Reardon_FabricationA.. - University of St Andrews
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1. Ina H amp Kobayashi S Fourier transform method of fringe pattern analysis for computer based topography and interferometry J Opt Soc Am 72 1 156 160 doi 10 1364 JOSA 72 000156 19822. Tarhan I Zinkin M P amp Watson G H Interferometric technique for the measurement of photonic band structure in colloidal crystals Opt Lett 20 14 1571 1573 doi 10 1364 OL 20 001571 1995 Galli M Marabelli F amp Guizzetti G Direct measurement of refractive index dispersion of transparent media by white light interferometry Appl Opt 42 19 3910 3914 doi 10 1364 A0 42 003910 2003 Galli M Bajoni D Marabelli F Andreani L C and Pavesi L amp Pucker G Photonic bands and group velocity dispersion in Si SiO photonic crystals from white light interferometry Phys Rev B 69 11 115107 doi 10 1103 PhysRevB 69 115107 2004 Vlasov Y A O Boyle M Hamann H F amp McNab S J Active control of slow light on a chip with photonic crystal waveguides Nature 438 65 69 doi 10 1038 nature04210 2005 Gomez lIglesias A O Brien D O Faolain L Miller A amp Krauss T F Direct measurement of the group index of photonic crystal waveguide via Fourier transform spectral interferometry Appl Phys Lett 90 26 261107 doi 10 1063 1 2752761 2007 Akahane Y Asano T Song B S amp Noda S High Q photonic nanocavity in a two dimensional photonic crystal Nature 425 944 947 doi 10 1038 nature02063 2003 McCutcheon M W Rieger G W et al Resonant scattering and second harmonic spectroscopy of planar photonic crystal microcavities Appl Phys Lett 87 22 221110 d
3. acetone and isopropanol Note that for safety reasons only glass beakers and metal tweezers should be used with the Piranha solution As Piranha solution can explode in contact with acetone or isopropanol it should be handled away from these reagents Facet Cleaving if preparing a photonic crystal slow light waveguide the sample requires facet cleaving Cleave the sample by following the same procedure as outlined in step 1 1 except that as small a scratch as possible should be used An SOI chip with 700um thick substrate can be reliably cleaved down to 4 Smm long samples 4 Photonic Crystal Slow light Waveguide Characterisation 4 1 4 2 Preliminary preparation of the setup connect the output of a CAUTION broadband amplified spontaneous emission ASE light source invisible IR radiation avoid unnecessary high powers cover beam path if possible to a 3dB fibre splitter and use each of the outputs to couple light into the two arms of a free space Mach Zehnder interferometeter MZI as shown in Figure 9 Use aspheric lenses to collimate the light output from the fibres In one of the arms of the interferometer use two additional aspheric lenses to couple the light beam in and out of the sample chip Place a polarisation beam splitter PBS in the sample arm to TE polarise the light inputting the sample Use aspheric lenses to couple the collimated output beams from both arms back into a second 3dB fibre splitter where they will
4. before attempting to work with important samples It is equally imperative that once a good cleave has been achieved the facets are not damaged the sample should only be lifted using the two edges parallel to the waveguides i e not by the end facet sides of the chip Sample lengths down to 2 3mm can be reliably achieved with manual cleaving of a 700um thick SOI chip For smaller samples we suggest thinning the substrate or use a different cleaving technique Although the protocol outlined in this paper is optimised for SOI the general principle behind the fabrication methods are also valid for the fabrication of devices into other semiconductors of course when changing from silicon careful consideration of etch tool etch chemistries and mask materials would need to be made The fabrication protocol of this paper is optimised for devices targeted at an operating centre wavelength of 1550nm however devices have also been prepared for the MidIR 2 7 3 5um regime using fabrication protocols based on the ones presented in this paper Slow light group index measurements The significance of the group index as the key parameter to measure slow light originates from the dispersion diagram or band structure w k typically used to describe the dispersion of a photonic crystal waveguide The local slope of the dispersion curve dw 0k corresponds to the group velocity v i e the speed at which the electromagnetic energy travels through the wavegu
5. devices are of great interest This paper outlines our fabrication technique and two optical characterisation methods namely interferometric waveguides and resonant scattering cavities Long Abstract 150 words minimum 400 words maximum 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 and the enhancement of both linear and nonlinear devices 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 realisation of optically and electrically pumped ultra low threshold lasers and the enhancement of nonlinear effects Furthermore passive filters and modulators 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 fabricati
6. the delay stage to make sure that the reference arm is shorter than the sample arm and results in a fringe spacing of about 5 to 10 fringes in a 10nm wavelength range see Figure 10a Finally perform this optimisation on the device that provides the maximum delayg and then keep the delay fixed throughout the measurement of the entire sample 4 3 4 4 4 5 4 6 Calibration run while still aligned on the blank waveguide run three scans on the OSA one scan for the interference spectrum and one scan for each of the two arms separately obtained by blocking the other arm Use a resolution of 0 05 0 1nm Record each measured spectrum Slow light data acquisition run and record three spectra as in step 4 3 for each photonic crystal waveguide on the chip Fourier data analysis the interference spectrum interferogram is mathematically expressed by I S R sqrt S R exp i iwt c c where S w and R are the spectral densities measured separately from the sample and reference arms respectively The delay 7 is set by the position of the delay stage in the reference arm The information on the dispersion of the photonic crystal waveguide is contained in the phase term which we must extract from the measured data Subtract the non interfering background S R q from the interferogram to isolate only the interfering term Calculate the Fourier transform of the interfering term the
