Name: fabrication and characterization of superconducting resonators
Uploaded: 2016-05-21T14:00:00.000+00:00
Duration: 10 min 26 s
Description: Watch this Scientific Journal Video about Fabrication and Characterization of Superconducting Resonators at JoVE.com

The authors acknowledge funding support from the National Aeronautics and Space Administration (NASA)&#39;s ROSES and APRA programs. GC also acknowledges the Universities Space Research Association for administering his appointment at NASA.

1. Microstrip Line Resonator Fabrication14 (Figure&#160;3)
<ol>
	<li>Clean a silicon-on-insulator (SOI) wafer, which has a 0.45-&#181;m-thick silicon device layer, with freshly mixed H2SO4:H2O2 (3:1) for 10 min. Then rinse the wafer in deionized water for 10 min and dry with a nitrogen gun. Immediately prior to subsequently processing, dip the wafer in H2O:HF (10:1) for 10 sec and rinse in deionized water for 5 min.</li>
	<li>Fabricate a lift-off mask, which consists of a germanium (Ge)/positive photoresist such as S-1811.15
	<ol>
		<li>Spin-coat the wafer with thinned positive photoresist bilayer (2 parts thinner-P : 1 part positive photoresist) at 4,000 rpm for 30 sec and then electron-beam deposit Ge.</li>
		<li>Pattern Ge using photo-lithography by first applying hexamethyldisilazane (HMDS) on the wafer for 1 min and then spin off the excess at 3,000 rpm for 30 sec.</li>
		<li>Spin on thinned positive photoresist (2 parts thinner-P : 1 part positive photoresist) at 2,000 rpm for 30 sec and bake it on a hot plate for 1 min at 110 &#176;C. Use a mask aligner to expose photoresist and spray develop resist with a tetramethyl ammonium hydroxide-based solution.</li>
		<li>Reactive-ion etch the Ge with an SF6/O2 plasma at 70 W. Ash underlying photoresist with O2 plasma to achieve undercut of photoresist.</li>
		<li>DC-magnetron sputter-deposit niobium (Nb) ground plane with 3.7 mT of argon (Ar) at 500 W and lift it off by placing the wafer inside an acetone-filled beaker for 4 hr.</li>
	</ol>
	</li>
	<li>Spin-coat bisbenzocyclobutene (BCB) at 4,000 rpm for 30 sec on the Nb-coated surface of the SOI wafer and to one surface of another silicon wafer. Bond the two BCB-coated surfaces together with 3 bar of pressure at 200 &#176;C.</li>
	<li>Manually flip wafer stack upside down to begin processing the backside of the SOI wafer.</li>
	<li>Etch the silicon handle wafer by mechanical lapping using Al2O3 slurry, followed by deep reactive ion etching using the Bosch process.16 Etch the buried SiO2 layer with H2O:HF (10:1) for 20 min.</li>
	<li>Deposit molybdenum nitride (Mo2N) using DC magnetron reactive sputtering at 700 W and 3.3 mT (Ar:N2 partial pressure = 7:1). Pattern resonators by spinning at 2,000 rpm for 30 sec and baking it at 180 &#176;C for 2 min followed by spinning thinned positive photoresist (2 parts thinner-P : 1 part positive photoresist) at 2,000 rpm for 30 sec. Develop photoresist in a tetramethyl ammonium hydroxide-based solution and ash in a reactive ion etcher. Etch Mo2N with a phosphoric acid-based solution.</li>
	<li>Fabricate a lift-off mask consisting of a Ge/PMMA bilayer by spinning on the polymethylmethacrylate (PMMA) at 5,000 rpm for 30 sec and baking it at 180 &#176;C for 2 min followed by electron-beam deposition of Ge. Sputter-deposit Nb transmission lines and lift off in acetone (refer to step 1.2 with the exception that the positive photoresist is substituted with PMMA).</li>
	<li>In some embodiments, radio-frequency (RF) sputter-deposit SiO2, pattern it by spinning with positive photoresist and etch in a hydrofluoric-acid based solution. Then, lift off a sputter-deposited Nb thin film using a germanium/positive photoresist liftoff mask as detailed in step 1.2.</li>
</ol>
2. Procedure for Installation of Microwave Resonator Chip in Test Package
<ol>
	<li>Design and machine a test package consisting of gold (Au)-coated copper cavity (with a base and lid) which matches resonator chip dimensions, feedline input and output locations. NOTE: The cavity size of the housing should be specified to support a single-mode operation with minimal parasitic coupling over the band of interest.</li>
	<li>Design and fabricate a controlled impedance microwave fan-out board17 to route the signals between the chip and Sub-Miniature version A (SMA) connectors.</li>
	<li>Insert the SMA connectors into the input and output of the test package so that the center conductor pin is aligned over the corresponding fan-out board contact pad. Apply a solder mask to protect against shorting, and apply solder in the region of the center conductor pin. Place the package on a hot plate and heat to 200 &#176;C for ~5 min to melt the solder. Let cool and then remove the solder mask.</li>
