The authors would like to thank Picometrix for providing the LT-GaAs substrate and gratefully acknowledge the financial support from Michigan Space Grant Consortium, DARPA Young Faculty Award managed by Dr. John Albrecht (contract # N66001-10-1-4027), NSF CAREER Award managed by Dr. Samir El-Ghazaly (contract # N00014-11-1-0096), ONR Young Investigator Award managed by Dr. Paul Maki (contract # N00014-12-1-0947), and ARO Young Investigator Award managed by Dr. Dev Palmer (contract # W911NF-12-1-0253).

1. Plasmonic Photoconductive Emitter Fabrication
<ol>
 <li>Fabricate plasmonic gratings.
 <ol>
 <li>Clean the semiconductor wafer by immersing in acetone (2 min) followed by isopropanol (2 min), and rinsing with deionized water (10 sec).</li>
 <li>Dry the sample with nitrogen and heat it on a hotplate at 115 &deg;C for 90 sec to remove any remaining water.</li>
 <li>Spin MicroChem 950K PMMA A4 on the sample at 4,000 rpm for 45 sec. Pre-bake the resist on a hotplate at 180 &deg;C for 3 min.</li>
 <li>Load the sample into an electron beam lithography tool (JEOL JBX-6300-FS). Expose the plasmonic grating pattern at a base dose around 650 &mu;C/cm2, using a 100 kV acceleration voltage.</li>
 <li>Develop PMMA by immersing the sample in a MIBK:IPA 1:3 mixture for 90 sec. Immediately transfer the sample to a solution of pure isopropanol for 60 sec.</li>
 <li>Rinse the sample with deionized water for 10 sec and then dry the sample with nitrogen.</li>
 <li>Load the sample into a plasma stripper (YES-CV200RFS). Descum the sample using 30 W RF power at 30 &deg;C with a 100 sccm O2 flow rate for 10 sec.</li>
 <li>Remove surface oxide by immersing in a HCl:H20 3:10 mixture for 30 sec. Immediately transfer the sample to a cascade rinse of deionized water for 4 min.</li>
 <li>Transfer the sample to a beaker of deionized water to minimize exposure to atmospheric oxygen before metal deposition.</li>
 <li>Take beaker containing the sample in deionized water to a metal evaporator (Denton SJ-20). Vent the chamber and then remove, dry, and load the sample into the chamber (these steps should be followed without interruption to prevent surface oxide formation on the sample).</li>
 <li>Pump the chamber to a pressure below 2x10-6 Torr. Deposit Ti/Au (50/450 &Aring;).</li>
 <li>Vent the chamber and remove the sample.</li>
 <li>In order to lift-off the deposited metal, place the sample on a Teflon holder in a beaker of acetone, cover, and leave overnight. Uncover the beaker, place it in an ultrasonic agitator, and wait until all unwanted metal is removed (typically 30 sec).</li>
 </ol>
 </li>
 <li>Deposit SiO2 passivation.

 <ol>
 <li>Clean the sample as in Steps 1.1.1 - 1.1.2.</li>
 <li>Load the sample in a plasma-enhanced chemical vapor deposition tool (GSI PECVD). Deposit 1500 &Aring; of SiO2 at 200 &deg;C.</li>
 </ol>
 </li>
 <li>Open contact vias through SiO2.

