2 and 3D Finite Difference Time Domain (FDTD) simulations and measurements of a hexagonal lattice 2D photonic crystal with holes having taper angles in the region of 3 to 7 degrees are performed. The results show a smoothing of band edges and increased losses. Reasonably good agreement between measured and modelled results is obtained2 and 3D Finite Difference Time Domain (FDTD) simulations and measurements of a hexagonal lattice 2D photonic crystal with holes having taper angles in the region of 3 to 7 degrees are performed. The results show a smoothing of band edges and increased losses. Reasonably good agreement between measured and modelled results is obtaine

Craddock, IJ

Cryan, MJ

M., Hill

M., Ronda

R., Varrazza

Railton, CJ

Rorison, JM

Yu, S

English

Explore Bristol Research

Cryan, MJ., M., H., R., V., M., R., Craddock, IJ., Yu, S., Railton, CJ., &Rorison, JM. (2003). Modelling and measurement of 2D photoniccrystals with tapered hole profiles. In 2003 International Conferenceon Transparent Optical Networks - ICTON 2003 (Vol. 1, pp. 344 -347). Institute of Electrical and Electronics Engineers (IEEE).https://doi.org/10.1109/ICTON.2003.1264651Peer reviewed versionLink to published version (if available):10.1109/ICTON.2003.1264651Link to publication record on the Bristol Research PortalPDF-documentUniversity of Bristol – Bristol Research PortalGeneral rightsThis document is made available in accordance with publisher policies. Please cite only thepublished version using the reference above. Full terms of use are available:http://www.bristol.ac.uk/red/research-policy/pure/user-guides/brp-terms/Tu.P.23 344 ICTON 2003 Modelling and Measurement of 2D Photonic Crystals with Tapered Hole Profiles M.J. Cryan, M. Hill, R. Varrazza, M. Mor0 Ronda, l.J.C Raddock, S. Yu, C.J. Railton and J. Rorison Centre for Communications Research, Deportment of Electronic and Electrical Engineering, University oJBristo1, Bristol, BS8 I UB. UK, email : m.cryan@bristol.ac.uk ABSTRACT 2 and 3 0  Finite Difference Time Domain (FDTD) simulations and measurements of o hexagonal lottice 2D photonic crystal with holes having taper angles in the region of 3 la 7 degrees are performed. The results show o smoothing of band edges and increosed losses. Reasonably good agreement between measured and modelled results is obtained. Keywords: Photonic Crystals, 3D-FDTD 1. INTRODUCTION Two dimensional Photonic Crystals (PCs) can be fabricated by a number of methods [ I ] .  These include Chemically Assisted Ion Beam Etching (CAIBE), Electron-Cyclotron Resonance-Reactive Ion Etching (ECR- RIE) and Inductively Coupled Plasma (ICP) Etching. Recently another approach has been investigated which uses Focused Ion Beam Etching (FIBE) [2]. This is a rapid, maskless etching technique which allows post processing of semiconductor devices [3]. Whilst it has the advantage of very fast turnaround times. the hole verticality is less good than other approaches with typical hole angles in InP in the range of three to seven degrees. Currently more advanced methods involving FIBE are being investigated to overcome this problem In order to characterise currently fabricated devices it is important to understand the effects of hole angle on PC based devices. In fact angled holes may prove useful in applications such TErTM mode conversion. Also as PC ~ based devices begin to approach real applications it is important to understand the impact of fabrication induced effects such as random hole position, radius and hole taper angle. To this end 3D FDTD modelling and measurements are used to study the effect of hole taper angle. 