7. 2 0kV 23 0mm x50 0k SE M Figure 1 Photonic crystal pattern in electron beam resist ZEP520A 4800 1 0kV 4 7 x60 0k SE U Figure 2 Photonic crystal pattern after sined in silicon Figure 4 Photonic crystal in silicon etched with TEE RIE recipe Of note are the straight vertical side walls and the little to no side wall roughness Figure 5 Photonic crystal sike wath id RIE chamber pressure Angled side walls are evident Figure 6 Photonic crystal etched with higher RF power and DC bias Holes show evidence of widening at surface Figure 7 Over etched photonic crystal Angled side walls due to resist breakdown and overall widening of holes evident 54800 2 0kV 9 7 Figure 8 Poorly optimised etch i e both pressure and time Mask break down has caused striations at the top of each hole 54800 delay stage reference path splitter Optical Spectrum Analyser Figure 9 Schematic of the Mach Zehnder interferometric setup used to measure transmission and group index curves of slow light photonic crystal waveguides sample a Ss a So a o Transmitted power nW o o a 0 1550 1560 1570 1580 1590 1600 Wavelength nm Figure 10 Measured interferograms of a a blank ridge waveguide and b an 80um long engineered slow light photonic crystal waveguide on the same chip The original data is shown as the grey curve in the background The black curv
8. Fabrication and Characterisation of Photonic Crystal Slow Light Waveguides and Cavities Authors Christopher P Reardon Isabella H Rey Karl Welna Liam O Faolain and Thomas F Krauss C P R LH R and K W contributed equally to this work Authors institution s affiliation s for each author Christopher P Reardon SUPA School of Physics amp Astronomy University of St Andrews St Andrews Scotland UK Tel 01334 467336 Fax 01334 463104 cr39 st andrews ac uk Isabella H Rey SUPA School of Physics amp Astronomy University of St Andrews St Andrews Scotland UK ir21 st andrews ac uk Karl Welna SUPA School of Physics amp Astronomy University of St Andrews St Andrews Scotland UK kw322 st andrews ac uk Liam O Faolain also known as William Whelan Curtin SUPA School of Physics amp Astronomy University of St Andrews St Andrews Scotland UK jww1 st andrews ac uk Thomas F Krauss SUPA School of Physics amp Astronomy University of St Andrews St Andrews Scotland UK tfk st andrews ac uk Corresponding author Christopher P Reardon Keywords Photonic Crystals Slow light Cavities Waveguides Silicon SOI Fabrication Characterisation Short Abstract 50 words maximum Use of photonic crystal slow light waveguides and cavities has been widely adopted by the photonics community in many differing applications Therefore fabrication and characterisation of these
9. between one photonic crystal hole to another can be on the order of nanometres which can be further reduced by increasing the speed at which the pattern is written As mentioned these issues can be further negated although never completely removed by allowing the system to settle after first loading the sample The Ar H2 plasma etch used to remove metal and silicon contaminants through ion bombardment followed by O plasma etch used for the removal of polymer and organic residue through plasma ashing described in section 3 1 of the protocol define a cleaning regimen that was developed to control contamination within the RIE chamber when etching the photonic crystals this cleaning is considered by us to be one of the most important steps in the fabrication of photonic crystal devices cleaning of the RIE chamber is paramount to repeatable reliable fabrication especially as in our case where the RIE is not used solely for the etching of silicon The Ar Hz plasma is seen to change from a blue grey colour indicating a contaminated chamber to a pink colour indicating that the chamber is free of contaminants a 10min plasma is normally sufficient The O plasma is then carried out for a further 5 10min depending on the cleanliness of the chamber at the beginning of the process i e Ar H plasma colour Although the previous method has not be conclusively proven we find that the colour of the plasma proves a useful indicator for chamber cleanlin
10. cing of 4fringes 10nm we are able to reliably measure group indices up to almost 100 also in 300um long engineered waveguides Figure 13 For longer waveguides the fringes become very dense very quickly and the resolution of the OSA will limit the maximum measurable group index Note however that for a fixed resolution and fringe spacing the maximum measurable group index does not scale linearly with waveguide length and may also be influenced by propagation loss dispersion For a very long waveguide we suggest to include next to it a short waveguide with identical design specifically for group index measurement In summary we have described a simple and powerful method for the experimental determination of the dispersion properties of slow light photonic crystal waveguides Our technique is based on the combination of frequency domain interferometry with Fourier transform analysis and allows for a direct single shot continuous mapping of the group index curve with no need for delay scans nonlinear fitting of data or determination of the position of fringe extrema 7 By using a broadband light source we are able to extract information from the sample over a large wavelength range and in a very stable and repeatable manner We can measure group indices in excess of 100 for both short and moderately long waveguides up to 200 250um which are values much higher than those needed for the useful application of slow light wave
