Name: fabrication of nanopillar-based split ring resonators for displacement current mediated resonances in terahertz metamaterials
Uploaded: 2017-03-23T14:00:00
Description: Watch this Scientific Journal Video about Fabrication of Nanopillar-Based Split Ring Resonators for Displacement Current Mediated Resonances in Terahertz Metamaterials at JoVE.com

This material is based upon work supported by a start-up fund at the University of Minnesota, Twin Cities. Parts of this work were carried out in the Characterization Facility, University of Minnesota, a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org) via the MRSEC program. A portion of this work was also carried out in the Minnesota Nano Center which receives partial support from the NSF through the NNCI program.

Caution: Several of the chemicals used in these syntheses are toxic, highly flammable, and may cause irritation and severe organ damage when touched or inhaled. Please wear appropriate personal protective equipment (PPE) when handling.
1. Preparation of the First Layer of Gold (Au) Nanopillar Arrays (Figure 2a-c and Figure 2e-g)
<ol>
	<li>Preparation of Copper (Cu) Seed layers for Au electroplating (Figure 2a, b and Figure 2e, f)
	<ol>
		<li>Use a 4&#34; high resistivity silicon (Si) wafer (resi...

<ol>
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This fabrication technique has significant advantages for creating nano-scale structures over existing methods such as E-beam lithography and self-assembly. First, nano-scale structures can be realized over a large area (an entire wafer) using a photomask that features nanopillar arrays, which is not practical with an E-beam lithography process. Second, the fabrication process uses a traditional wafer scale micro fabrication process, which is much faster, simpler, and cheaper compared to E-beam lithography. Third, the at...

Terahertz (THz) metamaterials (MMs) have been developed for biomedical sensors and frequency-agile devices1,2,3,4,5,6,7,8,9,10,11. In order to improve the sensitivity and frequency selectivity of the THz MM sensors, a nanopillar-based split ring resonator (SRR) has been designed using displacement current generated inside gold (Au) nanopillar arrays to excite THz resonances with ultra-high quality factors (Q-factors) (~450) (Figure 1)12. Even though nanopillar-based SRRs show high Q-factors and promising sensing abilities, fabrication of such nanostructures with high aspect ratios (more than 40) and nano-scale gaps (sub-10 nm) over a large area remains challenging13.
The most commonly used technique to fabricate nano-scale structures is electron-beam lithography (EBL)14,15,16,17. However, the resolution of EBL is still limited due to the beam spot size, electron scattering, properties of the resist, and the development process18,19. In addition, it is not practical to fabricate nanostructures using EBL over a large area due to a slow process time and large process costs20. Another strategy to achieve nanostructures is to use a self-assembly technique21,22. By self-assembling metal nanocubes (NCs) in a solution and utilizing the electrostatic interaction and the association of polymer ligands between NCs, a well-organized one-dimensional NC array with nano-scale gaps can be achieved23. The nano-gap size depends on the polymer ligands between the NCs and can be controlled by applying different polymer materials with different molecular weights24,25,26. Self-assembly is a powerful technique for achieving scalable and cost-efficient nanostructures23. However, the fabrication process is more complicated compared to conventional micro and nano fabrication processes, and the control of nano-gap sizes is not precise enough for electronic device applications. In order to successfully fabricate nanopillar-based SRRs, a novel fabrication method should be invented to achieve the following goals: i) the fabrication process is easy to apply and is compatible with conventional micro and nano fabrication processes; ii) fabrication over a large area is applicable; iii) nano-gap sizes can be easily and precisely controlled with a 0.1 nm resolution and can be scaled down to 10 nm or less.
A novel fabrication method is demonstrated using the combination of an electroplating process and an atomic layer deposition (ALD) process to fabricate nanopillar-based SRRs. Since electroplating is a self-filling process with low cost, it is easy to fabricate structures over a large area. ALD is a chemical vapor deposition (CVD) process that can be precisely controlled by the reaction cycle during the process. The resolution of ALD thin film can be 0.1 nm, and the thin film is uniformly coated with a&#160;high quality, which is suitable to create nano-scale gaps27,28. Nanopillar-based SRR array with 10 nm gaps or less can be successfully fabricated over an area of 6 mm &#215; 6 mm. Both simulated and measured THz transmission spectra show resonant behaviors with ultra-high Q-factors and large frequency shifts, which proves the feasibility of the nanopillar-based SRRs mediated by displacement current. The detailed fabrication process is described below in the protocol section, and the video protocol can help practitioners to understand the fabrication process and avoid common mistakes associated with the fabrication of nanopillar-based SRRs.

