Name: fabrication of nanopillar-based split ring resonators for displacement current mediated resonances in terahertz metamaterials
Uploaded: 2017-03-23T14:00:00.000+00:00
Duration: 10 min 28 s
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 (resistivity: 560 - 840 &#937;&#183;cm) as the substrate. The Si wafer is N-type doped and polished on one side (Figure 2a, e).</li>
		<li>Cut the Si wafer into 2 cm &#215; 2.5 cm pieces for later use.</li>
		<li>Deposit a 5 nm chromium (Cr) layer on the Si sample using an electron-beam (E-beam) evaporation process as an adhesion layer between the Si and Cu.</li>
		<li>Deposit a 10 nm Cu layer on top of the existing Cr layer using an E-beam evaporation process as the seed layer for Au electroplating (Figure 2b, f).</li>
	</ol>
	</li>
	<li>Electroplating the Au nanopillar array (Figure 2c, g)
	<ol>
		<li>Patterning the nanopillar array
		<ol>
			<li>Spin coat photoresist on the sample prepared in section 1.1 at 2,000 rpm for 60 s.</li>
			<li>Bake the sample on a hot plate at 115 &#176;C for 60 s.</li>
			<li>Expose the photoresist under ultraviolet (UV)-light (power of ~15 mW/cm2) with a Cr photomask that contains thousands of nanopillar patterns for 22 s.</li>
			<li>Develop with a developer for 90 s with agitation.</li>
			<li>Rinse the sample with deionized (DI) water and blow-dry the sample with an air gun.</li>
		</ol>
		</li>
		<li>Electroplating the Au nanopillar array
		<ol>
			<li>Remove the top section of the photoresist on the sample with acetone to expose the Cu seed layer for electrode connection.</li>
			<li>Connect the sample (Cu seed layer) to the negative terminal of a source meter using a clamp and a wire. In this case, the sample is the anode during the electroplating process.</li>
			<li>Connect a piece of platinum (Pt) coated Si (same size as the sample) to the positive terminal of the source meter. The Pt is the cathode during the electroplating process.</li>
			<li>Submerge both the Pt cathode and Cu anode in the Au electroplating solution. Keep the two electrodes facing each other with a distance of ~ 1 cm.</li>
			<li>Turn on the source meter and supply a constant voltage of 1.12 V. Electroplate Au on the sample for 8 min (deposition rate: ~ 100 nm/min).</li>
			<li>Rinse the sample with DI water, followed by acetone to remove the photoresist.</li>
			<li>Rinse the sample with DI water again and blow-dry with an air gun.</li>
			<li>Inspect the electroplated Au nanopillar array under a microscope.</li>
			<li>Measure the thickness of the Au nanopillars with a profilometer (The thickness of the Au nanopillars is ~800 nm). 
			NOTE: Constant current set-up can also be used to electroplate Au nanopillars. In both constant voltage and constant current set-ups, the ideal current and voltage used for Au electroplating can be achieved by trial and error.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
2. Creation of Nano-gaps between Au Nanopillars (Figure 2d, h)
<ol>
	<li>Removal of Cr and Cu layers
	<ol>
		<li>Submerge the sample in Cu etchant until the Cu color disappears.</li>
		<li>Rinse the sample with DI water and blow-dry with an air gun.</li>
		<li>Inspect the Au nanopillars under a microscope.</li>
		<li>Submerge the sample in Cr mask etchant for 10 s.</li>
		<li>Rinse the sample with DI water and blow-dry with an air gun.</li>
		<li>Inspect the Au nanopillars under a microscope.</li>
	</ol>
	</li>
	<li>Fabrication of nano-scale Aluminum Oxide (Al2O3) gaps
	<ol>
		<li>Heat the ALD system chamber to 200 &#176;C.</li>
		<li>Place the sample in the center of the chamber.</li>
		<li>Pump down the chamber to a vacuum and set the cycle number to 100 (deposition rate: ~1 &#197;/cycle).</li>
		<li>Sequentially and alternatively pulse trimethylaluminum (TMA) gas with a time period of 0.015 s and water (H2O) vapor with a time period of 0.015 s into the chamber to uniformly deposit Al2O3 layers on the sample. The time gap between each pulse is 5 s. The chamber pressure during TMA pulse is 10 Torr and the pressure during H2O vapor pulse is 2 Torr.</li>
		<li>Purge and vacuum the chamber between each cycle of the deposition. Deposit Al2O3 for 100 cycles and take out the sample from the chamber.</li>
		<li>Measure the thickness of the ALD Al2O3 using an ellipsometer.</li>
	</ol>
	</li>
</ol>