11. djust the micro block so that the sample is in focus and a cavity can be seen with the camera as in Figure 15 left Using an amplified spontaneous emission ASE source align the beam with the centre of the cavity Figure 15 right Flip away the illumination mirror and allow the output arm to enter the spectrometer monochromator with attached array detector Start a broad scan with a low to moderate resolution in order to identify the cavity peaks Obtain the coarse wavelength of the resonance in the ASE scan Figure 16a with an accuracy of Inm It is also possible to acquire the broad scan with a CAUTION tunable laser source TLS Figure 16b invisible IR radiation avoid unnecessary high powers cover beam path if possible One has to be careful that the resolution is set to the highest value in order to sample the line widths of every peak 5 2 Perform high resolution scans on the identified peaks connect the TLS to the input arm and attenuate the beam to a uW level Prepare for the high resolution scan by allowing the output arm to be collected by the photodetector and setting up a continuous sweep scan with a resolution of lpm for a 2nm range centred at the previously found resonance wavelength The importance of this step is to improve the signal to noise ratio SNR with the aim to obtain a Lorentzian line shape resonance change the xyz position of the micro block and re run the scan until the SNR is maximised and the line shape
12. e from the high resolution scan needs to be fitted As the coupling to the cavity is governed by Fano resonances we use the following Fano function to obtain a proper fit of the line shape Y yo A gt2 A AglAAYI14 2 A Ap AA where yo is an offset A an area constant q a dimensionless parameter that gives the ratio between the resonant and non resonant amplitudes 29 is the resonant wavelength and 4 is the full width half maximum FWHM of the resonance In general the fitting is easier the closer the line shape is to that of a Lorentzian because in the first fitting steps q can be fixed at 0 and oto the centre of the peak An example of such a Lorentzian line shape with a high SNR is shown in Figure 17a The wavelength is determined as 1562 162nm and q is 0 0891 The Q factor is calculated according to Q Ag AA so that Q results in 41 382 In the case that no Lorentizan line shape can be obtained during the measurement steps in 5 2 the fit is still possible but more difficult due to more unknown fitting parameters For example in Figure 17b the peak of the line shape does not correspond to the resonant wavelength indicated by the dashed line The Q factor however is close to that obtained in Figure 17a If the SNR is low the fitting error obviously increases and a Lorentzian line shape Figure 17c gives a more accurate Q factor than an asymmetric Fano line shape Figure 17d Tables and Figures 54800
13. e has been numerically filtered to remove Fabry Perot fringes ao Ss 70 2 x lt 5 o 2 10 50 E a 40 3 2 S 30 O z 15 x 20 10 20 0 1550 1560 1570 1580 1590 1600 Wavelength A nm Figure 11 Group index blue and transmission black grey curves of the same waveguide as in Figure 10 b The transmission curve is obtained by normalising to that of the blank ridge waveguide The black curve has been numerically filtered to remove Fabry Perot fringes Transmission dB 20 0 1530 1535 1540 1545 1550 1555 1560 1565 Wavelength A nm Figure 12 Group index blue and transmission black grey curves of an 80um long waveguide The transmission curve is obtained by normalising to that of a blank ridge waveguide on the same chip The black curve has been numerically filtered to remove Fabry Perot fringes N 3 g Transmission dB 8 SS Group index n gt 10 Bao 1540 1550 1560 1570 Wavelength A nm Figure 13 Group index blue and transmission black grey curves of a 300um long waveguide The transmission curve is obtained by normalising to that of a blank ridge waveguide on the same chip The black curve has been numerically filtered to remove Fabry Perot fringes Figure 14 Top view of multi functional characterisation setup with exchangeable element The probe beam from the input arm green is centred at the high NA objective that focuses it on the mounted sample Alignment to the cen
14. ess We have also found that by pre conditioning the etch chamber with the silicon etch gasses for 10min results in a more reliable process we believe this to be due to the etching gas flow rates stabilising and being adsorbed into the chamber walls during the pre condition period When under etching the sample to create membranes using hydrofluoric acid the access waveguides must be protected If the hydrofluoric acid comes into contact with the access waveguide it penetrates through the now etched trench either side of the waveguide and under etches the access waveguides for hundreds of micrometres In extreme cases the access waveguides may bend and break due to stresses rendering a complete chip useless As hydrofluoric acid is an isotropic etchant the etching time must be controlled to prevent lateral etching perpendicular to the photonic crystal waveguide from causing the membrane to bend due to release of the stress intrinsic to the silicon layer In extreme cases the excessive under etch can also cause the membrane to collapse Finally the creation of clean facets for the free space coupling of light into photonic crystal waveguides is extremely challenging If scratched cleaved carefully silicon will essentially follow a crystal plane forming a good facet in our experience techniques such as facet polishing are not required A bad facet can cause large coupling losses at each facet We recommend perfecting a cleaving technique
