We present new results that demonstrate advances in the performance achievable in photonic crystal/photonic wire micro-cavities. In one example, a quality-factor value as high as 147,000 has been achieved experimentally at a useful transmission level

De La Rue, R.M.

Md Zain, A.R.

Sorel, M.

English

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Advancing the performance of one-dimensional

photonic crystal/photonic wire micro-cavities in

silicon-on-insulator

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       Md Zain, A.R. and Sorel, M. and De La Rue, R.M. (2008) Advancing the performance of one-dimensional photonic crystal/photonic wire micro-cavities in silicon-on-insulator. In: Integrated Photonics and Nanophotonics Research and Applications, 13-16 July 2008, Boston, Massachusset.  http://eprints.gla.ac.uk/7018/  Deposited on: 24 August 2009   Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk Advancing the Performance of One-DimensionalPhotonic Crystal/Photonic Wire Micro-cavities inSilicon-on-InsulatorAhmad Rifqi Md Zain, Marc Sorel and Richard M. De La Rue.Optoelectronics Research Group, Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8QQ,Scotland U.K.r.delarue@elec.gla.ac.ukAbstract: We present new results that demonstrate advances in the performance achievable inphotonic crystal/photonic wire micro-cavities.  In one example, a quality-factor value as high as147,000 has been achieved experimentally at a useful transmission level.OCIS codes: 130.0130, 130.3120, 250.0250, 060.4080, 060.42301. IntroductionThe combination of narrow (photonic wire) optical waveguides and one-dimensionally periodic photoniccrystal structures in high refractive index semiconductors such as silicon, in the form of silicon-on-insulator(SOI) and epitaxial structures based on the III-V semiconductors, has been a topic of interest for a numberof years [1-4].Recent work has led to demonstrations of quality-factor (Q-factor) values approaching 60,000 [2] and thecombination of a Q-factor value approaching 20,000 together with a normalised transmission coefficient ofnearly 90% [3]. Observations using various versions of scanning near-field optical microscopy (SNOM)have also verified that such structures can exhibit optical confinement in very small volumes, resulting inextremely large Purcell-factor values (5). Key factors in achieving such high values of Q-factor andnormalised transmission are the incorporation of hole diameter tapering, careful displacement of holeposition in transition sections (both within and outside the cavity) and careful adjustment of the length ofthe cavity spacer section, aided by numerical modelling.Fig. 1:Scanning electron micrograph of typical microcavity structure using 5-hole periodic Bragg mirrors and tapered hole inputand output structures, both within and outside the cavity.2. Device Design, Fabrication and MeasurementIn this presentation we shall describe new results for the combined photonic wire/photonic crystal micro-cavity structure that include achievement of resonance quality-factor (Q-factor) values of well over100,000, in combination with usefully large transmission factors. Substantially higher transmissioncoefficient values have been achieved, in conjunction with somewhat smaller (c. 80,000) Q-factor values.A scanning electron micrograph of a typical cavity is presented in Fig.1. It shows a photonic wiremicrocavity that has a wire width of 500 nm, three-hole tapers at the input and output of the cavity – andfour-hole tapers for the mirrors within the cavity. The periodic parts of the mirrors are formed by fiveholes. The hole diameters and spacings chosen for the structure shown in Fig. 1 are similar to those quotedin reference [3]. Although we have also looked at the use of extended cavities with longer spacer sections,the present design retains the use of a short cavity having a spacer section length that is somewhat less thanthe optical wavelength in the medium - and is the configuration that can maximise the Purcell factor of themicro-cavity. The value of the Q-factor achievable is strongly dependent on the length of the cavity spacesection. Operation is always in (quasi-) TE polarization, i.e. with the predominant electric-field vectorcomponent being in-plane.As described previously, the pattern generation for our devices was carried out by direct-write electron-beam lithography (EBL) in a Vistec VB6 machine. A one-stage dry-etch transfer process, with minimalwriting time, was used in conjunction with a layer of hydrogen silsesquioxane (HSQ) negative resist totransfer the written pattern, with high precision and small edge-roughness [6], into the silicon waveguidelayer.Fig. 2(a): Computed transmission spectrum for micro-cavity with5-hole mirrors plus tapersFig. 2(b): Measured spectrum for the same micro-cavity, withexpanded spectrum inset.In our recent work, we have found it convenient to carry out computational modelling of the properties ofdifferent micro-cavity designs using a high-resolution calculation based on a two-dimensionally varyingmodel of the refractive index distribution, together with an effective refractive index calculated for theplanar silicon-on-insulator waveguide. The results of calculations that last on the order of a working day fortheir execution on a personal computer are shown in Fig. 2(a) and yield an estimated Q-factor value of177,000, at a wavelength of 1483.54 nm. The corresponding measured transmission spectrum appears inFig. 