	<li>Mount the resonator chip into the Au-coated copper package cavity such that the on-chip feedline output and input pads are close and aligned to the corresponding fan-out board coplanar waveguide (CPW) lines. Secure the chip with copper clips which make contact at the edges of the corners of the chip.</li>
	<li>Place superconducting Al wire bonds between the fan-out board and on-chip contact pads. Place a maximum number (~4 in the case presented here &#8212; see Figure&#160;4) of ~500-600-&#956;m-long, ~250-&#181;m-in-height wire bonds, to provide impedance match between the SMA connector input and outputs and the on-chip CPW feedline.</li>
	<li>After wire-bonding, with a multimeter check the DC resistance between the center pins of the input and output connectors, and between a center pin and ground, to confirm there is an electrical connection across the two center pins and an open connection between the center line and ground.</li>
</ol>
3. Procedure for Installation of Microwave Resonator in a Cryogenic Helium-3 Microwave Testbed
<ol>
	<li>Assemble the testbed as in the configuration shown in Figure 2, in which a series of SMA cables are routed from RT to the 0.3-K cold stage where the device will be mounted.</li>
	<li>Install copper (Cu) and superconducting niobium-titanium (NbTi) cables as shown in Figure 2 to provide low microwave loss and, in the case of the NbTi cables, a low thermal conductance. Use the NbTi cables as a thermal break between the 2-K and 0.3-K stages.</li>
	<li>Mount a cryogenic high electron mobility transistor (HEMT) amplifier at the 2-K stage on the output line for low-noise amplification in the band of the resonator device and install a circulator.</li>
	<li>Insert a cryogenic circulator on the output line at the input to this amplifier.</li>
	<li>Mount the packaged resonator devices onto a bracket bolted to the 0.3-K cold stage.</li>
	<li>Connect a microwave attenuator on the input side of the package to provide for matched termination and connect the appropriate SMA cables to this attenuator input and package output. Ensure that these controlled impedance terminations are well matched and are as close to the device under test as possible &#8212; they define the &#34;device calibration plane&#34; (see Figure 2).</li>
	<li>Close up the cryostat. Follow standard procedure to cool the devices to 0.3 K.</li>
</ol>
4. Procedure for Microwave Resonator Measurements
<ol>
	<li>Set the VNA to scan over a wide frequency band (10 MHz - 8 GHz, for the device considered here) at the device-under-test design frequencies. Adjust the power levels on the VNA to suitable levels for the device under test (~-30 dBm, for the device considered here). &#160; 
	NOTE: Ensure that the input RF power level is low enough so as not to exceed the critical current of the superconducting microwave resonator and superconducting feedline. Ensure that the power level is high enough to provide an adequate signal-to-noise ratio.</li>
	<li>Calibrate the flexible RF cables following standard Short-Open-Load-Thru (SOLT) procedure, following VNA software directions found in the VNA manual. Insert in shorted, open, terminated and thru standards at the output of each of the flexible cables, which route from the vector network analyzer and which will later be connected to the input of the cryostat for measurements. This calibration defines the &#34;instrument reference plane&#34; (e.g., see Figure 2).</li>
	<li>Following this SOLT calibration, verify the fidelity of the calibration by confirming that the transmission, S21, with the thru line connected measured with the VNA, has low residual errors (i.e., the response is at ~0 dB level and S11 and S22 are low, e.g., &#8804; -50 dB).</li>
	<li>Connect the flexible cables to the input and output lines of the cryostat.</li>
	<li>Turn on the cryogenic microwave amplifier by applying the required DC bias voltage as specified in the company-provided documentation for the microwave amplifier.</li>
	<li>First, complete a wideband scan of VNA (10 MHz - 8 GHz, for the device considered here) to observe the S21 baseline structure and to look for any sharp high-Q structures indicative of microwave resonators.</li>
	<li>Then, narrow the frequency range (to ~2 - 4 GHz, for the device considered here) and adjust the number of data points (~30,000 for the device considered here) of the VNA to scan over the resonator band. Use a frequency band wide enough to provide an adequate baseline span for later fits to this baseline to carry out an in-situ calibration (see discussion in Introduction). 
	NOTE: Depending on the noise level, increase the number of averages, or reduce the IF bandwidth to improve signal-to-noise.</li>
	<li>Save these VNA data scans of the complex transmission data to file for post-measurement in-situ calibration, and analysis and extraction of quality factors and resonance frequencies.11</li>
</ol>