 <ol>
 <li>Clean the sample as in Steps 1.1.1 - 1.1.2.</li>
 <li>Spin on HMDS at 4,000 rpm for 30 sec. Spin on Megaposit SPR 220-3.0 photoresist at 4,000 rpm for 30 sec. Pre-bake the resist on a hotplate at 115 &deg;C for 90 sec.</li>
 <li>Load the sample and mask plate into projection lithography stepper (GCA AutoStep 200). Align the sample and expose.</li>
 <li>Post-bake the exposed photoresist on a hotplate at 115 &deg;C for 90 sec.</li>
 <li>Develop resist in AZ 300 MIF developer for 60 sec.</li>
 <li>Immediately move the sample to a cascade rinse of deionized water for 4 min. Dry the sample with nitrogen.</li>
 <li>Load the sample into a reactive ion etcher (LAM 9400). Etch SiO2 using a TCP RF power of 500 W, a Bias RF power of 100 W, 15 sccm of SF6-, 50 sccm of C4F8, 50 sccm of He, 50 sccm of Ar for 80 sec.</li>
 <li>Remove the bulk of the photoresist by placing the sample in acetone (5 min) followed by isopropanol (2 min). Rinse in deionized water (10 sec). Dry with nitrogen.</li>
 <li>Remove the residual photoresist by loading the sample in a plasma stripper (YES-CV200RFS). Remove the photoresist using 800 W RF power at 30 &deg;C with a 100 sccm O2 flow rate for 5 min.</li>
 </ol>
 </li>
 <li>Fabricate antennas and bias lines.
 <ol>
 <li>Repeat Steps 1.3.1 - 1.3.6 to pattern antennas and bias lines.</li>
 <li>Repeat Steps 1.1.8 - 1.1.9 to remove surface oxide.</li>
 <li>Take the beaker containing the sample and deionized water to a metal evaporator (Denton SJ-20).</li>
 <li>Vent the chamber and then quickly remove, dry, and load the sample into the chamber.</li>
 <li>Pump the chamber to a pressure below 2x10-6 Torr. Deposit Ti/Au (10/4,000 &Aring;).</li>
 <li>Vent the chamber and remove the sample.</li>
 <li>Repeat Step 1.1.13 to lift-off the deposited metal.</li>
 </ol>
 </li>
 <li>Package the sample.
 <ol>
 <li>Glue the edges of a 12 mm diameter hyper-hemispherical silicon lens to a 2 inch aluminum washer with 8 mm hole.</li>
 <li>Glue a PCB board with metal traces, to which one can easily solder, to the aluminum washer.</li>
 <li>Mount the fabricated plasmonic photoconductive terahertz emitter prototypes on the silicon lens using thin epoxy.</li>
 <li>Wire bond the device contact pads to a PCB board glued on the same aluminum washer.</li>
 <li>Solder wires to the metal traces on the PCB board.</li>
 <li>Connect device contact pads to a parametric analyzer (Hewlett Packard 4155A) using wires soldered to the corresponding pads of the PCB board for testing purposes.</li>
 </ol>
 </li>
</ol>

