Fabbricazione di Microrisonatori silice Ultra High Quality Factor

Summary

Si descrivono l'uso di una tecnica di anidride carbonica reflow laser per fabbricare cavità risonanti silice, incluse autoportante microsfere e on-chip microtoroids. Il metodo di reflow rimuove le imperfezioni della superficie, che consente lunghi tempi di vita dei fotoni all'interno di entrambi i dispositivi. I dispositivi ultra risultanti hanno fattori di alta qualità, consentendo applicazioni che vanno dalle telecomunicazioni alla rilevazione di agenti biologici.

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 date^1-2. 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 (τ_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/τ_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>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. Microsfera Fabrication Selezionare una piccola quantità (circa 5 pollici) di fibra ottica, strip ~ 1.5 "rivestimento da una estremità e pulire sia con metanolo o etanolo (Figura 1a, b). Se disponibile, attaccatevi alla fine con una mannaia in fibra ottica. Se non disponibile, tagliato con tronchesi o forbici tali che ~ 0.5 "è a sinistra. Il vantaggio di utilizzare una fibra ottica mannaia è che produce molto liscia, taglio uniforme come in figura 1b.</strong…

Discussion

Come con qualsiasi struttura ottica, mantenere la pulizia ad ogni passo del processo di fabbricazione è di importanza critica. Come ci sono numerosi libri di testo scritti sul tema della litografia e di fabbricazione, i suggerimenti che seguono non intendono essere esaustivi, ma mettere in evidenza alcuni dei problemi più comuni, i ricercatori hanno dovuto affrontare. ^19-20

Poiché l'uniformità della periferia del microtoroid è determinata dalla uniformità del disco inizi…

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.