실리카 초고 품질 팩터 Microresonators의 제작

Summary

우리는 자유 서 microspheres 및 온칩 microtoroids 포함한 실리카 공진 충치를 조작하는 이산화탄소 레이저 reflow 기법의 사용을 설명합니다. reflow 메서드는 두 장치 사이 긴 광자 수명 있도록, 표면 결함을 제거합니다. 그 결과 디바이스는 통신에서 biodetection에 이르기까지 애플 리케이션을 가능하게, 초고 품질 요인이 있습니다.

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. Microsphere의 제조 한쪽에서 광섬유, 스트립 ~ 1.5 "cladding 소량 (약 5 인치)를 선택하고 메탄올 또는 에탄올 (그림 1A, B) 중 하나와 함께 청소. 광섬유 칼로 사용할 경우, 다니엘은 끝. 사용할 수없는 경우, 전선 절단기 또는 ~ 0.5 "가 남아있다는 등 가위로 잘라. 광섬유 식칼을 사용하는 장점은 그림 1B와 같이 매우 부드럽고 균일한 커팅을 생산한다는 것…

Discussion

모든 광학 구조와 마찬가지로 제조 공정의 모든 단계에서 청결을 유지하는 것은 매우 중요합니다. 리소그래피와 제조의 주제에 쓰여진 수많은 교과서가있는 바와 같이, 아래 제안은 종합 있겠지만 연구자들이 직면보다 일반적인 문제 몇 가지를 강조하기위한 것이 아닙니다. ^19-20

microtoroid의 주변의 균일도가 초기 디스크의 균일에 의해 결정되기 때문에 패턴이 매우 …

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.