15. guides for enhancing the performance of both linear and nonlinear devices Resonant scattering Photonic crystal cavities confine light in plane in two dimensions in contrast to photonic crystal waveguides where light is guided in one dimension This allows the storage of light within ultra small volumes which is described by an energy decay analogue to i e that of an electronic resonator In photonic systems this decay is associated with the photon lifetime of the cavity and is of exponential form hence resulting in a Lorentzian lineshape of the peak The ratio of the peak centre wavelength to the Full Width Half Maximum represents the Q factor An important feature of the RS technique is the polarisation maintaining property of the setup and especially that of the high NA objective Here lies the issue with the compatibility of having a high NA high collection efficiency while maintaining the polarisation because high NA objectives tend to mix polarisations This polarisation intermixing is responsible for small peaks and low SNR When the off resonance x polarised light arrives at the cavity Figure 18a it is back scattered through the objective and filtered out by the beam splitter analyser y polarised so that only a low level is seen at the detector In the case of polarisation intermixing some of the x polarised light is converted to the opposite polarisation and can pass the analyser thus increasing the background If then o
16. hnique was first demonstrated by McCutcheon et al and further developed by Galli et al Protocol Text Disclaimer The following protocol gives a general process flow covering the fabrication and characterisation techniques for photonic crystal waveguides and cavities The process flow is optimised for the specific equipment available in our laboratory and parameters may differ if other reagents or equipment is used 1 Sample Preparation 1 1 1 2 1 3 Sample Cleaving take the silicon on insulator SOI wafer and use a diamond scribe to scratch a line approximately 1 2mm 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 that of a microscope slide and apply even positive pressure to both sides of the scratch the wafer will cleave along the crystal plane at the scratch location Repeat this procedure to define the entire chip Sample Cleaning Place the sample into the CAUTION acetone using tweezers and clean in an ultrasonic bath for 1 2min Remove the sample from the acetone rinse any remaining acetone from the sample using CAUTION isopropanol 30s both acetone and isopropanol are flammable use good ventilation and avoid all ignition sources Dry the sample using a clean dry nitrogen gun Spin resist place the sample onto the spin coater Pipette electron sensitive resist CAUTION ZEP520A ZEP520A is flammable har
17. i 10 1364 OE 17 002944 2009 Corcoran B Monat C et al Green light emission in silicon through slow light enhanced third harmonic generation in photonic crystal waveguides Nature Photon 3 206 210 doi 10 1038 nphoton 2009 28 2009 Li J O Faolain L Rey I H amp Krauss T F Four wave mixing in photonic crystal waveguides slow light enhancement and limitations Opt Express 19 5 4458 4463 doi 10 1364 OE 19 004458 2010 Checoury X Han Z amp Boucaud P Stimulated Raman scattering in silicon photonic crystal waveguides under continuous excitation Phys Rev B 82 4 041308 doi 10 1103 PhysRevB 82 041308 2010 Nomura M Kumagai N Iwamoto S Ota Y amp Arakawa Y Photonic crystal nanocavity laser with a single quantum dot gain Opt Express 17 18 15975 15982 doi 10 1364 OE 17 015975 2009 Ellis B Mayer M A et al Ultralow threshold electrically pumped quantum dot photonic crystal nanocavity laser Nature Photon 24 297 300 doi 10 1038 nphoton 2011 51 2011 Galli M Gerace D et al Low power continuous wave generation of visible harmonics in silicon photonic crystal nanocavities Opt Express 18 25 26613 26624 doi 10 1364 OE 18 026613 2010 Notomi M Shinya A Mitsugi S Kira G Kuramochi E amp Tanabe T Optical bistable switching action of Si high Q photonic crystal nanocavities Opt Express 13 7 2678 2687 doi 10 1364 OPEX 13 002678 2005 Shamba
18. ide which can be equivalently described by the group index n c v Values of ng around 5 correspond to the fast light regime whereas higher values are typically considered to fall within in the slow light regime When building the slow light MZI setup it is important to make sure that all the fibres of the two arms of the interferometer are securely tied to the optical table as any movement or vibration will change the path lengths compromising the quality of the interferogram acquisition For the same reason the scan of the interferogram should be performed quickly or fluctuations of the phase will result in unwanted oscillations of the group index data The two arms of the MZI may also be realised entirely in free space to avoid fibres altogether as in Reference 26 a free space MZI will be more stable but also more difficult to align Depending on the resolution set and the strength of the Fabry Perot fringes the determination of the group index is affected by large uncertainty when the fringes converge very tightly Setting the delay stage to initially give 4 10fringes 10nm as detailed in step 4 2 of the protocol works well for photonic crystal waveguides of lengths 30 100um with relatively high group indices up to n gt 100 for engineered slow light waveguides see Figure 12 For band edge slow light the maximum measurable group indices tend to be lower for the same length due to the higher propagation losses With a fringe spa
19. is close to that of a Lorentzian as shown in the representative result section Representative 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 50x50um squares each with a differing base dose are used to determine the correct dose factor and development time for subsequent films When creating high resolution features using electron beam lithography it is beneficial to use as thin a film of resist as possible when etching the features into silicon however it is advisable to use the thickest possible film Balancing these opposing conditions is the goal of optimising the fabrication recipe we have found that a layer of ZEP 520A approximately 350nm thick gives the required high resolution while still withstanding the etching process Figure 2 shows an SEM image of a second sample which has been exposed developed and etched using the reactive ion etching RIE system the sample shows vertical sidewalls in each photonic crystal hole and no widening of the holes at either surface of the silicon Figure 3 is of a completed membrane photonic crystal device the hydrofluoric etch is somewhat isotropic in that it etches the