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The authors declare that they have no competing financial interests.

The single-flip fabrication process provides a means for realizing superconducting resonators on both sides of a thin 0.45-&#181;m single-crystal Si substrate. One may be motivated to use a single-crystal Si dielectric because it has more than an order of magnitude lower loss than deposited dielectrics (such as Si3N4) with loss tangents in the 4.0-6.5-GHz range &#60; 1 x 10-5. 23-24 The ability to pattern features on both sides of this substrate allows one to employ a microstri...

Advances in astrophysical instrumentation have recently introduced superconducting microwave resonators for the detection of infrared light.1-4 A superconducting resonator will respond to infrared radiation of energy E = hv &#62; 2&#916; (where h is Planck&#39;s constant, v is the radiation frequency and &#916; is the superconducting gap energy). When the resonator is cooled to a temperature well below the superconductor critical temperature, this incident radiation breaks Cooper pairs in the resonator volume and generates quasiparticle excitations. The increase in the density of quasiparticle excitations changes the kinetic inductance, and thus the complex surface impedance of the superconductor. This optical response is observed as a shift in the resonance frequency to lower frequency and a reduction in the quality factor of the resonator. In the canonical read-out scheme for a microwave kinetic inductance detector (MKID), the resonator is coupled to a microwave feedline and one monitors the complex transmission through this feedline at a single microwave frequency tone on resonance. Here, the optical response is observed as a change in both the amplitude and phase of transmission5 (Figure 1). Frequency-domain multiplexing schemes are capable of reading out arrays of thousands of resonators.6-7
To successfully design and implement superconducting-resonator-based instrumentation, the properties of these resonant structures need to be characterized accurately and efficiently. For example, precision measurements of the noise properties, quality factors Q, resonance frequencies (including their temperature dependence) and optical response properties of superconducting resonators are desired in the context of MKID device physics,8 quantum computing,9 and the determination of low-temperature materials properties.10
In all of these cases, the measurement of the circuit&#39;s complex transmission scattering parameters is desired. This work concentrates on the determination of the resonator&#39;s complex transmission coefficient, S21, whose amplitude and phase can be measured with a vector network analyzer (VNA). Ideally, the VNA reference plane (or test port) would be directly connected to the device under test (DUT), but a cryogenic setting normally requires the use of additional transmission line structures to realize a thermal break between RT (~300 K) and the cold stage (~0.3 K in this work; see Figure 2). Additional microwave components such as directional couplers, circulators, isolators, amplifiers, attenuators, and associated interconnecting cables may be needed to appropriately prepare, excite, read out and bias the device of interest. The phase velocities and dimensions of these components vary when cooling from room to cryogenic temperatures, and therefore they affect the observed response at the device calibration plane. These intervening components between the instrument and the device calibration plane influence the complex gain and need to be appropriately accounted for in the interpretation of the measured response.11
