Name: fabrication of silica ultra high quality factor microresonators
Uploaded: 2012-07-02T08:04:00
Description: Watch this Scientific Journal Video about Fabrication of Silica Ultra High Quality Factor Microresonators at JoVE.com

A. Maker was supported by an Annenberg Foundation Graduate Research Fellowship, and this work was supported by the National Science Foundation [085281 and 1028440].

1. Microsphere Fabrication
<ol>
 <li>Select a small amount (approximately 5 inches) of optical fiber, strip ~1.5" cladding from one end and clean with either methanol or ethanol (Figure 1a, b).</li>
 <li>If available, cleave the end with an optical fiber cleaver. If not available, cut with wire cutters or scissors such that ~0.5" is left. The advantage of using an optical fiber cleaver is that it produces a very smooth, uniform cut as in Figure 1b. Excessive roughness or defects...

<ol>
	<li>Armani, D. K., Kippenberg, T. J., Spillane, S. M., Vahala, K. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultra-high-Q+toroid+microcavity+on+a+chip.">Ultra-high-Q toroid microcavity on a chip.</a> Nature. 421, 925-928 (2003).</li><li>Gorodetsky, M. L., Savchenkov, A. A., Ilchenko, V. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultimate+Q+of+optical+microsphere+resonators.">Ultimate Q of optical microsphere resonators.</a> Optics Letters. 21, 453-455 (1996).</li><li>Armani, A. M., Kulkarni, R. P., Fraser, S. E., Flagan, R. C., Vahala, K. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Label-Free,+Single-Molecule+Detection+with+Optical+Microcavities.">Label-Free, Single-Molecule Detection with Optical Microcavities.</a> Science. 317, 783 (2007).</li><li>Choi, H. S., Ismail, S., Armani, A. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Studying+polymer+thin+films+with+hybrid+optical+microcavities.">Studying polymer thin films with hybrid optical microcavities.</a> Optics Letters. 36, 2152-2154 (2011).</li><li>Aoki, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Observation+of+strong+coupling+between+one+atom+and+a+monolithic+microresonator.">Observation of strong coupling between one atom and a monolithic microresonator.</a> Nature. 443, 671-674 (2006).</li><li>Hsu, H. -. S., Cai, C., Armani, A. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultra-low+threshold+Er:Yb+sol-gel+microlaser+on+silicon.">Ultra-low threshold Er:Yb sol-gel microlaser on silicon.</a> Optics Express. 17, 23265 (2009).</li><li>Zhu, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On-chip+single+nanoparticle+detection+and+sizing+by+mode+splitting+in+an+ultrahigh-Q+microresonator.">On-chip single nanoparticle detection and sizing by mode splitting in an ultrahigh-Q microresonator.</a> Nature Photonics. 4, 46-49 (2009).</li><li>Zhang, X., Choi, H. -. S., Armani, A. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultimate+quality+factor+of+silica+microtoroid+resonant+cavities.">Ultimate quality factor of silica microtoroid resonant cavities.