20. mful by inhalation and contact with skin and eyes should be avoided onto the sample use enough resist to completely cover the sample without the resist flowing over the edge Spin the sample so as to give an approx 350nm thick film and bake on a hotplate at 180 C for 10min We found this thickness to be the optimal thickness that balances resolution and etch resistance see later 2 Pattern Definition 2 1 2 2 2 3 2 4 Design using appropriate software simulate the required photonic crystal pattern A number of useful software packages are available including but not limited to MIT Photonic Bands MPB FullWAVE RSoft MIT Electromagnetic Equation Propagation MEEP Pattern Generation create the exposure files gds format in general and proximity error correct using appropriate software Pattern Exposure load the sample into the chamber of the electron beam lithography system LEO 1530 Raith Elphy and pump down Once vacuum has been achieved switch on the EHT supply and set to 30kV Leave the system in this state for 1hr to allow the sample stage and chamber to reach an equilibrium temperature Set up the exposure as indicated in the user manual of your specific electron beam lithography system Expose the sample using an appropriate basic step size e g 2nm this being the minimum pixel size that the system can expose a settling time of at least 1ms this being the time the system waits bet
21. n resonance light couples to the cavity the polarisation rotates for the fundamental cavity mode red arrow in Figure 18b and creates a y polarisation component This light is directed to the output arm and passes the analyser Again the y polarised light can convert to the opposite polarisation thus reducing the signal level Therefore an objective needs to be chosen so that polarisation intermixing is kept at a minimum For ultra high Q factor cavities such as a hetero structure cavity the emitted power is lower This situation can further reduce the SNR and the peak vanishes in the noise level A lock in configuration should then be used to lower the noise level not the background level in order to recover the peak Note that our setup Figure 14 is designed for multi functional cavity characterisation and in addition to RS includes micro photoluminescence and the generation of second and third harmonic frequencies Acknowledgments 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 Disclosures The authors have nothing to disclose Reagents Catalogue A Name Company Comments optional number latch Fisher A 0520 17 CAUTION flammable use good ventilation and avoid Scientific all ignition s
22. of both etch pressure and time this micrograph indicates that the etch mask has started to break down causing striations in the photonic crystal holes Each of these effects if not corrected manifests itself as a higher propagation loss in the final device propagation losses may arise from both a high density of scattering centres especially observed in devices with high side wall roughness and from a break in symmetry of the photonic crystal structure as seen in non verticality and hole widening Measured slow light group index curves A typical example interferogram measured from a blank waveguide is shown in Figure 10a The raw measured data is shown in grey and is affected by strong Fabry Perot fringes that result from the high reflectivity at the facets of the waveguide For clarity we have numerically filtered out the Fabry Perot fringes as shown in the black curve The fringes resulting from the interference of the sample and reference arms of the MZI setup are clearly visible and they are uniformly distributed over the entire wavelength range The interferogram from an 80um long engineered slow light photonic crystal waveguide on the same chip is shown in Figure 10b the fringes become denser at wavelengths higher than 1575nm marking the transition from the fast to the slow light regime Note that an increase of the group index corresponding to an increase of the sample arm optical length will always result in a monotonic reducti
23. oi 10 1063 1 2137898 2005 Galli M Portalupi S L Belotti M Andreani L C O Faolain L amp Krauss T F Light scattering and Fano resonances in high Q photonic crystal nanocavities Appl Phys Lett 94 7 71101 doi 10 1063 1 3080683 2009 W est R Strasser P Jungo M Robin F Erni D amp Jackel H An efficient proximity effect correction method for electron beam patterning of photonic crystal devices Microelectron Eng 67 68 182 188 doi 10 1016 S0167 9317 03 00070 4 2003 Tanaka Y Asano T Akahane Y Song B S amp Noda S Theoretical investigation of a two dimensional photonic crystal slab with truncated cone air holes Appl Phys Lett 82 11 1661 doi 10 1063 1 1559947 2003 Asano T Song B S amp Noda S Analysis of the experimental Q factors 1 million of photonic crystal nanocavities Opt Express 14 5 1996 2002 doil0 1364 OE 14 001996 2006 O Faolain L Schulz S A et al Loss engineered slow light waveguides Opt Express 18 26 27627 27638 doi 10 1364 OE 18 027627 2010 Joannopoulos J D Johnson S G Winn J N amp Meade R D Photonic crystals molding the flow of light Princeton University Press 2nd ed 2008 Li J White T P O Faolain L Gomez Iglesias A amp Krauss T F Systematic design of flat band slow light in photonic crystal waveguides Opt Express 16 9 6227 6232 doi 10 1364 OE 16 006227 2008 Takeda M
