to ensure the functional reliability of MEMS. This study presents a system based on digital holography as a tool for MEMS metrology. Digital holography has gained increasing attention in the past 20 years. With the fast development and decreasing cost of sensor arrays, resolution of such systems has increased broadening potential applications. Thus, it has attracted attention from both research and industry sides as a potential reliable tool for industrial metrology. Indeed, by recording the interference pattern between an object beam (which contains sample height information) and a reference beam on a CCD camera, one can retrieve the quantitative phase information of an object. However, most of digital holographic systems are bulky and thus not easy to implement on industry production lines. The novelty of the system presented is that it is lens-less and thus very compact. In this study, it is shown that the Compact Digital Holographic Microscope (CDHM) can be used to evaluate several characteristics typically consider as criteria in MEMS inspections. The surface profiles of MEMS in both static and dynamic conditions are presented. Comparison with AFM is investigated to validate the accuracy of the CDHM.

We present a compact reflection digital holographic system (CDHM) for inspection and characterization of MEMS devices. A lens-less design using a diverging input wave providing natural geometrical magnification is demonstrated. Both static and dynamic studies are presented.

A micro-electro-mechanical-system (MEMS) is a widely used component in many industries, including energy, biotechnology, medical, communications, and ...

Electroactive Polymer Nanoparticles Exhibiting Photothermal Properties

A method for the synthesis of electroactive polymers is demonstrated, starting with the synthesis of extended conjugation monomers using a three-step process that finishes with Negishi coupling. Negishi coupling is a cross-coupling process in which a chemical precursor is first lithiated, followed by transmetallation with ZnCl2. The resultant organozinc compound can be coupled to a dibrominated aromatic precursor to give the conjugated monomer. Polymer films can be prepared via electropolymerization of the monomer and characterized using cyclic voltammetry and ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy. Nanoparticles (NPs) are prepared via emulsion polymerization of the monomer using a two-surfactant system to yield an aqueous dispersion of the polymer NPs. The NPs are characterized using dynamic light scattering, electron microscopy, and UV-Vis-NIR-spectroscopy. Cytocompatibility of NPs is investigated using the cell viability assay. Finally, the NP suspensions are irradiated with a NIR laser to determine their effectiveness as potential materials for photothermal therapy (PTT).

A protocol is presented for the synthesis and preparation of nanoparticles consisting of electroactive polymers.

A method for the synthesis of electroactive polymers is demonstrated, starting with the synthesis of extended conjugation monomers using a three-step ...

Optical Trap Loading of Dielectric Microparticles In Air

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly scattered dielectric microparticles adhered to a glass substrate are momentarily detached using ultrasonic vibrations generated by a piezoelectric transducer (PZT). Then, the optical beam focused on a selected particle lifts it up to the optical trap while the vibrationally excited microparticles fall back to the substrate. A particle may be trapped at the nominal focus of the trapping beam or at a position above the focus (referred to here as the levitation position) where gravity provides the restoring force. After the measurement, the trapped particle can be placed at a desired position on the substrate in a controlled manner. 
In this protocol, an experimental procedure for selective optical trap loading in air is outlined. First, the experimental setup is briefly introduced. Second, the design and fabrication of a PZT holder and a sample enclosure are illustrated in detail. The optical trap loading of a selected microparticle is then demonstrated with step-by-step instructions including sample preparation, launching into the trap, and use of electrostatic force to excite particle motion in the trap and measure charge. Finally, we present recorded particle trajectories of Brownian and ballistic motions of a trapped microparticle in air. These trajectories can be used to measure stiffness or to verify optical alignment through time domain and frequency domain analysis. Selective trap loading enables optical tweezers to track a particle and its changes over repeated trap loadings in a reversible manner, thereby enabling studies of particle-surface interaction.

A protocol for launching and stably trapping selected dielectric microparticles in air is presented.

We demonstrate a method to trap a selected dielectric microparticle in air using radiation pressure from a single-beam gradient optical trap. Randomly ...