2. Plasmonic Photoconductive Emitter Characterization

<ol>
 <li>Device alignment.
 <ol>
 <li>Place the aluminum washer carrying the plasmonic photoconductive terahertz emitter prototypes on a rotation mount and tightly focus the optical pump from a Ti:Sapphire mode-locked laser (MIRA 900D V10 XW OPT 110V) onto the active area of each device.</li>
 <li>Adjust the rotation mount such that the electric field of the optical pump is oriented for efficient excitation of surface plasmon waves (normal to the plasmonic gratings).</li>
 <li>Use the parametric analyzer to simultaneously apply bias voltages to each device and measure the induced electrical current in each device. Confirm the optimum optical pump alignment and polarization adjustment by maximizing the photocurrent of each device under test.</li>
 </ol>
 </li>
 <li>Output power measurement.
 <ol>
 <li>Use an optical chopper (Thorlabs MC2000) to modulate the optical pump from the mode-locked pump laser incident on each device.</li>
 <li>Measure the output power of the plasmonic photoconductive terahertz emitter prototypes using a pyroelectric detector (Spectrum Detector, Inc. SPI-A-65 THz).</li>
 <li>Connect the output of the pyroelectric detector to a lock-in amplifier (Stanford Research Systems SR830) with the optical chopper's reference frequency to recover terahertz power data at low noise levels.</li>
 </ol>
 </li>
 <li>Radiation spectral characterization.
 <ol>
 <li>Start with a Ti:Sapphire mode-locked laser and use a beam splitter to split the output of the mode-locked laser into a pump beam and a probe beam.</li>
 <li>Use an electrooptic modulator (Thorlabs EO-AM-NR-C2) to modulate the optical beam in the pump path. Focus the pump beam onto the active area of the photoconductive emitter under test to generate terahertz radiation.</li>
 <li>Collimate the generated terahertz beam using a first polyethylene spherical lens. Focus the collimated terahertz beam using a second polyethylene spherical lens.</li>
 <li>Before the focus of the terahertz beam, combine the collimated terahertz beam with the probe optical beam using an ITO coated glass filter.</li>
 <li>Place a 1 mm thick, &lt;110&gt; ZnTe crystal mounted on a rotation stage at the combined focus of the optical and terahertz beam.</li>
 <li>Insert a controllable optical delay line in the optical probe path by using a motorized linear stage (Thorlabs NRT100) to vary the time delay between the optical and terahertz pulses interacting inside the ZnTe crystal.</li>
 <li>Using a half-waveplate in the probe path, rotate the polarization of the optical probe to be at a 45&deg; angle relative to the terahertz polarization direction.</li>
 <li>Use a quarter-waveplate after the ZnTe crystal, convert the optical beam polarization into circular polarization.</li>
 <li>Split the circularly polarized optical beam into two branches by a Wollaston prism. Measure the optical beam power in each branch using two balanced detectors connected to a lock-in amplifier.</li>
 <li>Connect the motorized delay line and lock-in amplifier to a computer. Write a Matlab script to iteratively move the position of the motorized delay line, pause, and read the signal magnitude from the lock-in amplifier.</li>
 <li>Convert the stage position to the time domain, through dividing the total optical delay length by the speed of light, followed by a discreet Fourier transform (using Matlab) to obtain the frequency domain data.</li>
 </ol>
 </li>
</ol>

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No conflicts of interest declared.

In this video article, we present a novel photoconductive terahertz generation technique that uses a plasmonic contact electrode configuration to enhance the optical-to-terahertz conversion efficiency by two orders of magnitude. The significant increase in the terahertz radiation power from the presented plasmonic photoconductive emitters is very valuable for future high-sensitivity terahertz imaging, spectroscopy and spectrometry systems used for advanced chemical identification, medical imaging, biological sensing, ast...

We present a novel photoconductive terahertz emitter that uses a plasmonic contact electrode configuration to enhance the optical-to-terahertz conversion efficiency by two orders of magnitude. Our technique addresses the most important limitations of conventional photoconductive terahertz emitters, namely low output power and poor power efficiency, which originate from the inherent tradeoff between high quantum efficiency and ultrafast operation of conventional photoconductors.
One of the key novelties in our design that led to this leapfrog performance improvement is to design a contact electrode configuration that accumulates a large number of photo-generated carriers in close proximity to the contact electrodes, such that they can be collected within a sub-picosecond timescale. In other words, the tradeoff between photoconductor ultrafast operation and high quantum efficiency is mitigated by spatial manipulation of the photo-generated carriers. Plasmonic contact electrodes offer this unique capability by (1) allowing light confinement into nanoscale device active areas between the plasmonic electrodes (beyond diffraction limit), (2) extraordinary light enhancement at the metal contact and photo-absorbing semiconductor interface 10, 11. Another important attribute of our solution is that it accommodates large photoconductor active areas without a considerable increase in the parasitic loading to the terahertz radiating antenna. Utilizing large photoconductor active areas enable mitigating the carrier screening effect and thermal breakdown, which are the ultimate limitations for the maximum radiation power from conventional photoconductive emitters. This video article is concentrated on the unique attributes of our presented solution by describing the governing physics, numerical modeling, and experimental verification. We experimentally demonstrate 50 times higher terahertz powers from a plasmonic photoconductive emitter in comparison with a similar photoconductive emitter with non-plasmonic contact electrodes.

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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.

To demonstrate the potential of plasmonic electrodes for terahertz power enhancement, we fabricated two terahertz emitters: a conventional (Figure 1a) and plasmonic (Figure 1b) photoconductive emitter incorporating plasmonic contact electrodes to reduce carrier transport times to contact electrodes. Both designs consist of an ultrafast photoconductor with 20 &mu;m gap between anode and cathode contacts, connected to a 60 &mu;m long bowtie antenna with maximum and minimum widths of 100 &m...