24. on of the fringe spacing as we have deliberately set the delay stage to make sure the reference arm is the shortest The corresponding group index curve is shown in Figure 11 blue curve from a value of around 5 in the fast light regime it increases to around 46 where it remains constant over a bandwidth of 6nm The group index curve shown here has been smoothened from the Fabry Perot noise by performing a running average on the phase term just before differentiation Note from Figure 10b that past the cutoff where the photonic crystal does not transmit light there are no fringes and therefore any resulting group index data at these wavelengths is a measurement artefact The transmission curve calculated as the ratio between the transmission of the photonic crystal waveguide and the blank waveguide is also shown as the black line in Figure 11 with a sharp cutoff clearly visible around 1594nm Figures 12 and 13 illustrate the capability of our measuring technique Figure 12 shows measured group indices in excess of 100 for an 80um long waveguide and Figure 13 shows a measured group index of almost 90 for a 300um long waveguide These waveguides were fabricated on the same chip as the waveguides of Figures 10 11 The pronounced dips appearing in the transmission curve when approaching the mode cutoff are believed to the signature of multiple scattering Cavities In order to obtain the resonant wavelength and the Q factor the line shap
25. on 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 characterisation of transmission and group index spectra Various methods have been reported most notably resolving the Fabry Perot fringes of the transmission spectrum and interferometric techniques oe Here we describe a direct broadband measurement technique combining spectral interferometry with Fourier transform analysis 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 sources or external waveguides directly coupled to the cavity 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 tec
26. ources Isopropanol Fisher i P 7500 15 CAUTION flammable use good ventilation and avoid Scientific all ignition sources Electron Beam Marubeni CAUTION flammable harmful by inhalation avoid ZEP520A resist Europe plc contact with skin and eyes Fisher CAUTION flammable and highly toxic use good Xylene Oe X 0100 17 ventilation avoid all ignition sources avoid contact with Scientific skin and eyes Microposit S1818 Chestech Ltd 10277866 CAUTION flammable and causes irritation to eyes G2 nose and respiratory tract Microposit CAUTION alkaline liquid and can cause irritation to Developer MF 319 EASE EPR eyes nose and respiratory tract Fisher CAUTION extremely corrosive readily destroys tissue Hydrofluoric Acid be ae 22333 5000 handle with full personal protective equipment rated for Scientific HF Microposit 1165 Chestech Ltd 110058734 CAUTION flammable and causes irritation to eyes IRemover nose and respiratory tract Fisher CAUTION corrosive and very toxic handle with Sulphuric Acid Scientific S 9120 PB17 personal protective equipment and avoid inhalation of vapours or mists _ Fisher CAUTION very hazardous in case of skin and eye Pye EON Scientific Bree contact handle with personal protective equipment Equipment Name Company Catalogue number Comments optional Silicon on Insulator Soitec G8P 110 01 wafer Diamond Scribe J amp M Diamond Tool Inc HS 415 Microscope slides Fisher Scientific FB58622 Beaker
27. recombine Connect one of the outputs to an infrared detector and use the reading of the detector to maximise the light coupling into the sample connect the other output to an optical spectrum analyser OSA The two arms of the MZI should have approximately the same optical length when in the presence of the sample make sure that the fibres in the two arms of the MZI have the same nominal length and include a tunable delay stage in the reference arm to allow for fine adjustment of its length In the sample arm mount the aspheric lenses onto xyz precision stages to obtain the best coupling into the sample Adjust reference arm length couple the light beam to a blank i e without photonic crystal ridge waveguide of the same type as the access waveguides that feed light inside the photonic crystals within the same chip in the sample arm Run a continuous scan on the OSA and observe the measured wavelength spectra If the two arms of the MZI have approximately the same optical length the spectra exhibit fringes due to constructive and destructive interference these fringes will not appear if the arms of the MZI have very different optical lengths gt cm The fringe spacing is inversely proportional to the difference in optical path length between the two arms Move the delay stage to make the reference arm shorter and observe the fringes in the OSA if they become denser sparser the reference arm is shorter longer than the sample arm Set
28. s coupled resonators and photonic crystals a comparison IEEE Photon J 2 2 181 194 doi 10 1109 JPHOT 2010 2044989 2010 Ishikura N Baba T Kuramochi E amp Notomi M Large tunable fractional delay of slow light pulse and its application to fast optical correlator Opt Express 19 24 24102 24108 doi 10 1364 OE 19 024102 2011 Beggs D M Rey I H Kampfrath T Rotenberg N Kuipers L amp Krauss T F Ultrafast tunable optical delay line based on indirect photonic transitions Phys Rev Lett 108 21 213901 doi 10 1103 PhysRevLett 108 213901 2012 Beggs D M White T P O Faolain L amp Krauss T F Ultracompact and low power optical switch based on silicon photonic crystals Opt Lett 33 2 147 149 doi 10 1364 OL 33 000147 2008 10 11 12 13 14 15 16 17 18 19 20 21 Nguyen H C Sakai Y Shinkawa M Ishikura N amp Baba T 10Gb s operation of photonic crystal silicon optical modulators Opt Express 19 14 13000 13007 doi 10 1364 OE 19 013000 2011 Kampfrath T Beggs D M White T P Melloni A Krauss T F amp Kuipers L Ultrafast adiabatic manipulation of slow light in a photonic crystal Phys Rev A 81 4 043837 doi 10 1103 PhysRevA 81 043837 2010 Monat C Corcoran B et al Slow light enhancement of nonlinear effects in silicon engineered photonic crystal waveguides Opt Express 17 4 2944 2953 do