In theory, a scheme is needed that sets the measurement reference plane, identical to the one employed during calibration, at the DUT. To reach this target, one could measure the calibration standards over multiple cool-downs; however, this poses constraints on the stability of the VNA and the repeatability of the cryogenic instrument, which are difficult to attain. To mitigate these concerns, one could place the necessary standards in the cooled test environment and switch between them. This is, for example, similar to what is found in microwave probe stations, where the sample and calibration standards are cooled to 4 K by a continuous liquid helium flow or a closed-cycle refrigeration system.12 This method was demonstrated at sub-kelvin temperatures but requires a low-power, high-performance microwave switch in the test band of interest.13
An in-situ calibration procedure is therefore desired which accounts for the instrumental transmission response between the VNA reference plane and the device calibration plane (Figure 2) and which overcomes the limitations of the methods described above. This cryogenic calibration method, presented and discussed in detail in Cataldo et al.11, allows one to characterize multiple resonators over a frequency range wide compared to the resonator line width and inter-resonator spacing with an accuracy of ~1%. This paper will focus on the details of the sample fabrication and preparation processes, experimental test set-up and measurement procedures used to characterize superconducting microwave resonators with planar line geometries.11

<table><tbody><tr><td colspan="4">&lt;strong&gt;Protocol Section 1&lt;/strong&gt;</td></tr><tr><td>Microposit S-1811 Photoresist</td><td>Shipley</td><td></td><td></td></tr><tr><td>BCB</td><td>Dow</td><td></td><td>3022-35</td></tr><tr><td>SOI wafers</td><td>SOITec</td><td></td><td>Fabricated with SmartCut&lt;sup&gt;TM&lt;/sup&gt; process</td></tr><tr><td>Mo</td><td>Kamis</td><td></td><td>99.99%</td></tr><tr><td>Nb</td><td>Kamis</td><td></td><td>99.95% (excludes Ta)</td></tr><tr><td>E-6 metal etch w/AES</td><td>Fujifilm</td><td></td><td>CPG Grade</td></tr><tr><td>Acetone</td><td>JT Baker</td><td>9005-05</td><td>CMOS Grade</td></tr><tr><td>HF dip (1:10)</td><td>JT Baker</td><td>5397-03</td><td></td></tr><tr><td>PMMA</td><td>Microchem</td><td></td><td>950 PMMA A2</td></tr><tr><td colspan="4">&lt;strong&gt;Protocol Section 2&lt;/strong&gt;</td></tr><tr><td>GE 7031</td><td>General Electric</td><td></td><td>Low-temperature adhesive</td></tr><tr><td colspan="4">&lt;strong&gt;Protocol Sections 3-4&lt;/strong&gt;</td></tr><tr><td>Cryogenic Microwave Amplifier</td><td>MITEQ</td><td>AF S3-02000400-08-CR-4</td><td>2-4 GHz, gain ~30&amp;nbsp;dB</td></tr><tr><td>NbTi Semi-rigid SMA cables</td><td>Coax. Co.</td><td>SC-086/50-NbTi-NbTi</td><td></td></tr><tr><td>Circulator</td><td>PamTech</td><td>CTD1229K</td><td>return loss &gt; -20 dB from 2-4 GHz</td></tr><tr><td>RF attenuator</td><td>Weinschel</td><td>Model-4M</td><td>7 dB attenuation</td></tr><tr><td>Flexible SMA cables</td><td>Teledyne-Storm</td><td>R94-240</td><td>ACCU-TEST</td></tr><tr><td>Vector Network Analyzer</td><td>Agilent</td><td>N5242A PNA-X</td><td></td></tr><tr><td>Liquid He-4 cryogen</td><td>Praxair</td><td></td><td></td></tr><tr><td>Liquid N&lt;sub&gt;2&lt;/sub&gt; cryogen</td><td>Praxair</td><td></td><td></td></tr></tbody></table>

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.