</a> Applied Physics Letters. 96, 153304 (2010).</li><li>Vernooy, D. W., Ilchenko, V. S., Mabuchi, H., Streed, E. W., Kimble, H. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-Q+measurements+of+fused-silica+microspheres+in+the+near+infrared.">High-Q measurements of fused-silica microspheres in the near infrared.</a> Optics Letters. 23, 247-249 (1998).</li><li>Saleh, B. E. A., Teich, M. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Fundamentals of Photonics. , (2007).</li><li>Ilchenko, V. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Crystal+quartz+optical+whispering-gallery+resonators.">Crystal quartz optical whispering-gallery resonators.</a> Optics Letters. 33, 1569-1571 (2008).</li><li>Soteropulos, C., Hunt, H., Armani, A. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+binding+kinetics+using+whispering+gallery+mode+microcavities.">Determination of binding kinetics using whispering gallery mode microcavities.</a> Applied Physics Letters. 99, 103703 (2011).</li><li>Barclay, P. E., Srinivasan, K., Painter, O., Lev, B., Mabuchi, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Integration+of+fiber-coupled+high-Q+SiNx+microdisks+with+atom+chips.">Integration of fiber-coupled high-Q SiNx microdisks with atom chips.</a> Applied Physics Letters. 89, (2006).</li><li>Srinivasan, K., Painter, O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mode+coupling+and+cavity-quantum-dot+interactions+in+a+fiber-coupled+microdisk+cavity.">Mode coupling and cavity-quantum-dot interactions in a fiber-coupled microdisk cavity.</a> Physical Review. A. 75, (2007).</li><li>Xu, Q. F., Lipson, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=All-optical+logic+based+on+silicon+micro-ring+resonators.">All-optical logic based on silicon micro-ring resonators.</a> Optics Express. 15, 924-929 (2007).</li><li>Martin, A. L., Armani, D. K., Yang, L., Vahala, K. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Replica-molded+high-Q+polymer+microresonators.">Replica-molded high-Q polymer microresonators.</a> Optics Letters. 29, 533-535 (2004).</li><li>Chao, C. Y., Guo, L. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Polymer+microring+resonators+fabricated+by+nanoimprint+technique.">Polymer microring resonators fabricated by nanoimprint technique.</a> Journal of Vacuum Science Technology B. 20, 2862-2866 (2002).</li><li>Armani, A. M., Armani, D. K., Min, B., Vahala, K. J., Spillane, S. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultra-high-Q+microcavity+operation+in+H2O+and+D2O.">Ultra-high-Q microcavity operation in H2O and D2O.</a> Applied Physics Letters. 87, 151118 (2005).</li><li>Kovacs, G. T. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Micromachined Transducers Sourcebook. , (1998).</li><li>Kovacs, G. T. A., Maluf, N. I., Petersen, K. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bulk+Micromaching+of+Silicon.">Bulk Micromaching of Silicon.</a> Proceedings of the IEEE. 86, 1536-1551 (1998).</li></ol>