3. Preparation of the Second Layer of an Au Thin Film (Figure 2i-l and Figure 2m-p)
<ol>
	<li>Preparation of Cu Seed layers for Au electroplating (Figure 2i, m)
	<ol>
		<li>Place the sample in the center of an E-beam evaporator sample holder.</li>
		<li>Turn off the rotation of the sample in the E-beam evaporator.</li>
		<li>Deposit a 5 nm Cr layer on the sample to act as an adhesion layer between Al2O3 and Cu. Use an E-beam evaporation process without sample rotation.</li>
		<li>Deposit 10 nm Cu on top of the existing Cr layer using an E-beam evaporation process without sample rotation as the seed layer for Au electroplating.</li>
	</ol>
	</li>
	<li>Electroplating the Au thin film (Figure 2j, n)
	<ol>
		<li>Connect the sample (Cu seed layer) to the negative terminal of the source meter using a clamp and a wire. In this case, the sample is the anode during the electroplating process.</li>
		<li>Connect the Pt cathode to the positive terminal of the source meter.</li>
		<li>Submerge both the Pt cathode and Cu anode in the Au electroplating solution. Keep the two electrodes facing each other with a distance of ~ 1 cm.</li>
		<li>Turn on the source meter and set up constant voltage of 1.35 V and electroplate Au on the sample for 16 min.</li>
		<li>Rinse the sample with DI water and blow-dry with an air gun.</li>
		<li>Inspect the electroplated Au and the previously electroplated Au nanopillar array under a microscope.</li>
		<li>Measure the thickness of the Au nanopillars with a profilometer (Thickness of the Au nanopillars is ~400 nm). 
		NOTE: Similar to the Au electroplating in section 1.2.2, constant current set-up can also be used to electroplate Au thin film. In both constant voltage and constant current set-ups, the ideal current and voltage used for Au electroplating can be achieved by trial and error.</li>
	</ol>
	</li>
	<li>Removal of Cr and Cu layers (Figure 2k, o)
	<ol>
		<li>Submerge the sample in Cu etchant for 10 s.</li>
		<li>Rinse the sample with DI water and blow-dry with an air gun.</li>
		<li>Inspect the Au nanopillars under a microscope.</li>
		<li>Submerge the sample in Cr mask etchant for 10 s.</li>
		<li>Rinse the sample with DI water and blow-dry with an air gun.</li>
		<li>Inspect the Au nanopillars under a microscope. 
		NOTE: Alternatively, submerge the sample in Au electroplating solution again to deposit an extra layer of Au on top of the second electroplated Au layer after the removal of Cr and Cu (step 3.3). This extra Au layer increases the total thickness of the second Au layer and ensures good contact between the Au layer and the Al2O3 layer (Figure 2i, p).</li>
	</ol>
	</li>
</ol>
4. Definition of C-shape SRR (Figure 2q-s and Figure 2u-w)
<ol>
	<li>Patterning the C-shape SRR (Figure 2q, u)
	<ol>
		<li>Spin coat a photoresist on the sample at 2,000 rpm for 60 s.</li>
		<li>Bake the sample on hot plate of 115 &#176;C for 60 s.</li>
		<li>Expose photoresist under UV-light (power of ~15 mW/cm2) with a Cr photomask for 22 s.</li>
		<li>Develop with a developer for 90 s with agitation.</li>
		<li>Rinse the sample with DI water and blow-dry the sample with an air gun.</li>
	</ol>
	</li>
	<li>C-shape definition using ion mill (Figure 2r, v and Figure 2s, w)
	<ol>
		<li>Attach the sample on an ion mill sample holder using double-sided Cu conductive tape.</li>
		<li>Cool down the ion mill chamber to 6 &#176;C.</li>
		<li>Ion mill the sample with a beam voltage of 300 V and a beam current of 125 mA for ~30 min.</li>
		<li>Take out the sample and inspect the Au nanopillars outside the C-shape.</li>
		<li>Repeat step 4.2.3 and 4.2.4 if Au is still visible outside the C-shape.</li>
		<li>Sonicate the sample in acetone to remove the photoresist.</li>
		<li>Rinse the sample with DI water and blow-dry with an air gun.</li>
		<li>Inspect the sample under a microscope.</li>
		<li>Repeat step 4.2.6 and 4.2.7 if photoresist is not fully removed. 
		NOTE: Alternatively, apply oxygen resist softening steps to the photoresist before removing the photoresist. However, a sonication bath is the most effective method to remove photoresist if applicable.</li>
	</ol>
	</li>
</ol>
5. Removal of Al2O3 for Air Nano-gaps (Figure 2t, x)
<ol>
	<li>Submerge the sample in 5% hydrogen fluoride (HF) solution for 5 min to remove Al2O3.</li>
	<li>Rinse the sample with DI water and blow-dry with an air gun.</li>
</ol>