2(b) – and yields an estimated value of 147,000, at a wavelength of 1479.705 nm. We believe that thefact that the computation involves the effective index approximation, rather than a fully three-dimensionalcalculation, and the uncertainty in the specification of the actual dimensions of the fabricated devicetogether imply a satisfactory level of agreement in the estimates of both Q-factor and resonancewavelength. We have addressed issues of reproducibility for photonic-wire based device structuresproduced using electron-beam lithography in reference [6], which indicates that careful attention to HSQresist cycle life-time can significantly increase the accuracy with which a target wavelength can beapproached. Achieving a specific wavelength with the accuracy implied by our ability to measure it, and bythe line-width of the resonance, seems likely to require the addition of a tuning mechanism, e.g. electro-optic [7] or thermal tuning [8,9].An important aspect of the structures that we have fabricated, as well as similar ones published by otherworkers, is that the silicon waveguide core is firmly anchored to the silicon substrate via the silica buffer(lower-cladding) layer. But we have also begun work on an air-bridge version of our micro-cavity devicestructure, using a suitably adjusted design. For the air-bridge structures, conventional ultra-violet (UV)photolithography has been used to define windows in the silicon waveguide layer, through which the silicalower cladding layer could be selectively removed using buffered hydrofluoric acid (HF). Air-bridge/membrane structures have been used to obtain the highest Q-factor values achieved in waveguidephotonic crystal-based micro-cavities – and we are investigating the extent to which the modified geometryimplied by surrounding the entire photonic wire waveguide with air affects the Q-factor values that areachievable. We shall report results obtained in the conference presentation.3. ConclusionsThe Q-factor and transmission coefficient combination achieved in this work are probably still some wayfrom the best that can eventually be obtained. With sufficiently low propagation losses obtained throughsmooth side-walls - and micro-cavity designs based around a greater number of holes in the periodic partsof the cavity mirrors, substantially higher cavity resonance quality factors should be possible. Furtherattention to the design of the tapered sections will contribute both to increases in the Q-factor and to thetransmission coefficient.We acknowledge the partial support of the EPSRC in the U.K. and of the European Community.4. References1. A.S. Jugessur, P. Pottier, R.M. De La Rue, ‘One-dimensional periodic photonic crystal microcavity filters with transition mode-matching features, embedded in ridge waveguides’, Electronics Letters, 39(4), 367-369, 20th Feb (2003).2. P. Velha, E. Picard, T. Charvolin, E. Hadji, J. C. Rodier, P. Lalanne and D. Peyrade, ‘Ultra-High Q/V Fabry-Perot microcavity  onSOI substrate’, Optics Express, 15(24), 16090-16096, 26th November (2007).3. Ahmad Rifqi Md Zain, Marco Gnan, Harold M. H. Chong, Marc Sorel and Richard M. De La Rue, ‘Tapered Photonic CrystalMicrocavities Embedded in Photonic Wire Waveguides With Large Resonance Quality-Factor and High Transmission’, IEEE Photon.Technol. Letts, 20(1), 6 – 8, 1st January (2008).4. Richard De La Rue, Harold Chong, Marco Gnan, Nigel Johnson, Iraklis Ntakis, Pierre Pottier, Marc Sorel, Ahmad Md Zain, HuaZhang, Edilson Camargo, Chongjun Jin, Mario Armenise and Caterina Ciminelli, ‘Photonic crystal and photonic wire nano-photonicsbased on silicon-on-insulator’, New Journal of Physics, 8, 256, (2006).5. Benoit Cluzel, Loïc Lalouat, Philippe Velha, Emmanuel Picard, David Peyrade, Jean-Claude Rodier, Thomas Charvolin, PhilippeLalanne, Frédérique de Fornel and Emmanuel Hadji, ‘A near-field actuated optical nanocavity’, Optics Express, 16(1), 279 – 286, 7thJanuary (2008).6. M. Gnan, S. Thoms, D.S. Macintyre, R.M. De La Rue and M. Sorel, ‘Fabrication of low-loss photonic wires in silicon-on-insulatorusing hydrogen silsesquioxane electron-beam resist’, Electron. Letts., 44(2), 115 - 116, 17th Jan (2008).7. C. A. Barrios, V. R. Almeida, and M. Lipson, “Low-power-consumption short-length and high-modulation-depth siliconelectrooptic modulator,” J. Lightw. Technol., 21(4), 1089–1098, Apr. (2003).8. H. M. H. Chong and R. M. De La Rue, “Tuning of photonic crystal waveguide microcavity by thermooptic effect,” IEEE Photon.Technol. Lett., 16(6), 1528–1530, June (2004).9. M. W. Geis, S. J. Spector, R. C. Williamson, and T. M. Lyszczarz, ‘Sub-microsecond sub-milliwatt silicon-on-insulator thermoopticswitch,” IEEE Photon. Technol. Lett., 16(11), 2514–2516, Nov. (2003).

Ahmad Md Zain, Hua Zhang, Edilson Camargo, Chongjun Jin, Mario Armenise and Caterina Ciminelli, ‘Photonic crystal and photonic wire nano-photonics based on silicon-on-insulator’,

De La Rue, ‘One-dimensional periodic photonic crystal microcavity filters with transition modematching features, embedded in ridge waveguides’,

De La Rue, ‘Tapered Photonic Crystal Microcavities Embedded in Photonic Wire Waveguides With Large Resonance Quality-Factor and High Transmission’,

De La Rue, “Tuning of photonic crystal waveguide microcavity by thermooptic effect,”

Fabrication of low-loss photonic wires in silicon-on-insulator using hydrogen silsesquioxane electron-beam resist’,

Low-power-consumption short-length and high-modulation-depth silicon electrooptic modulator,”

Sub-microsecond sub-milliwatt silicon-on-insulator thermooptic switch,”

Thomas Charvolin, Philippe Lalanne, Frédérique de Fornel and Emmanuel Hadji, ‘A near-field actuated optical nanocavity’,

Advancing the performance of one-dimensional photonic crystal/photonic wire micro-cavities in silicon-on-insulator

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