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		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Silicon Wafer</td>
			<td style="width:295px;">Siltronic AG</td>
			<td style="width:199px;">N/A</td>
			<td style="width:437px;">100mm diameter, N-type, one-side polish, resitivity: 560-840 &#937;&#8226;cm</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Chromium</td>
			<td style="width:295px;">Kurt J. Lesker Company</td>
			<td style="width:199px;">EVMCR35J</td>
			<td>99.95% pure</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Copper</td>
			<td style="width:295px;">Kurt J. Lesker Company</td>
			<td style="width:199px;">EVMCU40QXQJ</td>
			<td>99.99% pure</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">E-Beam Evaporator System</td>
			<td style="width:295px;">Rocky Mountain Vacuum Tech.</td>
			<td style="width:199px;">N/A</td>
			<td>RME-2000</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">S1813 Positive Photoresist</td>
			<td style="width:295px;">Microposit</td>
			<td style="width:199px;">10018348</td>
			<td>N/A</td>
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			<td height="21" style="height:21px;width:311px;">Spinner</td>
			<td style="width:295px;">Best Tools</td>
			<td style="width:199px;">S0114031123</td>
			<td>SMART COATER 100</td>
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			<td height="21" style="height:21px;width:311px;">Mask Aligner</td>
			<td style="width:295px;">Midas</td>
			<td style="width:199px;">MDA-400LJ</td>
			<td>N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Digital Hot Plate</td>
			<td style="width:295px;">Thermo Scientific</td>
			<td style="width:199px;">HP131725</td>
			<td>Super-Nuvoa series, maximum temperature: 370 degree C</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">MF319 Developer</td>
			<td style="width:295px;">Microposit</td>
			<td style="width:199px;">10018042</td>
			<td>N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Acetone</td>
			<td style="width:295px;">Fisher Chemical</td>
			<td style="width:199px;">A18P-4</td>
			<td style="width:437px;">N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Isopropyl Alcohol</td>
			<td style="width:295px;">Fisher Chemical</td>
			<td style="width:199px;">A416-4</td>
			<td>N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Gold 25 ES RTU</td>
			<td style="width:295px;">Technic Inc.</td>
			<td style="width:199px;">391427</td>
			<td>N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Source Meter</td>
			<td style="width:295px;">Keithley</td>
			<td style="width:199px;">N/A</td>
			<td>2612 System SourceMeter</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Microscope</td>
			<td style="width:295px;">Omax</td>
			<td style="width:199px;">NJF-120A</td>
			<td>N/A</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Profilometer</td>
			<td style="width:295px;">Tencor Instruments</td>
			<td style="width:199px;">N/A</td>
			<td>Alpha-Step 200</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">APS Copper Etchant 100</td>
			<td style="width:295px;">Transfene Company, Inc.</td>
			<td style="width:199px;">N/A</td>
			<td>N/A</td>
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			<td height="21" style="height:21px;width:311px;">CE-5 M Chromium Mask Etchant</td>
			<td style="width:295px;">Transfene Company, Inc.</td>
			<td style="width:199px;">N/A</td>
			<td>N/A</td>
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		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Atomic Layer Deposition System</td>
			<td style="width:295px;">Cambridge Nano Tech inc.</td>
			<td style="width:199px;">N/A</td>
			<td>Savannah series</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:311px;">Ion Mill Etching System</td>
			<td style="width:295px;">Intlvac Thin Film</td>
			<td style="width:199px;">N/A</td>
			<td>Nanoquest series</td>
		</tr>
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			<td height="21" style="height:21px;width:311px;">Ultrasonic Cleaner</td>
			<td style="width:295px;">Crest Ultrasonics</td>
			<td style="width:199px;">N/A</td>
			<td>Powersonic series</td>
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			<td height="21" style="height:21px;width:311px;">Hydrofluoric Acid</td>
			<td style="width:295px;">Sigma-Aldrich</td>
			<td style="width:199px;">244279</td>
			<td>Diluted to 5%</td>
		</tr>
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			<td height="42" style="height:42px;width:311px;">Field Emission Gun Scanning Electron Microscope</td>
			<td style="width:295px;">Jeol Ltd.</td>
			<td style="width:199px;">N/A</td>
			<td>JEOL 6700 series</td>
		</tr>
	</tbody>
</table>

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.