2. MODELLING FDTD is a fullwave method which allows arbitrary combinations of high index contrast dielectrics and as such is ideally suited to the study of PC based devices. The FDTD code used in this work is a mature in-house code which has been used to study losses in PC waveguides [4]. The structure to be studied is shown in plan view in figure I .  It consists of a hexagonal lattice of air holes etched into a 3 layer slab guide. The PC has a lattice constant of 3640111 and a radius to lattice constant ratio (da) at the core of the waveguide of 0.22. Propagation is in the gamma-M direction. The waveguide consists of an upper cladding of 400nm, refractive index=3.17, acentral core of 2 4 0 ~  refractive index=3.46 and a lower cladding layer of IOOGnm, refractive index=3.17. Input and output ridge waveguides are used to couple power into the structure. Two different ridge widths are used, fmtly a narrow width for coupling to PC waveguides and secondly a wider ridge for modelling purposes. It should be noted that since the structure is symmetrical and being excited symmetrically only half the structure need be simulated and an electric wall can be placed at the centre of the structure when TE excitation is being used. Initially a quasi-2D version of the 3D code was used to study the structure - in order to decrease runtimes [SI. Translight [6], a Transfer Matrix Method code was used to validate these initial simulations, the results are shown in figure 3. Figure 1 Plon view of Photonic Crystol structure, narrow ridge guides. Figure 2. Plon view of Photonic Crystalstructure. wide ridge guides. Figure 3 shows quite good agreement in the case of wide ridge guide and less good with the narrow guide. This is as expected since Translight assumes an infinite plane wave to be incident on the structure. The MITOW guide results show that increased losses result, here most likely due to dimaction from the m o w  guide, however, for PC waveguide applications MITOW guides are required for good coupling. 0-7803-7816-4/03/$17.00 02003 IEEE Authorized licensed use limited to: UNIVERSITY OF BRISTOL. Downloaded on January 12, 2009 at 10:29 from IEEE Xplore.  Restrictions apply.ICTON 2003 345 Tu.P.23 Figure 3. Transmission coefficient for structures in$gures I and 2 compared m'fh translighf, r/a 4 2 2 .  Having validated the 2D code, the 3D structure can now be studied. Here the structurc is excited with the fbndamental TE mode of the input waveguide, derived from a 2D FDTD mode solvcr. Figure 4 shows a vertical cross section through the Structure with tapered boles and figure 5 shows an individual hole showing how its constructured from a set of cylinders. A number of simulations are performed, fitly, vertical holes with no taper and tapered boles with angles of 3 and 7 degrees. Figure 4. Vertical crass section through PCshrchrre with tapered holes. Figure 5. 3 0  view of isolated hole. I dom i m m  t m m  ( m m  i s n o 0  iamm iaDm 17mm tmm l e "  W"(MII Figure 6. Compariron of transmission coefficient for 2 0  FDTD and 3 0  FDTD with vertical and fapered holes Figure 6 shows the simulated results for the 3D structure, with the 2D results added for comparison. The 3D vertical holes results have an r/a value of 0.22 in order to compare with the 2D results. The results show that in going to f ~ t e  depth, vertical boles, losses are increased as might be expected. When tapered boles are introduced a very smooth band edge around 155Onm results with again higher losses. Authorized licensed use limited to: UNIVERSITY OF BRISTOL. Downloaded on January 12, 2009 at 10:29 from IEEE Xplore.  