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The authors have nothing to disclose.

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 atomic scale nano-gaps can be easily created by an ALD process with precisely controlled feature sizes.
Cr and Cu E-beam evaporations without sample rotation allow Cr and Cu deposition directly onto the substrate with minimized sidewall deposition. This is crucial for the following Au electroplating process because Au can only be electroplated on the Cu seed layer that is connected to the source meter. Since the Cu layers on top of the Au nanopillars are disconnected with the Cu layer on the substrate, Au cannot be electroplated on the substrate. The quality and thickness of the electroplated Au depend on the electroplating voltage/current and the electroplating time. Higher voltage/current leads to a high deposition rate. However, high voltage/current can also result in low quality Au deposition. Electroplated Au with low quality has a lower electrical conductivity compared to standard Au material as well as lots of voids in Au, which reduces the intensity of the displacement current circulating the SRR, leading to a weak resonant behavior and a lower magnitude of the resonant peaks. Therefore, a suitable voltage/current is essential to achieve high quality Au nanopillars. Electroplating time and voltage/current should be also precisely controlled to make sure that the thickness of the Au thin film (the second layer of Au) is less than that of the Au nanopillars (the first layer of Au).
100 cycles of ALD can achieve about 10 nm thick Al2O3 layer on both the Si substrate and the sidewalls of the Au nanopillars. The deposition rate and quality of the Al2O3 deposited by ALD depends on the reaction temperature inside the chamber. A reaction temperature of above 200 &#176;C is recommended to achieve high quality Al2O3 films. The cycle number and temperature can be precisely controlled to obtain Al2O3 layer with desired thickness. The size of the nano-gaps (Figure 3h) is critical for achieving high Q-factors of the nanopillar-based SRRs. An increase of nano-gap size increases the energy storage inside the nano-gaps, which leads to a high Q-factor. However the size of nano-gaps cannot be increased without limitation. When nano-gap sizes exceed about 50 nm, the displacement current between Au nanopillars drops dramatically and is unable to pass through the nano-gaps, leading to the disappearance of the resonant responses. In addition, if the size of the Al2O3 nano-gaps is less than 2 nm, the electroplating voltage for Au deposition can breakdown the dielectric barrier (Al2O3 nano-gaps), resulting in the conduction between Au nanopillars and the Au electroplating solution, which leads to a second Au layer electroplated on top of the Au nanopillars (a first gold layer). This limit leads to the difficulty to achieve ultra-thin Al2O3 gaps without breaking the dielectric barrier between Au nanopillars.
A finite element method (FEM) was used to simulate the SRRs (Figure 3i and 3k). Three resonance peaks in the transmission spectra are known as the first (1st) mode, second (2nd) mode, and third (3rd) mode of the SRR. The transmission spectra of the nanopillar-based SRRs with 10 nm Al2O3 gaps and 10 nm air gaps were measured using a THz time-domain spectroscopy (Figure 3j and 3l). All the measured transmission spectra match the simulated data, which proves that the fabricated nanopillar-based SRRs meet the expected design.
The combination of continuous metal thin films and dielectric nano-scale gaps provide structures for more energy storage compared to traditional film-based SRRs, which results in ultra-high Q-factors of around 450 (more than 45 times higher than the Q-factor of the traditional thin-film-based SRRs) and large frequency shifts (around 17 times larger than the frequency shift of the thin-film-based SRRs). The unique fabrication technique shown in this video journal allows for the fabrication of thousands of nanopillar forming SRRs over a large area. Since the formation of Au nanopillars largely increases the surface areas of the SRRs and the number of nano-scale gaps between Au nanopillars enhances the amount of the energy storage (electric charges), ultra-high Q-factors can be achieved leading to a high sensitivity. In addition, the substances applied to the nanopillar-based SRRs are presented inside the nano-gaps contribute to the permittivity changes of the nano-gaps, resulting in large frequency shifts of the nanopillar-based SRRs, which leads to a high selectivity. Thus the nanopillar-based SRRs fabricated using Au electroplating and ALD techniques are ideal for highly acute chemical and biomolecular sensing units.