29. s Fisher Scientific FB33109 Tweezers SPI Supplies PT006 AB Ultrasonic Bath Camlab 1161436 Spin Coater aoe Micro Systems EMS 4000 Pipette Fisher Scientific FB55343 E beam Lithography Raith Gmbh Raith 150 System Reactive Ion Etching Proprietary In house I System Designed UV Mask Aligner Karl Suss MJB 3 ASE source Amonics ALS CL 15 B FA e A radiation Single mode fibres Thorlabs P1 SMF28E FC 2 C WD AL 50 H 2210 35 3dB fibre splitters Thorlabs FC FC Aspheric lenses New Focus 5720 C XYZ stages Melles Griot 17AMB003 MD Polarising beamsplitter Thorlabs PBS104 cube IR detector New Focus 2033 100xObjective Nikon BD Plan 100x Oscilloscope Tektronix TDS1001B ppucal Specta Advantest Q8384 Analyser IR sensor card Newport F IRC2 TLS source Agilent 81940A Se ON aes radiation IR Camera Electrophysics 7290A IR Detector New Focus 2153 Digital Multimeter Agilent 34401A Illumination Stocker Yale Lite Mite IMonochromator Spectral Products DK480 Array Detector Andor DU490A 1 7 GIF Fibre Thorlabs 31L02 References Baba T Kawasaki T Sasaki H Adachi J amp Mori D Large delay bandwidth product and tuning of slow light pulse in photonic crystal coupled waveguide Opt Express 16 12 9245 9253 doi 10 1364 OE 16 009245 2008 Melloni A Canciamilla A et al Tunable delay lines in silicon photonic
30. silica layer nearly equally in every direction A relatively tight control of the under etch must therefore be maintained or too long an etch will cause the photonic crystal membranes to collapse In Figure 4 we present a close up image of an optimised photonic crystal etch the vertical sidewalls the lack of striations and edge wall roughness are clear to see This etch was performed using the following parameters etch pressure 5 x 10 mBar etch time 1min 40s RF power 19W producing a DC bias of 210V and 50 50 gas ratio of SFe and CHF3 Increasing the RIE etching pressure above optimum i e to 5 9 x 10 mBar an increase of 18 introduces an angle in the photonic crystal wall as can be seen in Figure 5 This effect becomes more prevalent as the pressure is increased further On the other hand increasing the RF power which results in a larger DC bias i e RF power of 22W resulting in a DC bias of 232V increases of 15 and 10 respectively causes a faster break down of the etch mask producing a widening of the top of the photonic crystal holes as can be seen in Figure 6 Figure 7 shows the result of over etching a photonic crystal the longer etch time i e 2min 20s 40 increment allowing the resist to completely break down resulting in the widening of the photonic crystal holes creating both of the above effects i e photonic crystal hole widening and angled sidewalls Finally in Figure 8 we see the results of poor optimisation
31. t G Rivoire K Lu J Hatami F amp Vu kovi J Tunable wavelength second harmonic generation from GaP photonic crystal cavities coupled to fiber tapers Opt Express 18 12 12176 12184 doi 10 1364 OE 18 012176 2010 Fan S Villeneuve P R Joannopoulos J D amp Haus H A Channel drop filters in photonic crystals Opt Express 3 1 4 11 doi 10 1364 OE 3 000004 1998 Tanabe T Nishiguchi K Kuramochi E amp Notomi M Low power and fast electro optic silicon modulator with lateral p i n embedded photonic crystal nanocavity Opt Express 17 25 22505 22513 doi 10 1364 OE 17 022505 2009 Nozaki K Tanabe T et al Sub femtojoule all optical switching using a photonic crystal nanocavity Nature Photon 4 477 483 doi 10 1038 nphoton 2010 89 2010 Notomi M Yamada K Shinya A Takahashi J Takahashi C amp Yokohama I Extremely large group velocity dispersion of line defect waveguides in photonic crystal slabs Phys Rev Lett 87 25 253902 doi 10 1103 PhysRevLett 87 253902 2001 Labilloy D Benisty H Weisbuch C Smith C J M Krauss T F Houdr R amp Oesterle U Finely resolved transmission spectra and band structure of two dimensional photonic crystals using emission from InAs quantum dots Phys Rev B 59 3 1649 1652 doi 10 1103 PhysRevB 59 1649 1999 22 23 24 25 26 2T 28 29 30 31 32 33 34 35 36 Inang
32. tching load the sample into the RIE main chamber and pump the system down to a background pressure of lt 3x10 mBar to ensure the chamber is free of water vapour Begin the etch by pre conditioning the chamber with the etching gasses namely CHF and SF set the flow rate of both gasses to 100sccm i e set a gas ratio of 1 1 and using the throttle bring the chamber pressure to 5x10 mBar allow the gasses to flow for at least 10min After pre conditioning set the RF power to approximately 20W and ignite a plasma etch the sample for approximately 2min the etch rate of silicon for these etch parameters is approximately 150nm min while ensuring that a chamber pressure of 5x10 mBar is maintained A DC bias between 200 220V should be achieved throughout the etching period Sample Cleaning to remove remaining electron sensitive resist after dry etching clean the sample by rinsing in CAUTION 1165 Remover 1165 is flammable and can cause irritation to eyes nose and respiratory tract with ultrasonic agitation for 1 2min followed by acetone and isopropanol as outlined above step 1 2 Membrane Isolation spin coat the sample with UV sensitive photo resist CAUTION Microposit 1818 G2 S1818 G2 is both flammable and causes irritation to eyes nose and respiratory tract see step 1 3 Using an appropriate photomask define windows within the resist above the photonic crystal patterns using the UV mask aligner Expose the sample for appro
33. term sqrt SR exp i wr and its complex conjugate correspond to peaks centred at t t and t t respectively Filter numerically one of the two terms and transform back to the frequency domain Differentiate the phase q wr of the resulting data with respect to to obtain Az the difference in group delay between the two arms The group index n c V with v the group velocity is given by Ng Ga z Ate clL Neal where Ar is obtained from the calibration data taken from the blank waveguide L is the photonic crystal waveguide length and Mncar 2 7 is the effective index of the reference ridge waveguide The contribution to the delay from the various optical elements of the setup is taken into account in the calibration run and is therefore subtracted in this step Transmission curve calculate the transmission curve by normalising the sample spectrum of a photonic crystal waveguide to that of the blank waveguide 5 Photonic Crystal Cavity Characterisation 5 1 Setup the preparation of the setup Figure 14 for RS includes switching of the exchangeable element to the polarising beam splitter inserting a polariser in the input arm as well as an analyser in the output arm flip a mirror into the probe arm to allow the use of a near infrared source allow the illumination of the sample Mount the sample vertically with a 45 orientation to axis of the polarizer Figure 18 on a differential driven xyz micro block and a