Restrictions apply.Tu.P.23 346 ICTON 2003 3. MEASUREMENTS Figure 7 shows the structure that has been fabricated using FIBE [2]. The PC structure has been etched into a 1 1 9  slab waveguide with dimensions estimated to be equal to those given in the modelling section above. Figure 7. Fabricated 2 0  Photonic Crystal shucfure. The inset in figure 7 shows how the bulk material has been etched away such that the holes are not required to be as deep The tratlsmission rcspoll~e of the struclure has been measured using the set up shown in figure 8. Power is coupled into the structure h m  a fibre lense. Power Polariser Mirr I Objective Meter D.U.T. Variable Aperture / Lensed Fibre Holder Variable Mirr ’ \  / Rotation Stage xyz Stages Sample holder Tunable Laser Source Figure 8. Trammission measurement set-up Authorized licensed use limited to: UNIVERSITY OF BRISTOL. Downloaded on January 12, 2009 at 10:29 from IEEE Xplore.  Restrictions apply.ICTON 2003 347 Tu.P.23 U wavelemgh /“In Figure 9.Measured transmission data for structure shown in jigure 7 .  Two seperate measurements of the same device are shown. .” i x a w  w o m  iw.m w w ~  1m.m im.w W.nh.”lm) Figure 10. Zoom offigure 6 for comparison with measured data. Figure 9 shows the measured transmission data, a slope in the order of 3dB is observed. The compares reasonably well with a simulated slope for the 7 degree angled hole of 4dB. The ripples occurring in the measured data appear to be due to a Fabry-Perot effect and it is interesting to note that the ripple period is changing substantially across the band, this could be due to the highly dispersive nature of the PC section. 3D simulations will be carried out coupling power from air into the smcture to further study these Fabry-Perot effects. The relative levels of the signals are somewhat different this is due to the fact thai the 3D simulation are excited from a narrow ridge guide, whereas the measurements use a slabguide. Work is now under way to use FIB etching to defme input and ouput waveguides enabling improved comparison, also 3D simulation with wide ridge guides are being carried out. 4. CONCLUSIONS This paper has used both 2D and 3D FDTD and measurements to study the effects of hole tapering in Photonic Crystal strcutures. It has been shown that the tapering has the effect of smoothing out the band edges and increasing the losses outside the band gap, also evidence for strong dipersive behaviour has been observed. A number of further developments are being carried out including the use of deep etched ridge guides for coupling and advanced FIB based processing for decreasing hole tapering. Also a study of the T W M  mode conversion properties of the tapered hole PCs is being carried out. ACKNOWLEDGEMENTS The authors would like to acknowledge the PHOTON consortium funded by UK EPSRC for partial support of the work. RE FE RE N C E S [ I ]  R.Houdre et al, “Optical study of bi-dimensional photonic crystals structures fabricated by inductively coupled plasma etching in InP-based waveguides”, Photonic and Electromagnetic Crystal Structures, PECS- IV, UCLA, Los Angeles, USA, October 2002 [2] M. Hill et al. “Focused Ion Beam Fabrication of Photonic Crystal Structures”, CLEO-Europe, Munich 2003. [3] LSargent et al. “Investigation of 2-D-Lattice Distributed Reflector Lasers”, IEEE Journal of Quantum Electronic, Vo1.38, Nov.2002 141 M.J.Cryan et al, “Analysis of Losses in 2D Photonic Crystal Waveguides Using the 3D Finite Difference Time Domain (FDTD) Method”, IEEE LEOS Nov. 200 [5] M.J.Cryan et al, “Design and Simulation of a Photonic Crystal Waveguide Filter Using the FDTD Method”, IEEE LEOS Nov. 2002 [6] Translight, A.Reynolds, University of Glasgow. . Authorized licensed use limited to: UNIVERSITY OF BRISTOL. Downloaded on January 12, 2009 at 10:29 from IEEE Xplore.  Restrictions apply.

