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Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Published: November 30th, 2012



1School of Physics & Astronomy, University of St Andrews

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).

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 signals1-4 and the enhancement of both linear5-7 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 modulators18-19 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 spectrum20-21 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

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.......

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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 developmen.......

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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.......

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The authors gratefully acknowledge Dr Matteo Galli, Dr Simone L. Portalupi and Prof. Lucio C. Andreani from the University of Pavia for helpful discussions related to the RS technique and the execution of measurements.


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Name Company Catalog Number Comments
Name Company Catalogue number Comments (optional)
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.
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  

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