As with any optical structure, maintaining cleanliness at every step of the fabrication process is of critical importance. As there are numerous textbooks written on the topic of lithography and fabrication, the suggestions below are not intended to be comprehensive, but highlight a few of the more common issues researchers have faced.19-20
Because the uniformity of the microtoroid's periphery is determined by the uniformity of the initial disk, it is very important to pattern very ...

<table border="1">
 <tbody>
 <tr>
 <td>Name of the part</td>
 <td>Company</td>
 <td>Catalogue number</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>Fiber scribe</td>
 <td>Newport</td>
 <td>F-RFS</td>
 <td>Optional</td>
 </tr>
 <tr>
 <td>Optical fiber</td>
 <td>Newport</td>
 <td>F-SMF-28</td>
 <td>Any type of optical fiber can be used.</td>
 </tr>
 <tr>
 <td>Fiber coating stripper</td>
 <td>Newport</td>
 <td>F-STR-175</td>
 <td>Wire strippers can also be used</td>
 </tr>
 <tr>
 <td>Ethanol</td>
 <td>Any vendor</td>
 <td>Solvent-level purity</td>
 <td>Methanol or Isopropanol are substitutes</td>
 </tr>
 </tbody>
</table>
Table 1. Microsphere Fabrication Materials.
<table border="1">
 <tbody>
 <tr>
 <td>Name of the reagent</td>
 <td>Company</td>
 <td>Catalogue number</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>Silicon wafers with 2&mu;m thermally grown silica</td>
 <td>WRS Materials</td>
 <td>n/a</td>
 <td>We use intrinsic8, &lt;100&gt;, 4" diameter</td>
 </tr>
 <tr>
 <td>HMDS (Hexamethyldisilazane)</td>
 <td>Aldrich</td>
 <td>440191</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Photoresist</td>
 <td>Shipley</td>
 <td>S1813</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Developer</td>
 <td>Shipley</td>
 <td>MF-321</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Buffered HF - Improved</td>
 <td>Transene</td>
 <td>n/a</td>
 <td>The improved buffered HF gives a smoother, better quality etch than plain BOE or HF</td>
 </tr>
 <tr>
 <td>Acetone, Methanol, Isopropanol</td>
 <td>Any vendor</td>
 <td>99.8% purity</td>
 <td>&nbsp;</td>
 </tr>
 </tbody>
</table>
Table 2. Microtoroid Fabrication Materials.
<table border="1">
 <tbody>
 <tr>
 <td>Equipment Name</td>
 <td>Manufacturer</td>
 <td>Catalogue number</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>Spinner</td>
 <td>Solitec</td>
 <td>5110-ND</td>
 <td>Any spinner can be used.</td>
 </tr>
 <tr>
 <td>Aligner</td>
 <td>Suss Microtec</td>
 <td>MJB 3</td>
 <td>Any aligner can be used.</td>
 </tr>
 <tr>
 <td>XeF2 etcher</td>
 <td>Advanced Communication Devices, Inc.</td>
 <td>#ADCETCH2007</td>
 <td>&nbsp;</td>
 </tr>
 </tbody>
</table>
Table 3. Microtoroid Fabrication Equipment.
<table border="1">
 <tbody>
 <tr>
 <td>Name of the part</td>
 <td>Company</td>
 <td>Catalogue number</td>
 <td>Comments</td>
 </tr>
 <tr>
 <td>CO2 Laser</td>
 <td>Synrad</td>
 <td>Series 48</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>3-Axis stage</td>
 <td>OptoSigma</td>
 <td>120-0770</td>
 <td>Available from other vendors as well.</td>
 </tr>
 <tr>
 <td>Si Reflector 1" diameter)</td>
 <td>II-VI</td>
 <td>308325</td>
 <td>Available from other vendors as well.</td>
 </tr>
 <tr>
 <td>Kinematic gimbal mount (for Si reflector)</td>
 <td>Thor Labs</td>
 <td>KX1G</td>
 <td>Available from other vendors as well.</td>
 </tr>
 <tr>
 <td>Beam combiner (1" diameter)</td>
 <td>Meller Optics</td>
 <td>L19100008-B0 </td>
 <td>Available from other vendors as well.</td>
 </tr>
 <tr>
 <td>4" Focal length Lens (1" diameter)</td>
 <td>Meller Optics or II-VI</td>
 <td>&nbsp;</td>
 <td>Available from other vendors as well</td>
 </tr>
 <tr>
 <td>Assorted posts, lens mounts</td>
 <td>Thor Labs, Newport, Edmund Optics or Optosigma</td>
 <td>&nbsp;</td>
 <td>&nbsp;</td>
 </tr>
 <tr>
 <td>Zoom 6000 machine vision system</td>
 <td>Navitar</td>
 <td>n/a</td>
 <td>Requires generic USB camera and computer for real-time imaging. This is purchased as a kit.</td>
 </tr>
 <tr>
 <td>Focuser for Zoom 6000 system</td>
 <td>Edmund Optics</td>
 <td>54-792</td>
 <td>Available from other vendors as well.</td>
 </tr>
 <tr>
 <td>X-Z Axis Positioners for Zoom 6000</td>
 <td>Parker Daedal</td>
 <td>CR4457, CR4452, 4499</td>
 <td>CR4457 is X-axis, CR4452 is Z-axis, 4499 is mounting bracket.</td>
 </tr>
 </tbody>
</table>
Table 4. CO2 Laser Reflow Set-up.

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.

Watch this Scientific Journal Video about Fabrication of Silica Ultra High Quality Factor Microresonators at JoVE.com

Fabrication of Silica Ultra High Quality Factor Microresonators

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 ...

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