Modelling and measurement of 2D photonic crystals with tapered hole profiles

                          Cryan, M. J., M., H., R., V., M., R., Craddock, I. J., Yu, S., ... Rorison, J. M.(2003). Modelling and measurement of 2D photonic crystals with taperedhole profiles. In 2003 International Conference on Transparent OpticalNetworks - ICTON 2003. (Vol. 1, pp. 344 - 347). Institute of Electrical andElectronics Engineers (IEEE). 10.1109/ICTON.2003.1264651Link to published version (if available):10.1109/ICTON.2003.1264651Link to publication record in Explore Bristol ResearchPDF-documentUniversity of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-terms.htmlTake down policyExplore Bristol Research is a digital archive and the intention is that deposited content should not beremoved. However, if you believe that this version of the work breaches copyright law please contactopen-access@bristol.ac.uk and include the following information in your message:• Your contact details• Bibliographic details for the item, including a URL• An outline of the nature of the complaintOn receipt of your message the Open Access Team will immediately investigate your claim, make aninitial judgement of the validity of the claim and, where appropriate, withdraw the item in questionfrom public view.Tu.P.23 344 ICTON 2003 Modelling and Measurement of 2D Photonic Crystals with Tapered Hole Profiles M.J. Cryan, M. Hill, R. Varrazza, M. Mor0 Ronda, l.J.C Raddock, S. Yu, C.J. Railton and J. Rorison Centre for Communications Research, Deportment of Electronic and Electrical Engineering, University oJBristo1, Bristol, BS8 I UB. UK, email : m.cryan@bristol.ac.uk ABSTRACT 2 and 3 0  Finite Difference Time Domain (FDTD) simulations and measurements of o hexagonal lottice 2D photonic crystal with holes having taper angles in the region of 3 la 7 degrees are performed. The results show o smoothing of band edges and increosed losses. Reasonably good agreement between measured and modelled results is obtained. Keywords: Photonic Crystals, 3D-FDTD 1. INTRODUCTION Two dimensional Photonic Crystals (PCs) can be fabricated by a number of methods [ I ] .  These include Chemically Assisted Ion Beam Etching (CAIBE), Electron-Cyclotron Resonance-Reactive Ion Etching (ECR- RIE) and Inductively Coupled Plasma (ICP) Etching. Recently another approach has been investigated which uses Focused Ion Beam Etching (FIBE) [2]. This is a rapid, maskless etching technique which allows post processing of semiconductor devices [3]. Whilst it has the advantage of very fast turnaround times. the hole verticality is less good than other approaches with typical hole angles in InP in the range of three to seven degrees. Currently more advanced methods involving FIBE are being investigated to overcome this problem In order to characterise currently fabricated devices it is important to understand the effects of hole angle on PC based devices. In fact angled holes may prove useful in applications such TErTM mode conversion. Also as PC ~ based devices begin to approach real applications it is important to understand the impact of fabrication induced effects such as random hole position, radius and hole taper angle. To this end 3D FDTD modelling and measurements are used to study the effect of hole taper angle. 2. MODELLING FDTD is a fullwave method which allows arbitrary combinations of high index contrast dielectrics and as such is ideally suited to the study of PC based devices. The FDTD code used in this work is a mature in-house code which has been used to study losses in PC waveguides [4]. The structure to be studied is shown in plan view in figure I .  It consists of a hexagonal lattice of air holes etched into a 3 layer slab guide. The PC has a lattice constant of 3640111 and a radius to lattice constant ratio (da) at the core of the waveguide of 0.22. Propagation is in the gamma-M direction. The waveguide consists of an upper cladding of 400nm, refractive index=3.17, acentral core of 2 4 0 ~  refractive index=3.46 and a lower cladding layer of IOOGnm, refractive index=3.17. Input and output ridge waveguides are used to couple power into the structure. Two different ridge widths are used, fmtly a narrow width for coupling to PC waveguides and secondly a wider ridge for modelling purposes. It should be noted that since the structure is symmetrical and being excited symmetrically only half the structure need be simulated and an electric wall can be placed at the centre of the structure when TE excitation is being used. Initially a quasi-2D version of the 3D code was used to study the structure - in order to decrease runtimes [SI. Translight [6], a Transfer Matrix Method code was used to validate these initial simulations, the results are shown in figure 3. Figure 1 Plon view of Photonic Crystol structure, narrow ridge guides. Figure 2. Plon view of Photonic Crystalstructure. wide ridge guides. Figure 3 shows quite good agreement in the case of wide ridge guide and less good with the narrow guide. This is as expected since Translight assumes an infinite plane wave to be incident on the structure. The MITOW guide results show that increased losses result, here most likely due to dimaction from the m o w  guide, however, for PC waveguide applications MITOW guides are required for good coupling. 0-7803-7816-4/03/$17.00 02003 IEEE Authorized licensed use limited to: UNIVERSITY OF BRISTOL. Downloaded on January 12, 2009 at 10:29 from IEEE Xplore.  