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.

<table><tbody><tr><td>Silicon Wafer</td><td>Siltronic AG</td><td>N/A</td><td>100 mm diameter, N-type, one-side polish, resitivity: 560-840 &amp;Omega;&amp;bull;cm</td></tr><tr><td>Chromium</td><td>Kurt J. Lesker Company</td><td>EVMCR35J</td><td>99.95% pure</td></tr><tr><td>Copper</td><td>Kurt J. Lesker Company</td><td>EVMCU40QXQJ</td><td>99.99% pure</td></tr><tr><td>E-Beam Evaporator System</td><td>Rocky Mountain Vacuum Tech.</td><td>N/A</td><td>RME-2000</td></tr><tr><td>S1813 Positive Photoresist</td><td>Microposit</td><td>10018348</td><td>N/A</td></tr><tr><td>Spinner</td><td>Best Tools</td><td>S0114031123</td><td>SMART COATER 100</td></tr><tr><td>Mask Aligner</td><td>Midas</td><td>MDA-400LJ</td><td>N/A</td></tr><tr><td>Digital Hot Plate</td><td>Thermo Scientific</td><td>HP131725</td><td>Super-Nuvoa series, maximum temperature: 370 &amp;deg;C</td></tr><tr><td>MF319 Developer</td><td>Microposit</td><td>10018042</td><td>N/A</td></tr><tr><td>Acetone</td><td>Fisher Chemical</td><td>A18P-4</td><td>N/A</td></tr><tr><td>Isopropyl Alcohol</td><td>Fisher Chemical</td><td>A416-4</td><td>N/A</td></tr><tr><td>Gold 25 ES RTU</td><td>Technic Inc.</td><td>391427</td><td>N/A</td></tr><tr><td>Source Meter</td><td>Keithley</td><td>N/A</td><td>2612 System SourceMeter</td></tr><tr><td>Microscope</td><td>Omax</td><td>NJF-120A</td><td>N/A</td></tr><tr><td>Profilometer</td><td>Tencor Instruments</td><td>N/A</td><td>Alpha-Step 200</td></tr><tr><td>APS Copper Etchant 100</td><td>Transfene Company, Inc.</td><td>N/A</td><td>N/A</td></tr><tr><td>CE-5 M Chromium Mask Etchant</td><td>Transfene Company, Inc.</td><td>N/A</td><td>N/A</td></tr><tr><td>Atomic Layer Deposition System</td><td>Cambridge Nano Tech inc.</td><td>N/A</td><td>Savannah series</td></tr><tr><td>Ion Mill Etching System</td><td>Intlvac Thin Film</td><td>N/A</td><td>Nanoquest series</td></tr><tr><td>Ultrasonic Cleaner</td><td>Crest Ultrasonics</td><td>N/A</td><td>Powersonic series</td></tr><tr><td>Hydrofluoric Acid</td><td>Sigma-Aldrich</td><td>244279</td><td>Diluted to 5%</td></tr><tr><td>Field Emission Gun Scanning Electron Microscope</td><td>Jeol Ltd.</td><td>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 shows the cross-section scheme of nanopillar-based SRRs with both Al2O3 nano-gaps and air nano-gaps. SEM images were collected for the nanopillar-based SRR array and nano-scale gaps between Au nanopillars (Figure 2af, 2ag, 3e-h). Both simulated and measured transmission spectra of the samples with Al2O3 nano-gaps and air nano-gaps were shown (Figure 3i-l).
<img alt="Figure 1" src="/files/ftp_upload/55289/55289fig1.jpg" /> 
Figure 1: illustration of nanopillar-based SRRs mediated by displacement current. (a, b) Displacement current (Id) induced between two metal plates and two nanopillars by electric fields E. (c) Schematic of nanopillar-based SRRs defined by thousands of Au nanopillars (H: height of the nanopillar, A: facing area; d: nano-gap size; l: width of nanopillar and &#949;: permittivity in the nano-gaps). (d) Q-factor of thin-film-based SRR and nanopillar-based SRR. A Q-factor of around 450 can be achieved with a nanopillar-based SRR with a nano-gap size of 10 nm. The figure is adapted with permission from Advanced Optical Materials12. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55289/55289fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/55289/55289fig2.jpg" /> 
Figure 2: Fabrication schemes of nanopillar-based SRRs.&#160; (a-t) 3D animation and cross-section schematics of the fabrication process of nanopillar-based SRRs. (y-ac) Optical images of nanopillar-based SRRs at different fabrication steps. (ad-ag) SEM images of nanopillar-based SRRs at different fabrication steps as well as a 5 nm Al2O3 gaps (ag). The figure is adapted with permission from Advanced Optical Materials12. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55289/55289fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/55289/55289fig3.jpg" /> 
Figure 3: Characterizations of nanopillar-based SRRs. (a-d) Fabrication schemes of nanopillar-based SRRs. (e-h) SEM images of nanopillar-based SRRs. (i) Simulated transmission spectra of Al2O3 nano-gap nanopillar-based SRRs. (j) Measured transmission spectra of Al2O3 nano-gap nanopillar-based SRRs. (k) Simulated transmission spectra of air nano-gap nanopillar-based SRRs. (l) Measured transmission spectra of air nano-gap nanopillar-based SRRs. The figure is adapted with permission from Advanced Optical Materials12. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/55289/55289fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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 ...

