Summary

Fabricage van Silica Ultra High Quality Factor microresonators

Published: July 02, 2012
doi:

Summary

Wij beschrijven het gebruik van een kooldioxide laser reflow techniek silica trilholten, zoals vrijstaande microsferen en on-chip microtoroids fabriceren. De reflow-methode verwijdert oneffenheden, waardoor lange levensduur foton in beide apparaten. De resulterende apparaten hebben een zeer hoge kwaliteit factoren, waardoor toepassingen, variërend van telecommunicatie tot biodetection.

Abstract

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>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 (τo) 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/τo). 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>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.

Protocol

1. Microsphere Fabrication Kies een kleine hoeveelheid (ongeveer 5 cm) van optische vezel, strip ~ 1.5 "bekleding van de ene kant en schoon te maken met of methanol of ethanol (figuur 1a, b). Indien beschikbaar, klieven het einde met een optische vezel hakmes. Indien niet, geknipt met draadschaar of schaar zodat ~ 0.5 "blijft. Het voordeel van een optische vezel mes is dat een zeer gladde, uniforme snede als in figuur 1b produceert. Overmatige ruwheid of d…

Discussion

Zoals bij elke optische structuur, behoud van netheid in elke fase van het fabricageproces is van cruciaal belang. Omdat er veel verschillende leerboeken geschreven over het onderwerp van de lithografie en de fabricage, worden de suggesties hieronder niet bedoeld om volledig te zijn, maar een paar van de meest voorkomende problemen onderzoekers hebben geconfronteerd markeren 19-20.

Omdat de uniformiteit van de omtrek microtoroid wordt bepaald door de uniformiteit van de eerste sch…

Disclosures

The authors have nothing to disclose.

Acknowledgements

A. Maker werd ondersteund door een Annenberg Foundation Graduate Research Fellowship, en dit werk werd ondersteund door de National Science Foundation [085281 en 1028440].

Materials

Name of the part Company Catalogue number Comments
Fiber scribe Newport F-RFS Optional
Optical fiber Newport F-SMF-28 Any type of optical fiber can be used.
Fiber coating stripper Newport F-STR-175 Wire strippers can also be used
Ethanol Any vendor Solvent-level purity Methanol or Isopropanol are substitutes

Table 1. Microsphere Fabrication Materials.

Name of the reagent Company Catalogue number Comments
Silicon wafers with 2μm thermally grown silica WRS Materials n/a We use intrinsic8, <100>, 4″ diameter
HMDS (Hexamethyldisilazane) Aldrich 440191  
Photoresist Shipley S1813  
Developer Shipley MF-321  
Buffered HF – Improved Transene n/a The improved buffered HF gives a smoother, better quality etch than plain BOE or HF
Acetone, Methanol, Isopropanol Any vendor 99.8% purity  

Table 2. Microtoroid Fabrication Materials.

Equipment Name Manufacturer Catalogue number Comments
Spinner Solitec 5110-ND Any spinner can be used.
Aligner Suss Microtec MJB 3 Any aligner can be used.
XeF2 etcher Advanced Communication Devices, Inc. #ADCETCH2007  

Table 3. Microtoroid Fabrication Equipment.

Name of the part Company Catalogue number Comments
CO2 Laser Synrad Series 48  
3-Axis stage OptoSigma 120-0770 Available from other vendors as well.
Si Reflector 1″ diameter) II-VI 308325 Available from other vendors as well.
Kinematic gimbal mount (for Si reflector) Thor Labs KX1G Available from other vendors as well.
Beam combiner (1″ diameter) Meller Optics L19100008-B0 Available from other vendors as well.
4″ Focal length Lens (1″ diameter) Meller Optics or II-VI   Available from other vendors as well
Assorted posts, lens mounts Thor Labs, Newport, Edmund Optics or Optosigma    
Zoom 6000 machine vision system Navitar n/a Requires generic USB camera and computer for real-time imaging. This is purchased as a kit.
Focuser for Zoom 6000 system Edmund Optics 54-792 Available from other vendors as well.
X-Z Axis Positioners for Zoom 6000 Parker Daedal CR4457, CR4452, 4499 CR4457 is X-axis, CR4452 is Z-axis, 4499 is mounting bracket.

Table 4. CO2 Laser Reflow Set-up.

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Cite This Article
Maker, A. J., Armani, A. M. Fabrication of Silica Ultra High Quality Factor Microresonators. J. Vis. Exp. (65), e4164, doi:10.3791/4164 (2012).

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