Watch this Scientific Journal Video about Design, Fabrication, and Experimental Characterization of Plasmonic Photoconductive Terahertz Emitters at JoVE.com

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

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 ...

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14.8K Views.  University of Michigan. 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.

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

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 ...

Template Directed Synthesis of Plasmonic Gold Nanotubes with Tunable IR Absorbance

A nearly parallel array of pores can be produced by anodizing aluminum foils in acidic environments1, 2. Applications of anodic aluminum oxide (AAO) membranes have been under development since the 1990's and have become a common method to template the synthesis of high aspect ratio nanostructures, mostly by electrochemical growth or pore-wetting. Recently, these membranes have become commercially available in a wide range of pore sizes and densities, leading to an extensive library of functional nanostructures being synthesized from AAO membranes. These include composite nanorods, nanowires and nanotubes made of metals, inorganic materials or polymers 3-10. Nanoporous membranes have been used to synthesize nanoparticle and nanotube arrays that perform well as refractive index sensors, plasmonic biosensors, or surface enhanced Raman spectroscopy (SERS) substrates 11-16, as well as a wide range of other fields such as photo-thermal heating 17, permselective transport 18, 19, catalysis 20, microfluidics 21, and electrochemical sensing 22, 23. Here, we report a novel procedure to prepare gold nanotubes in AAO membranes. Hollow nanostructures have potential application in plasmonic and SERS sensing, and we anticipate these gold nanotubes will allow for high sensitivity and strong plasmon signals, arising from decreased material dampening 15.

Solution-suspendable gold nanotubes with controlled dimensions can be synthesized by electrochemical deposition in porous anodic aluminum oxide (AAO) membranes using a hydrophobic polymer core. Gold nanotubes and nanotube arrays hold promise for applications in plasmonic biosensing, surface-enhanced Raman spectroscopy, photo-thermal heating, ionic and molecular transport, microfluidics, catalysis and electrochemical sensing.

A nearly parallel array of pores can be produced by anodizing aluminum foils in acidic environments1, 2. Applications of anodic aluminum oxide (AAO) ...

Chemistry

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 ...

Terahertz Microfluidic Sensing Using a Parallel-plate Waveguide Sensor

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic samples with a wide range of possible sensor designs such as interferometers and resonators 1,2. Most of the existing RI sensing applications focus on biological materials in aqueous solutions in visible and IR frequencies, such as DNA hybridization and genome sequencing. At terahertz frequencies, applications include quality control, monitoring of industrial processes and sensing and detection applications involving nonpolar materials.
Several potential designs for refractive index sensors in the terahertz regime exist, including photonic crystal waveguides 3, asymmetric split-ring resonators 4, and photonic band gap structures integrated into parallel-plate waveguides 5. Many of these designs are based on optical resonators such as rings or cavities. The resonant frequencies of these structures are dependent on the refractive index of the material in or around the resonator. By monitoring the shifts in resonant frequency the refractive index of a sample can be accurately measured and this in turn can be used to identify a material, monitor contamination or dilution, etc.
The sensor design we use here is based on a simple parallel-plate waveguide 6,7. A rectangular groove machined into one face acts as a resonant cavity (Figures 1 and 2). When terahertz radiation is coupled into the waveguide and propagates in the lowest-order transverse-electric (TE1) mode, the result is a single strong resonant feature with a tunable resonant frequency that is dependent on the geometry of the groove 6,8. This groove can be filled with nonpolar liquid microfluidic samples which cause a shift in the observed resonant frequency that depends on the amount of liquid in the groove and its refractive index 9.
Our technique has an advantage over other terahertz techniques in its simplicity, both in fabrication and implementation, since the procedure can be accomplished with standard laboratory equipment without the need for a clean room or any special fabrication or experimental techniques. It can also be easily expanded to multichannel operation by the incorporation of multiple grooves 10. In this video we will describe our complete experimental procedure, from the design of the sensor to the data analysis and determination of the sample refractive index.