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7.7K Views.  University of Minnesota. A protocol for the design and fabrication of a novel nanopillar-based split ring resonator (SRR) is presented.

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

Fabrication of Silica Ultra High Quality Factor Microresonators

Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the quality factor (Q) of the cavity, can be in excess of 500ns for cavities with Q factors above 100 million. As a result of their low material losses, silica microcavities have demonstrated some of the longest photon lifetimes to date1-2. Since a portion of the circulating light extends outside the resonator, these devices can also be used to probe the surroundings. This interaction has enabled numerous experiments in biology, such as single molecule biodetection and antibody-antigen kinetics, as well as discoveries in other fields, such as development of ultra-low-threshold microlasers, characterization of thin films, and cavity quantum electrodynamics studies.3-7
The two primary silica resonant cavity geometries are the microsphere and the microtoroid. Both devices rely on a carbon dioxide laser reflow step to achieve their ultra-high-Q factors (Q&gt;100 million).1-2,8-9 However, there are several notable differences between the two structures. Silica microspheres are free-standing, supported by a single optical fiber, whereas silica microtoroids can be fabricated on a silicon wafer in large arrays using a combination of lithography and etching steps. These differences influence which device is optimal for a given experiment.
Here, we present detailed fabrication protocols for both types of resonant cavities. While the fabrication of microsphere resonant cavities is fairly straightforward, the fabrication of microtoroid resonant cavities requires additional specialized equipment and facilities (cleanroom). Therefore, this additional requirement may also influence which device is selected for a given experiment.
Introduction
An optical resonator efficiently confines light at specific wavelengths, known as the resonant wavelengths of the device. 1-2 The common figure of merit for these optical resonators is the quality factor or Q. This term describes the photon lifetime (&tau;o) within the resonator, which is directly related to the resonator's optical losses. Therefore, an optical resonator with a high Q factor has low optical losses, long photon lifetimes, and very low photon decay rates (1/&tau;o). As a result of the long photon lifetimes, it is possible to build-up extremely large circulating optical field intensities in these devices. This very unique property has allowed these devices to be used as laser sources and integrated biosensors.10
A unique sub-class of resonators is the whispering gallery mode optical microcavity. In these devices, the light is confined in circular orbits at the periphery. Therefore, the field is not completely confined within the device, but evanesces into the environment. Whispering gallery mode optical cavities have demonstrated some of the highest quality factors of any optical resonant cavity to date.9,11 Therefore, these devices are used throughout science and engineering, including in fundamental physics studies and in telecommunications as well as in biodetection experiments. 3-7,12
