Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Summary

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).

Abstract

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 signals^1-4 and the enhancement of both linear^5-7 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 modulators^18-19 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 spectrum^20-21 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

Protocol

Disclaimer: The following protocol gives a general process flow covering the fabrication and characterization techniques for photonic crystal waveguides and cavities. The process flow is optimized for the specific equipment available in our laboratory, and parameters may differ if other reagents or equipment is used.

1. Sample Preparation

1. Sample Cleaving - take the silicon-on-insulator (SOI) wafer and use a diamond scribe to scratch a line approximately 1-2 mm long from the edge of the silicon surface, ensuring that the scratch extends over the edge of the wafer. Align the scratch to a straight edge (e.g.

Results

Fabricated samples

Figure 1 shows a scanning electron microscope (SEM) image of an exposed and developed pattern in electron beam resist - it is evident from the "clean" edge between the resist and the silicon substrate that complete exposure/development has been accomplished. Exposure of dose test patterns, consisting of simple repeated shapes (in our case 50 × 50 μm squares), each with a differing base dose, are used to determine the correct dose factor and development time fo.......

Discussion

Sample fabrication

Our choice of electron-beam resist (i.e. ZEP 520A) is due to its simultaneously high resolution and etch resistance. We believe that ZEP 520A may be affected by the UV light emitted from overhead laboratory lights; as such we recommend placing spin-coated samples in UV opaque containers while moving them from one laboratory to another.

Moving onto defining the photonic crystal pattern, before exposing the sample we have found that allowing t.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Acetone	Fisher Scientific	A/0520/17	CAUTION: flammable, use good ventilation and avoid all ignition sources.
Isopropanol	Fisher Scientific	P/7500/15	CAUTION: flammable, use good ventilation and avoid all ignition sources.
Electron Beam resist	Marubeni Europe plc.	ZEP520A	CAUTION: flammable, harmful by inhalation, avoid contact with skin and eyes.
Xylene	Fisher Scientific	X/0100/17	CAUTION: flammable and highly toxic, use good ventilation, avoid all ignition sources, avoid contact with skin and eyes.
Microposit S1818 G2	Chestech Ltd.	10277866	CAUTION: flammable and causes irritation to eyes, nose and respiratory tract.
Microposit Developer MF-319	Chestech Ltd.	10058721	CAUTION: alkaline liquid and can cause irritation to eyes, nose and respiratory tract.
Hydrofluoric Acid	Fisher Scientific	22333-5000	CAUTION: extremely corrosive, readily destroys tissue; handle with full personal protective equipment rated for HF.
Microposit 1165 Remover	Chestech Ltd.	10058734	CAUTION: flammable and causes irritation to eyes, nose and respiratory tract.
Sulphuric Acid	Fisher Scientific	S/9120/PB17	CAUTION: corrosive and very toxic; handle with personal protective equipment and avoid inhalation of vapours or mists.
Hydrogen Peroxide	Fisher Scientific	BPE2633-500	CAUTION: very hazardous in case of skin and eye contact; handle with personal protective equipment.
Equipment
Silicon-on-Insulator wafer	Soitec	G8P-110-01
Diamond Scribe	J M Diamond Tool Inc.	HS-415
Microscope slides	Fisher Scientific	FB58622
Beakers	Fisher Scientific	FB33109
Tweezers	SPI Supplies	PT006-AB
Ultrasonic Bath	Camlab	1161436
Spin-Coater	Electronic Micro Systems Ltd.	EMS 4000
Pipette	Fisher Scientific	FB55343
E-beam Lithography System	Raith Gmbh	Raith 150
Reactive Ion Etching System	Proprietary In-house Designed	--
UV Mask Aligner	Karl Suss	MJB-3
ASE source	Amonics	ALS-CL-15-B-FA	CAUTION: invisible IR radiation.
Single mode fibers	Thorlabs	P1-SMF28E-FC-2
3 dB fiber splitters	Thorlabs	C-WD-AL-50-H-2210-35-FC/FC
Aspheric lenses	New Focus	5720-C
XYZ stages	Melles Griot	17AMB003/MD
Polarizing beamsplitter cube	Thorlabs	PBS104
IR detector	New Focus	2033
100× Objective	Nikon	BD Plan 100x
Oscilloscope	Tektronix	TDS1001B
Optical Spectrum Analyzer	Advantest	Q8384
IR sensor card	Newport	F-IRC2
TLS source	Agilent	81940A	CAUTION: invisible IR radiation.
IR Camera	Electrophysics	7290A
IR Detector	New Focus	2153
Digital Multimeter	Agilent	34401A
Illumination	Stocker Yale	Lite Mite
Monochromator	Spectral Products	DK480
Array Detector	Andor	DU490A-1.7
GIF Fiber	Thorlabs	31L02