The response of a half-wave Mo2N resonator (Figure 5) fabricated on a 0.45-&#181;m single-crystal silicon dielectric was validated with this methodology. In this instance, coupling to a Nb coplanar waveguide (CPW) feedline for read-out is achieved via capacitive coupling through a sputter-deposited SiO2 dielectric, in the &#34;H&#34; shaped region at one of the open ends of the resonator (see Protocol section 1.6). In other instances, capacitive coup...

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Fabrication and Characterization of Superconducting Resonators

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 ...

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10.4K Views.  NASA Goddard Space Flight Center. 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.

Video: Fabrication and Characterization of Superconducting Resonators

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 ...

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals1-4 and the enhancement of both linear5-7 and nonlinear devices.8-11
Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically12 and electrically13 pumped ultra-low threshold lasers and the enhancement of nonlinear effects.14-16 Furthermore, passive filters17 and modulators18-19 have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption.
To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues.
The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum20-21 and interferometric techniques.22-25 Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.26 Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans.
When characterising photonic crystal cavities, techniques involving internal sources21 or external waveguides directly coupled to the cavity27 impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.28 and further developed by Galli et al.29

Use of photonic crystal slow light waveguides and cavities has been widely adopted by the photonics community in many differing applications. Therefore fabrication and characterization of these devices are of great interest. This paper outlines our fabrication technique and two optical characterization methods, namely: interferometric (waveguides) and resonant scattering (cavities).

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view ...

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 ...

Fabrication and Testing of Microfluidic Optomechanical Oscillators

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including microresonators. However, because of the increased acoustic radiative losses during direct liquid immersion of optomechanical devices, almost all published optomechanical experiments have been performed in solid phase. This paper discusses a recently introduced hollow microfluidic optomechanical resonator. Detailed methodology is provided to fabricate these ultra-high-Q microfluidic resonators, perform optomechanical testing, and measure radiation pressure-driven breathing mode and SBS-driven whispering gallery mode parametric vibrations. By confining liquids inside the capillary resonator, high mechanical- and optical- quality factors are simultaneously maintained.

Parametric optomechanical excitations have recently been experimentally demonstrated in microfluidic optomechanical resonators by means of optical radiation pressure and stimulated Brillouin scattering. This paper describes the fabrication of these microfluidic resonators along with methodologies for generating and verifying optomechanical oscillations.

Cavity optomechanics experiments that parametrically couple the phonon modes and photon modes have been investigated in various optical systems including ...

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 ...

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

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

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

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

Fabrication of 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.

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, ...

Design and Characterization Methodology for Efficient Wide Range Tunable MEMS Filters

Here, we demonstrate the advantages of the laser Doppler vibrometer (LDV) over conventional techniques (the network analyzer), as well as the techniques to create an application-based microelectromechanical systems (MEMS) filter and how to use it efficiently (i.e., tuning the tuning-capability and avoiding both failure and stiction). LDV enables crucial measurements that are impossible with the network analyzer, such as higher mode detection (highly sensitive biosensor application) and resonance measurement for very small devices (fast prototyping). Accordingly, LDV was used to characterize the frequency tuning range and resonance frequency at different modes of the MEMS filters built for this study. This wide range frequency tuning mechanism is based simply on Joule heating from the embedded heaters and relatively high thermal stress with respect to the temperature of a fixed-fixed beam. However, we demonstrate that another limitation of this method is the resulting high thermal stress, which can burn the devices. Further improvement was achieved and shown for the first time in this study, such that the tuning capability was increased by 32% through an increase in the applied DC bias voltage (25 V to 35 V) between the two adjacent beams. This important finding eliminates the need for the extra Joule heating at the wider frequency tuning range. Another possible failure is through stiction and requirement of structure optimization: We propose a simple and easy technique of low frequency square wave signal application that can successfully separate the beams and eliminates the need for the more sophisticated and complicated methods given in the literature. The above findings necessitate a design methodology, and so we also provide an application-based design.