Optical microcavities can be fabricated from a wide range of materials and in a wide variety of geometries. A few examples include silica and silicon microtoroids, silicon, silicon nitride, and silica microdisks, micropillars, and silica and polymer microrings.13-17 The range in quality factor (Q) varies as dramatically as the geometry. Although both geometry and high Q are important considerations in any field, in many applications, there is far greater leverage in boosting device performance through Q enhancement. Among the numerous options detailed previously, the silica microsphere and the silica microtoroid resonator have achieved some of the highest Q factors to date.1,9 Additionally, as a result of the extremely low optical loss of silica from the visible through the near-IR, both microspheres and microtoroids are able to maintain their Q factors over a wide range of testing wavelengths.18 Finally, because silica is inherently biocompatible, it is routinely used in biodetection experiments.
In addition to high material absorption, there are several other potential loss mechanisms, including surface roughness, radiation loss, and contamination loss.2 Through an optimization of the device size, it is possible to eliminate radiation losses, which arise from poor optical field confinement within the device. Similarly, by storing a device in an appropriately clean environment, contamination of the surface can be minimized. Therefore, in addition to material loss, surface scattering is the primary loss mechanism of concern.2,8
In silica devices, surface scattering is minimized by using a laser reflow technique, which melts the silica through surface tension induced reflow. While spherical optical resonators have been studied for many years, it is only with recent advances in fabrication technologies that researchers been able to fabricate high quality silica optical toroidal microresonators (Q&gt;100 million) on a silicon substrate, thus paving the way for integration with microfluidics.1
The present series of protocols details how to fabricate both silica microsphere and microtoroid resonant cavities. While silica microsphere resonant cavities are well-established, microtoroid resonant cavities were only recently invented.1 As many of the fundamental methods used to fabricate the microsphere are also used in the more complex microtoroid fabrication procedure, by including both in a single protocol it will enable researchers to more easily trouble-shoot their experiments.

We describe the use of a carbon dioxide laser reflow technique to fabricate silica resonant cavities, including free-standing microspheres and on-chip microtoroids. The reflow method removes surface imperfections, allowing long photon lifetimes within both devices. The resulting devices have ultra high quality factors, enabling applications ranging from telecommunications to biodetection.

Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the ...

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 ...

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 ...

Implementation of a Reference Interferometer for Nanodetection

A thermally and mechanically stabilized fiber interferometer suited for examining ultra-high quality factor microcavities is fashioned. After assessing its free spectral range (FSR), the module is put in parallel with a fiber taper-microcavity system and then calibrated through isolating and eliminating random shifts in the laser frequency (i.e. laser jitter noise). To realize the taper-microcavity junction and to maximize the optical power that is transferred to the resonator, a single-mode optical fiber waveguide is pulled. Solutions containing polystyrene nanobeads are then prepared and flown to the microcavity in order to demonstrate the system&#8217;s ability to sense binding to the surface of the microcavity. Data is post-processed via adaptive curve fitting, which allows for high-resolution measurements of the quality factor as well as the plotting of time-dependent parameters, such as resonant wavelength and split frequency shifts. By carefully inspecting steps in the time-domain response and shifting in the frequency-domain response, this instrument can quantify discrete binding events.