Fabrication schemes show each step (Figure 2a-x). Optical images (Figure 2y-ac) and scanning electron microscope (SEM) images (Figure 2ad-ag) were collected for the nanopillar-based SRRs at different fabrication steps. Animations (Figure 2a-c) illustrate the first layer of electroplated Au nanopillars and the second layer of electroplated Au films as well as the nano-gaps created between them. Figure 2d s...

Watch this Scientific Journal Video about Fabrication of Nanopillar-Based Split Ring Resonators for Displacement Current Mediated Resonances in Terahertz Metamaterials at JoVE.com

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

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 ...

This protocol uses gold electroplating and atomic layer deposition methods to create novel nanopillar-based split ring resonator meta materials with nanoscale gaps. Each fabricated split ring resonator consists of thousands of gold nanopillars and is driven by displacement current across the nanogaps between them which excites terahertz resonances. The magnitude of displacement current is proportional to the size of the nanogaps.The size of the nanogaps can be tuned by changing the cycles of the atomic layer deposition process which results in changes to the resonant frequencies and amplitude. An enhanced quality factor of approximately 450 can be achieved, which is 40 times more than the quality factor of 11 that can be achieved by thin-film split ring resonators. A frequency shift 17 times larger than that of thin film-based resonators can also be observed.Both simulated and measured transmission spectra demonstrate that the nanopillar-based meta materials are ideal candidates for high sensitive sensors and frequency-agile terahertz devices. Analytical results from scanning electron microcopy show the fabricated nanopillar-based split ring resonator, two adjacent gold nanopillars, as well as the 10 nanometer aluminum oxide gaps between them. Nanopillar-based split resonators contain gold nanopillars and 10 nanometer aluminum oxide gap.This structure excite inductive and capacitive resonance via displacement current. They provide more energies to it, enhance the energies towards these to high quality factor, and large frequency shift. Because of the high Q value and the large frequency shift, the nanopillar-based split resonator are suitable for various application, including biomedical and chemical sensors, and frequency tuning devices.The main advantage of this technique over existing methods like electron beam lithography and self assembly is that this method combines a sequential electricoplating process and atomic layer deposition to realize nanoscale structures over a large area, with only conventional microfabrication process. The fabrication process is much faster and simpler compared to electron beam lithography and self assembly. High aspect ratio nano structures can be easily created with precisely controlled feature sizes.First, deposit and adhesion promoter layer and a seed layer onto a flat substrate. Then, pattern the nanopillars using photolithography and electroplating. Uniformly coat a nanoscale dielectric layer on the entire structure, using atomic layer deposition.A second adhesion promoter layer and seed layer are then deposited, followed by an electroplating deposition. Finally, create a C-shape split-ring resonator by partially etching the nanopillars and dielectric nanogaps, using photolithography and ion milling. Then, remove the photo resist using acetone in an ultrasonication bath.Begin the substrate preparation with a four-inch high resistivity silicone wafer. Clean the silicone wafer with acetone, isopropyl alcohol, and deionized water. Then, blow dry with nitrogen gas.Use an electron beam evaporator to deposit a five nanometer chromium adhesion promoter layer and a 10 nanometer copper seed layer. Cut the wafer into two centimeter by 2.5 centimeter pieces. Then, spin coat a two micrometer layer of S1813 Photoresist, at 2, 000 RPM, for 60 seconds.Bake the substrate at 115 degrees celsius for 60 seconds. Now, expose the Photoresist to the nanopillar mask using a mask aligner, with approximately 15 milliwatts per centimeter squared of ultraviolet light, for 22 seconds. The nanopillar mask features five millimeter by five millimeter nanopillar arrays.Develop the Photoresist in MF-319 developer with agitation, for 90 seconds. Rinse the sample with deionized water, and then blow dry it with nitrogen gas. Remove the top section of the Photoresist on the substrate using acetone to expose the copper seed layer for electrode connection.Then, dip the sample in gold electroplating solution and electroplate an 800 nanometer gold layer for approximately eight minutes. Remove the Photoresist using acetone. Rinse with deionized water, and blow dry with nitrogen gas.Remove the chromium and copper layer on the surface of the sample by dipping the sample in chromium etchant for 10 seconds, followed by copper etchant for 10 seconds. Now, uniformly coat the sample with a 10 nanometer aluminum oxide layer using atomic layer deposition. Use an electron beam evaporator to deposit a second five nanometer chromium adhesion promoter layer, as well as a second 10 nanometer copper seed layer, on top of the 10 nanometer aluminum oxide layer.Dip the sample in gold electroplating solution for 16 minutes, to electroplate a 400 nanometer gold film on the substrate. Spin coat a two micrometer layer of S1813 Photoresist at 2, 000 RPM for 60 seconds. Bake the substrate at 115 degrees celsius for 60 seconds.Expose the Photoresist to the C-shape split-ring resonator mask using a mask aligner with approximately 15 milliwatts per centimeter squared of UV light, for 22 seconds. Develop in MF-319 developer for 90 seconds. Etch the structures outside the Photoresist pattern using an ion mill system for approximately 30 minutes.Remove the Photoresist using acetone in an ultrasonication bath, then rinse the sample with acetone, isopropyl alcohol, and deionized water, then blow dry with nitrogen gas. Alternatively, if air nanogaps are preferred, dip the sample in 5%hydrogen fluoride solution for five minutes, to remove the aluminum oxide. Inspect the nanopillar-based split ring resonators under an optical microscope.Optical imaging shows two five millimeter by five millimeter nanopillar-based split ring resonator arrays on a silicone substrate. The zoomed-in, optical microscope image shows the C-shape nanopillar-based split ring resonator arrays. Gold nanopillar and aluminum oxide nanogaps can be observed in the zoomed-in image of a single split ring resonator.Analytical results from scanning electron microscopy also showed the fabricated nanopillar-based split ring resonator, two adjacent gold nanopillars, as well as aluminum oxide gaps between them. Both five nanometer aluminum oxide gaps and 10 nanometer aluminum oxide gaps can be easily fabricated by controlling the cycles of the aluminum oxide atomic layer deposition process. These results show that this fabrication process can produce thousands of nanopillar-based split ring resonators on a large-scale substrate.The process allows for precise control over the size of the nanogaps, and can easily produce high aspect ratio nanostructures, compared to electron beam lithography and self assembly. The nanopillar-based split ring resonators contain thousands of gold nanopillars and nanoscale gaps. The resonant frequencies and amplitudes of the nanopillar-based split ring resonators can be easily tuned by changing the size of the nanogaps, which makes them suitable for various applications, including biomedical sensors, and frequency-agile devices.After watching this video, you should have a good understanding of how to make gold nanopillar-based split ring resonators using a two-step gold electroplating and anatomic layer deposition process to create aluminum oxide nanoscale gaps between gold nanopillars. It is important to understand the working principle of the process and choose the right materials and thicknesses for each material used in the process.