A protocol for a fixed-fixed beam design using a laser Doppler vibrometer (LDV), including the measurement of frequency tuning, modification of tuning capability, and avoidance of device failure and stiction, is presented. The superiority of the LDV method over the network analyzer is demonstrated due to its higher mode capability.

Here, we demonstrate the advantages of the laser Doppler vibrometer (LDV) over conventional techniques (the network analyzer), as well as the techniques ...

Rapid Repetition Rate Fluctuation Measurement of Soliton Crystals in a Microresonator

Temporal solitons have attracted great interest in the past decades for their behavior in a steady state, where the dispersion is balanced by the nonlinearity in a propagation Kerr medium. The development of dissipative Kerr solitons (DKSs) in high-Q microcavities drives a novel, compact, chip-scale soliton source. When DKSs serve as femtosecond pulses, the repetition rate fluctuation can be applied to ultrahigh precision metrology, high-speed optical sampling, and optical clocks, etc. In this paper, the rapid repetition rate fluctuation of soliton crystals (SCs), a special state of DKSs where particle-like solitons are tightly packed and fully occupy a resonator, is measured based on the well-known delayed self-heterodyne method. The SCs are generated using a thermal-controlled method. The pump is a frequency fixed laser with a linewidth of 100 Hz. The integral time in frequency fluctuation measurements is controlled by the length of the delay fiber. For a SC with a single vacancy, the repetition rate fluctuations are ~53.24 Hz within 10 &#181;s and ~509.32 Hz within 125 &#181;s, respectively.

Here, we present a protocol to generate soliton crystals in a butterfly-packaged micro-ring resonator using a thermal tuned method. Further, the repetition rate fluctuations of a soliton crystal with a single vacancy are measured using a delayed self-heterodyne method.

Temporal solitons have attracted great interest in the past decades for their behavior in a steady state, where the dispersion is balanced by the ...

Fabrication of Surface Acoustic Wave Devices on Lithium Niobate

Manipulation of fluids and particles by acoustic actuation at small scale is aiding the rapid growth of lab-on-a-chip applications. Megahertz-order surface acoustic wave (SAW) devices generate enormous accelerations on their surface, up to 108 m/s2, in turn responsible for many of the observed effects that have come to define acoustofluidics: acoustic streaming and acoustic radiation forces. These effects have been used for particle, cell, and fluid handling at the microscale&#8212;and even at the nanoscale. In this paper we explicitly demonstrate two major fabrication methods of SAW devices on lithium niobate: the details of lift-off and wet etching techniques are described step-by-step. Representative results for the electrode pattern deposited on the substrate as well as the performance of SAW generated on the surface are displayed in detail. Fabrication tricks and troubleshooting are covered as well. This procedure offers a practical protocol for high frequency SAW device fabrication and integration for future microfluidics applications.

Two fabrication techniques, lift-off and wet etching, are described in producing interdigital electrode transducers upon a piezoelectric substrate, lithium niobate, widely used to generate surface acoustic waves now finding broad utility in micro to nanoscale fluidics. The as-produced electrodes are shown to efficiently induce megahertz order Rayleigh surface acoustic waves.&#160;

Manipulation of fluids and particles by acoustic actuation at small scale is aiding the rapid growth of lab-on-a-chip applications. Megahertz-order ...