A reference interferometer technique, which is designed to remove undesirable laser jitter noise for nanodetection, is utilized for probing an ultra-high quality factor microcavity. Instructions for assembly, setup, and data acquisition are provided, alongside the measurement process for specifying the cavity quality factor.

A thermally and mechanically stabilized fiber interferometer suited for examining ultra-high quality factor microcavities is fashioned. After assessing ...

Utilization of Plasmonic and Photonic Crystal Nanostructures for Enhanced Micro- and Nanoparticle Manipulation

A method to manipulate the position and orientation of submicron particles nondestructively would be an incredibly useful tool for basic biological research. Perhaps the most widely used physical force to achieve noninvasive manipulation of small particles has been dielectrophoresis(DEP).1 However, DEP on its own lacks the versatility and precision that are desired when manipulating cells since it is traditionally done with stationary electrodes. Optical tweezers, which utilize a three dimensional electromagnetic field gradient to exert forces on small particles, achieve this desired versatility and precision.2 However, a major drawback of this approach is the high radiation intensity required to achieve the necessary force to trap a particle which can damage biological samples.3 A solution that allows trapping and sorting with lower optical intensities are optoelectronic tweezers (OET) but OET's have limitations with fine manipulation of small particles; being DEP-based technology also puts constraint on the property of the solution.4,5
This video article will describe two methods that decrease the intensity of the radiation needed for optical manipulation of living cells and also describe a method for orientation control. The first method is plasmonic tweezers which use a random gold nanoparticle (AuNP) array as a substrate for the sample as shown in Figure 1. The AuNP array converts the incident photons into localized surface plasmons (LSP) which consist of resonant dipole moments that radiate and generate a patterned radiation field with a large gradient in the cell solution. Initial work on surface plasmon enhanced trapping by Righini et al and our own modeling have shown the fields generated by the plasmonic substrate reduce the initial intensity required by enhancing the gradient field that traps the particle.6,7,8 The plasmonic approach allows for fine orientation control of ellipsoidal particles and cells with low optical intensities because of more efficient optical energy conversion into mechanical energy and a dipole-dependent radiation field. These fields are shown in figure 2 and the low trapping intensities are detailed in figures 4 and 5. The main problems with plasmonic tweezers are that the LSP's generate a considerable amount of heat and the trapping is only two dimensional. This heat generates convective flows and thermophoresis which can be powerful enough to expel submicron particles from the trap.9,10 The second approach that we will describe is utilizing periodic dielectric nanostructures to scatter incident light very efficiently into diffraction modes, as shown in figure 6.11 Ideally, one would make this structure out of a dielectric material to avoid the same heating problems experienced with the plasmonic tweezers but in our approach an aluminum-coated diffraction grating is used as a one-dimensional periodic dielectric nanostructure. Although it is not a semiconductor, it did not experience significant heating and effectively trapped small particles with low trapping intensities, as shown in figure 7. Alignment of particles with the grating substrate conceptually validates the proposition that a 2-D photonic crystal could allow precise rotation of non-spherical micron sized particles.10 The efficiencies of these optical traps are increased due to the enhanced fields produced by the nanostructures described in this paper.

Plasmonic tweezers and photonic crystal nanostructures are shown to produce useful enhancements in the efficiency and orientation control of optically trapping micro- and nano-particles.

A method to manipulate the position and orientation of submicron particles nondestructively would be an incredibly useful tool for basic biological ...

Bioengineering

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 ...

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using femtosecond laser-induced ablation. We show that thousands of periodic nano-craters are fabricated on an optical nanofiber by irradiating with just a single femtosecond laser pulse. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. A key aspect of such a nanofabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. Moreover, the single-shot fabrication makes it immune to mechanical instabilities and other fabrication imperfections. Such periodic nano-craters on nanofiber, act as a 1-D PhC and enable strong and broadband reflection while maintaining the high transmission out of the stopband. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber. The strong confinement of the field, both transverse and longitudinal, in the nanofiber-based PhC cavities and the efficient integration to the fiber networks, may open new possibilities for nanophotonic applications and quantum information science.

We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using ...