The procedure for implementing a refractive index sensor for terahertz frequencies based on a grooved parallel-plate waveguide geometry is described here. The method yields a measurement of the refractive index of a small volume of liquid through monitoring of the shift in the resonant frequency of the waveguide structure

Refractive index (RI) sensing is a powerful noninvasive and label-free sensing technique for the identification, detection and monitoring of microfluidic ...

Simulation, Fabrication and Characterization of THz Metamaterial Absorbers

Metamaterials (MM), artificial materials engineered to have properties that may not be found in nature, have been widely explored since the first theoretical1 and experimental demonstration2 of their unique properties. MMs can provide a highly controllable electromagnetic response, and to date have been demonstrated in every technologically relevant spectral range including the optical3, near IR4, mid IR5 , THz6 , mm-wave7 , microwave8 and radio9 bands. Applications include perfect lenses10, sensors11, telecommunications12, invisibility cloaks13 and filters14,15. We have recently developed single band16, dual band17 and broadband18 THz metamaterial absorber devices capable of greater than 80% absorption at the resonance peak. The concept of a MM absorber is especially important at THz frequencies where it is difficult to find strong frequency selective THz absorbers19. In our MM absorber the THz radiation is absorbed in a thickness of ~ &lambda;/20, overcoming the thickness limitation of traditional quarter wavelength absorbers. MM absorbers naturally lend themselves to THz detection applications, such as thermal sensors, and if integrated with suitable THz sources (e.g. QCLs), could lead to compact, highly sensitive, low cost, real time THz imaging systems.

This protocol outlines the simulation, fabrication and characterization of THz metamaterial absorbers. Such absorbers, when coupled with an appropriate sensor, have applications in THz imaging and spectroscopy.

Metamaterials (MM),  artificial materials engineered to have properties that may not be found in  nature, have been widely explored since the first ...

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 ...

Colloidal Synthesis of Nanopatch Antennas for Applications in Plasmonics and Nanophotonics

We present a method for colloidal synthesis of silver nanocubes and the use of these in combination with a smooth gold film, to fabricate plasmonic nanoscale patch antennas. This includes a detailed procedure for the fabrication of thin films with a well-controlled thickness over macroscopic areas using layer-by-layer deposition of polyelectrolyte polymers, namely poly(allylamine) hydrochloride (PAH) and polystyrene sulfonate (PSS). These polyelectrolyte spacer layers serve as a dielectric gap in between silver nanocubes and a gold film. By controlling the size of the nanocubes or the gap thickness, the plasmon resonance can be tuned from about 500 nm to 700 nm. Next, we demonstrate how to incorporate organic sulfo-cyanine5 carboxylic acid (Cy5) dye molecules into the dielectric polymer gap region of the nanopatch antennas. Finally, we show greatly enhanced fluorescence of the Cy5 dyes by spectrally matching the plasmon resonance with the excitation energy and the Cy5 absorption peak. The method presented here enables the fabrication of plasmonic nanopatch antennas with well-controlled dimensions utilizing colloidal synthesis and a layer-by-layer dip-coating process with the potential for low cost and large-scale production. These nanopatch antennas hold great promise for practical applications, for example in sensing, ultrafast optoelectronic devices and for high-efficiency photodetectors.

A protocol for the colloidal synthesis of silver nanocubes and fabrication of plasmonic nanoscale patch antennas with sub-10 nm gaps is presented.

We present a method for colloidal synthesis of silver nanocubes and the use of these in combination with a smooth gold film, to fabricate plasmonic ...