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Unit 1

Electromagnetic Induction

SCIENTISTS IN ACTION

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 ...

Microwave Photonics Systems Based on Whispering-gallery-mode Resonators

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various areas of technology and applied science, such as aerospace and communication engineering, sensing, metrology, nonlinear photonics, and quantum optics. In this article, we present the principal techniques used in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators. First detailed in this article is the protocol for resonator polishing, which is based on a grind-and-polish technique close to the ones used to polish optical components such as lenses or telescope mirrors. Then, a white light interferometric profilometer measures surface roughness, which is a key parameter to characterize the quality of the polishing. In order to launch light in the resonator, a tapered silica fiber with diameter in the micrometer range is used. To reach such small diameters, we adopt the "flame-brushing" technique, using simultaneously computer-controlled motors to pull the fiber apart, and a blowtorch to heat the fiber area to be tapered. The resonator and the tapered fiber are later approached to one another to visualize the resonance signal of the whispering gallery modes using a wavelength-scanning laser. By increasing the optical power in the resonator, nonlinear phenomena are triggered until the formation of a Kerr optical frequency comb is observed with a spectrum made of equidistant spectral lines. These Kerr comb spectra have exceptional characteristics that are suitable for several applications in science and technology. We consider the application related to ultra-stable microwave frequency synthesis and demonstrate the generation of a Kerr comb with GHz intermodal frequency.

The customized techniques developed in our lab to build microwave photonics systems based on ultra-high Q whispering gallery mode resonators are presented. The protocols to obtain and characterize these resonators are detailed, and an explanation of some of their applications in microwave photonics is given.

Microwave photonics systems rely fundamentally on the interaction between microwave and optical signals. These systems are extremely promising for various ...

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 ...