34. tre is obtained by illuminating the cavity with a light source yellow and visualising the cavity with the camera The signal form the output arm red can then be directed to a free space spectrometer or a fibre coupled detector Figure 15 Captured image as appears on screen with beam off left and on right The beam is clearly aligned with the centre of the cavity a 1540 1550 1560 1570 1540 1550 1560 1570 b Wavelength i nm Figure 16 Initial broadband scan to identify the cavity resonances In both the ASE a and the TLS b scans a strong resonance is visible above 1560nm Voltage V o a 22 4 1962156 A gy te a 1562156 b 20 Q 41382 dj ye 43639 A b 18 q 0 0891 Ji q 1 845 i 16 fi 14 4 zo f 4 1 i J a x Ri 12 atte ij 10 BR i osd pon i J i A arati os 06 19620 1562 1 1562 2 15620 1582 1 1562 2 Voltage V ore 4 1562 164 Q 43855 ae a 0 0003 2 1562 161 i d Q 32052 5 Wavelength 2 nm Figure 17 High resolution scans with the TLS a Lorentzian line shape high SNR b Near Lorentzian line shape with high SNR c Lorentzian line shape with low SNR d Asymmetric Fano line shape with low SNR The dashed lines indicate the resonance wavelength Detector Sample e gt x Figure 18 Arrangement of polarising optics a to the sample b from Reference 29 The polariser orientates the polarisation in
35. ween moving the beam and exposing the particular portion of the pattern and an area dose of 55uAcm Sample Development using CAUTION Xylene Xylene is both flammable and highly toxic work in a well ventilated area away from ignition sources and avoid contact with skin and eyes at a temperature of 23 C develop the sample for 45s Rinse in isopropanol 3 Patten Transfer 3 1 3 2 3 3 3 4 RIE Chamber Cleaning Set the flow rates of argon and hydrogen to 200sccm Throttle down the pump via a butterfly valve to achieve chamber pressure of 1x10 mBar Set the RF power to 100W ignite the plasma and run for at least 10min a DC bias of approximately 700V should be observed After switching off the Ar H2 plasma allow the chamber to pump for approximately Imin Set the flow rate of oxygen into the chamber to 200sccm and again throttle the chamber pressure down to 1x10 mBar Ignite a second plasma of oxygen with a power of 100W and run for Smin After these procedures the chamber will be free of contaminants such as polymer residues from any previous dry etch We perform this procedure before every change in etch recipe to ensure maximum repeatability This procedure is optimised for our system which consists of a parallel plate cathode loaded RIE with a main chamber 12 inches in diameter by 14 inches in height including a 12 inch port with both throttling valve and turbo molecular pump attached Photonic Crystal E
36. x direction and the beam splitter only reflects y polarised light with the analyser further increasing the SNR of y to x polarised light at the output arm Reprinted with permission from Appl Phys Lett 94 071101 Copyright 2009 American Institute of Physics Discussion Sample fabrication Our choice of electron beam resist i e ZEP 520A is due to its simultaneously high resolution and etch resistance Finally 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 the electron beam lithography system to settle for at least an hour after loading reduces mis alignment errors during writing this is due to the sample stage and vacuum chamber not being at the same temperature immediately after loading As photonic crystal patterns along with access waveguides may take several hours to write a small drift in the stage relative to the chamber even at only nanometres per photonic crystal hole results in significant stitching and possibly pattern distortion errors with respect to photonic crystal tolerances This error is random in nature from one exposure to another but can be as high as 100 nm min absolute positional error however relative positional error i e
37. ximately 30 45s Develop the resist in CAUTION Microposit Developer MF 319 MF 319 is an alkaline liquid and can cause irritation to eyes nose and respiratory tract for 30 45s rinsing afterwards in de ionised water Prepare a plastic beaker with a mixture of CAUTION 1 5 Hydrofluoric acid 1 1499 mL 48 51 HF HF is extremely corrosive and readily destroys tissue when handling use full personal protective equipment rated for HF to de ionised water Note that for safety reasons only plastic beakers and tweezers should be used with Hydrofluoric acid Submerge the sample in the Hydrofluoric acid mixture for 15min After etching rinse the sample thoroughly in de ionised water Remove the remaining photo resist using acetone and isopropanol see step1 2 from this stage and onwards ultrasonic agitation cannot be used To ensure the sample is as clean as possible follow the acetone and isopropanol wash with a rinse in CAUTION Piranha solution 3 5 Piranha solution is very energetic potentially explosive and attacks organic materials when handling use full personal protective equipment 3 1 CAUTION sulphuric acid sulphuric acid is corrosive and very toxic when handling use personal protective equipment and avoid inhalation of vapours or mists to CAUTION hydrogen peroxide hydrogen peroxide is very hazardous in case of skin and eye contact when handling use personal protective equipment for 5min then rinse the sample in de ionised water
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