Fabrication of Nanopillar-Based Split Ring Resonators for Displacement Current Mediated Resonances in Terahertz Metamaterials

Terahertz (THz) split ring resonator (SRR) metamaterials (MMs) has been studied for gas, chemical, and biomolecular sensing applications because the SRR is not affected by environmental characteristics such as the temperature and pressure surrounding the resonator. Electromagnetic radiation in THz frequencies is biocompatible, which is a critical condition especially for the application of the biomolecular sensing. However, the quality factor (Q-factor) and frequency responses of traditional thin-film based split ring resonator (SRR) MMs are very low, which limits their sensitivities and selectivity as sensors. In this work, novel nanopillar-based SRR MMs, utilizing displacement current, are designed to enhance the Q-factor up to 450, which is around 45 times higher than that of traditional thin-film-based MMs. In addition to the enhanced Q-factor, the nanopillar-based MMs induce a larger frequency shifts (17 times compared to the shift obtained by the traditional thin-film based MMs). Because of the significantly enhanced Q-factors and frequency shifts as well as the property of biocompatible radiation, the THz nanopillar-based SRR are ideal MMs for the development of biomolecular sensors with high sensitivity and selectivity without inducing damage or distortion to biomaterials. A novel fabrication process has been demonstrated to build the nanopillar-based SRRs for displacement current mediated THz MMs. A two-step gold (Au) electroplating process and an atomic layer deposition (ALD) process are used to create sub-10 nm scale gaps between Au nanopillars. Since the ALD process is a conformal coating process, a uniform aluminum oxide (Al2O3) layer with nanometer-scale thickness can be achieved. By sequentially electroplating another Au thin film to fill the spaces between Al2O3 and Au, a close-packed Au-Al2O3-Au structure with nano-scale Al2O3 gaps can be fabricated. The size of the nano-gaps can be well defined by precisely controlling the deposition cycles of the ALD process, which has an accuracy of 0.1 nm.

A protocol for the design and fabrication of a novel nanopillar-based split ring resonator (SRR) is presented.

Terahertz (THz) split ring resonator (SRR) metamaterials (MMs) has been studied for gas, chemical, and biomolecular sensing applications because the SRR ...

Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography

Within recent years, the field of plasmonics has exploded as researchers have demonstrated exciting applications related to chemical and optical sensing in combination with new nanofabrication techniques. A plasmon is a quantum of charge density oscillation that lends nanoscale metals such as gold and silver unique optical properties. In particular, gold and silver nanoparticles exhibit localized surface plasmon resonances-collective charge density oscillations on the surface of the nanoparticle-in the visible spectrum. Here, we focus on the fabrication of periodic arrays of anisotropic plasmonic nanostructures. These half-shell (or nanocup) structures can exhibit additional unique light-bending and polarization-dependent optical properties that simple isotropic nanostructures cannot. Researchers are interested in the fabrication of periodic arrays of nanocups for a wide variety of applications such as low-cost optical devices, surface-enhanced Raman scattering, and tamper indication. We present a scalable technique based on colloidal lithography in which it is possible to easily fabricate large periodic arrays of nanocups using spin-coating and self-assembled commercially available polymeric nanospheres. Electron microscopy and optical spectroscopy from the visible to near-infrared (near-IR) was performed to confirm successful nanocup fabrication. We conclude with a demonstration of the transfer of nanocups to a flexible, conformal adhesive film.

We demonstrate the fabrication of periodic gold nanocup arrays using colloidal lithographic techniques and discuss the importance of nanoplasmonic films.

Within recent years, the field of plasmonics has exploded as researchers have demonstrated exciting applications related to chemical and optical sensing ...

Determination of the Excitation and Coupling Rates Between Light Emitters and Surface Plasmon Polaritons

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising from metallic periodic arrays without involving time-resolved techniques. We have formulated the rates by quantities that can be measured by simple optical measurements. The instrumentation based on angle- and polarization-resolved reflectivity and photoluminescence spectroscopy will be described in detail here. Our approach is intriguing due to its simplicity, which requires routine optics and several mechanical stages, and thus is highly affordable to most of the research laboratories.

This protocol describes the instrumentation for determining the excitation and coupling rates between light emitters and Bloch-like surface plasmon polaritons arising from periodic arrays.

We have developed a unique method to measure the excitation and coupling rates between the light emitters and surface plasmon polaritons (SPPs) arising ...