Fabrication and Characterization of Superconducting Resonators

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors (MKIDs) for the detection of faint astrophysical signatures, as well as for quantum computing applications and materials characterization. In this paper, procedures are presented for the fabrication and characterization of thin-film superconducting microwave resonators. The fabrication methodology allows for the realization of superconducting transmission-line resonators with features on both sides of an atomically smooth single-crystal silicon dielectric. This work describes the procedure for the installation of resonator devices into a cryogenic microwave testbed and for cool-down below the superconducting transition temperature. The set-up of the cryogenic microwave testbed allows one to do careful measurements of the complex microwave transmission of these resonator devices, enabling the extraction of the properties of the superconducting lines and dielectric substrate (e.g., internal quality factors, loss and kinetic inductance fractions), which are important for device design and performance.

Superconducting microwave resonators are of interest for detection of light, quantum computing applications and materials characterization. This work presents a detailed procedure for fabrication and characterization of superconducting microwave resonator scattering parameters.

Superconducting microwave resonators are of interest for a wide range of applications, including for their use as microwave kinetic inductance detectors ...

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 ...

Stimulated Stokes and Antistokes Raman Scattering in Microspherical Whispering Gallery Mode Resonators

Dielectric microspheres can confine light and sound for a length of time through high quality factor whispering gallery modes (WGM). Glass microspheres can be thought as a store of energy with a huge variety of applications: compact laser sources, highly sensitive biochemical sensors and nonlinear phenomena. A protocol for the fabrication of both the microspheres and coupling system is given. The couplers described here are tapered fibers. Efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility &#935;(3) interactions in triply resonant silica microspheres is presented in this paper. The interactions here reported are: Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman Scattering (SARS). A proof of the cavity-enhanced phenomenon is given by the lack of correlation among the pump, signal and idler: a resonant mode has to exist in order to obtain the pair of signal and idler. In the case of hyperparametric oscillations (four wave mixing and stimulated anti-stokes Raman scattering), the modes must fulfill the energy and momentum conservation and, last but not least, have a good spatial overlap.

Efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility &#935;(3) interactions in triply resonant silica microspheres is presented in this paper. The interactions here reported are: Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman Scattering (SARS).

Dielectric microspheres can confine light and sound for a length of time through high quality factor whispering gallery modes (WGM). Glass microspheres ...

Measurement of Quantum Interference in a Silicon Ring Resonator Photon Source

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit manipulation, and integrated single-photon detectors. Here, we present the key aspects of preparing and testing a silicon photonic quantum chip with an integrated photon source and two-photon interferometer. The most important aspect of an integrated quantum circuit is minimizing loss so that all of the generated photons are detected with the highest possible fidelity. Here, we describe how to perform low-loss edge coupling by using an ultra-high numerical aperture fiber to closely match the mode of the silicon waveguides. By using an optimized fusion splicing recipe, the UHNA fiber is seamlessly interfaced with a standard single-mode fiber. This low-loss coupling allows the measurement of high-fidelity photon production in an integrated silicon ring resonator and the subsequent two-photon interference of the produced photons in a closely integrated Mach-Zehnder interferometer. This paper describes the essential procedures for the preparation and characterization of high-performance and scalable silicon quantum photonic circuits.

Silicon photonic chips have the potential to realize complex integrated quantum systems. Presented here is a method for preparing and testing a silicon photonic chip for quantum measurements.

Silicon photonic chips have the potential to realize complex integrated quantum information processing circuits, including photon sources, qubit ...

Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, interface precipitation, and mini-emulsion. In all methods, well-defined, micrometer-sized spheres are obtained from a self-assembling process in solution. The vapor diffusion method can result in spheres with the highest sphericity and surface smoothness, yet the types of the polymers able to form these spheres are limited. On the other hand, in the mini-emulsion method, microspheres can be made from various types of polymers, even from highly crystalline polymers with coplanar, &#960;-conjugated backbones. The photoluminescent (PL) properties from single isolated microspheres are unusual: the PL is confined inside the spheres, propagates at the circumference of the spheres via the total internal reflection at the polymer/air interface, and self-interferes to show sharp and periodic resonant PL lines. These resonating modes are so-called "whispering gallery modes" (WGMs). This work demonstrates how to measure WGM PL from single isolated spheres using the micro-photoluminescence (&#181;-PL) technique. In this technique, a focused laser beam irradiates a single microsphere, and the luminescence is detected by a spectrometer. A micromanipulation technique is then used to connect the microspheres one by one and to demonstrate the intersphere PL propagation and color conversion from coupled microspheres upon excitation at the perimeter of one sphere and detection of PL from the other microsphere. These techniques, &#181;-PL and micromanipulation, are useful for experiments on micro-optic application using polymer materials.

Protocols for the synthesis of microspheres from polymers, the manipulation of microspheres, and micro-photoluminescence measurements are presented.