Restrictions apply.ICTON 2003 345 Tu.P.23 Figure 3. Transmission coefficient for structures in$gures I and 2 compared m'fh translighf, r/a 4 2 2 .  Having validated the 2D code, the 3D structure can now be studied. Here the structurc is excited with the fbndamental TE mode of the input waveguide, derived from a 2D FDTD mode solvcr. Figure 4 shows a vertical cross section through the Structure with tapered boles and figure 5 shows an individual hole showing how its constructured from a set of cylinders. A number of simulations are performed, fitly, vertical holes with no taper and tapered boles with angles of 3 and 7 degrees. Figure 4. Vertical crass section through PCshrchrre with tapered holes. Figure 5. 3 0  view of isolated hole. I dom i m m  t m m  ( m m  i s n o 0  iamm iaDm 17mm tmm l e "  W"(MII Figure 6. Compariron of transmission coefficient for 2 0  FDTD and 3 0  FDTD with vertical and fapered holes Figure 6 shows the simulated results for the 3D structure, with the 2D results added for comparison. The 3D vertical holes results have an r/a value of 0.22 in order to compare with the 2D results. The results show that in going to f ~ t e  depth, vertical boles, losses are increased as might be expected. When tapered boles are introduced a very smooth band edge around 155Onm results with again higher losses. Authorized licensed use limited to: UNIVERSITY OF BRISTOL. Downloaded on January 12, 2009 at 10:29 from IEEE Xplore.  Restrictions apply.Tu.P.23 346 ICTON 2003 3. MEASUREMENTS Figure 7 shows the structure that has been fabricated using FIBE [2]. The PC structure has been etched into a 1 1 9  slab waveguide with dimensions estimated to be equal to those given in the modelling section above. Figure 7. Fabricated 2 0  Photonic Crystal shucfure. The inset in figure 7 shows how the bulk material has been etched away such that the holes are not required to be as deep The tratlsmission rcspoll~e of the struclure has been measured using the set up shown in figure 8. Power is coupled into the structure h m  a fibre lense. Power Polariser Mirr I Objective Meter D.U.T. Variable Aperture / Lensed Fibre Holder Variable Mirr ’ \  / Rotation Stage xyz Stages Sample holder Tunable Laser Source Figure 8. Trammission measurement set-up Authorized licensed use limited to: UNIVERSITY OF BRISTOL. Downloaded on January 12, 2009 at 10:29 from IEEE Xplore.  Restrictions apply.ICTON 2003 347 Tu.P.23 U wavelemgh /“In Figure 9.Measured transmission data for structure shown in jigure 7 .  Two seperate measurements of the same device are shown. .” i x a w  w o m  iw.m w w ~  1m.m im.w W.nh.”lm) Figure 10. Zoom offigure 6 for comparison with measured data. Figure 9 shows the measured transmission data, a slope in the order of 3dB is observed. The compares reasonably well with a simulated slope for the 7 degree angled hole of 4dB. The ripples occurring in the measured data appear to be due to a Fabry-Perot effect and it is interesting to note that the ripple period is changing substantially across the band, this could be due to the highly dispersive nature of the PC section. 3D simulations will be carried out coupling power from air into the smcture to further study these Fabry-Perot effects. The relative levels of the signals are somewhat different this is due to the fact thai the 3D simulation are excited from a narrow ridge guide, whereas the measurements use a slabguide. Work is now under way to use FIB etching to defme input and ouput waveguides enabling improved comparison, also 3D simulation with wide ridge guides are being carried out. 4. CONCLUSIONS This paper has used both 2D and 3D FDTD and measurements to study the effects of hole tapering in Photonic Crystal strcutures. It has been shown that the tapering has the effect of smoothing out the band edges and increasing the losses outside the band gap, also evidence for strong dipersive behaviour has been observed. A number of further developments are being carried out including the use of deep etched ridge guides for coupling and advanced FIB based processing for decreasing hole tapering. Also a study of the T W M  mode conversion properties of the tapered hole PCs is being carried out. ACKNOWLEDGEMENTS The authors would like to acknowledge the PHOTON consortium funded by UK EPSRC for partial support of the work. RE FE RE N C E S [ I ]  R.Houdre et al, “Optical study of bi-dimensional photonic crystals structures fabricated by inductively coupled plasma etching in InP-based waveguides”, Photonic and Electromagnetic Crystal Structures, PECS- IV, UCLA, Los Angeles, USA, October 2002 [2] M. Hill et al. “Focused Ion Beam Fabrication of Photonic Crystal Structures”, CLEO-Europe, Munich 2003. [3] LSargent et al. “Investigation of 2-D-Lattice Distributed Reflector Lasers”, IEEE Journal of Quantum Electronic, Vo1.38, Nov.2002 141 M.J.Cryan et al, “Analysis of Losses in 2D Photonic Crystal Waveguides Using the 3D Finite Difference Time Domain (FDTD) Method”, IEEE LEOS Nov. 200 [5] M.J.Cryan et al, “Design and Simulation of a Photonic Crystal Waveguide Filter Using the FDTD Method”, IEEE LEOS Nov. 2002 [6] Translight, A.Reynolds, University of Glasgow. . Authorized licensed use limited to: UNIVERSITY OF BRISTOL. Downloaded on January 12, 2009 at 10:29 from IEEE Xplore.  Restrictions apply.

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Modelling and measurement of 2D photonic crystals with tapered hole profiles

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