Planar Photonic Crystal Microcavities for Add/Drop Filter Functionality H. Chong and R. M De La Rue Department of Electronics and Electrical Engineering, University of Glasgow, Rankine Building, Oakfield Avenue, Glasgow G12 8LT, SCOTLAND
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Abstract We have fabricated and measured hexagonally shaped microcavities within triangular symmetry 2D photonic crystal regions of silicon-on-insulator (SOI) planar waveguides, using reduced diameter in-filling holes. Device structures based on such microcavities have been realised using direct input and output coupling through channel waveguides with sizegraded holes in the channels at both the input and output of the microcavity. A measured Qvalue of ~ 800 has been obtained in a structure with a transmission resonance wavelength of 1542.6 nm. Keywords: Photonic crystal, cavity, WDM, Add/Drop Filters, silicon-on-insulator (SOI). Introduction Photonic crystal microcavities have been studied widely for their ability to confine light selectively at different operating wavelengths. This property is clearly highly desirable in the application areas of wavelength division multiplexing and channel add/drop filtering. Photonic crystal cavities with high Q-values have been reported by several authors [1,2,3,6,7] and various techniques have been used to couple light into the cavities [3,4]. We have fabricated and measured microcavities with partially in-filled holes of different diameter [6] that exhibit selective control of the cavity mode excitation and resonance frequency. The cavities are excited by direct coupling of light via a channel waveguide with size-graded holes [8]. The photonic crystal microcavities were fabricated in SOI material and a Q-value of greater than 800 has been measured.
(a) (b) Figure 1. (a) Hexagonal cavity in-filled 185 nm diameter holes with single size-graded holes and (b) Hexagonal cavity in-filled 185 nm diameter holes with two size-graded holes.
The microcavity uses a hexagonal lattice period of 420 nm and hole diameter of 280 nm. The smaller in-filling holes have diameters of 185 nm, as shown in Fig.1. The small change in the modal volume produced by the in-filling holes in the cavity is used to tune the resonance frequency [6]. Size-graded holes of 240 nm and 185 nm in the input and output channel waveguide confine the light at resonance and enhance the Q-factor and transmission of the
cavity by reducing unwanted reflection due to mismatch and through minimisation of vertical leakage [8,9]. The SOI waveguide structure has a silicon core thickness of 340 nm, with a lower cladding of 3 µm. Reactive ion etching was used for pattern transfer and the gas used for the silicon layer was SiCl4. The microcavities were measured with TE-polarised light in the S- to L-band wavelength range of 1480 nm to 1580 nm. Light is coupled into and out the waveguide using the end-fire approach and detected using a germanium-diode detector. Figure 2 shows measured TE transmission spectra for (i) a hexagonal cavity with in-filling holes of 185 nm diameter and one input/output reduced-size hole and (ii) an in-filled hexagonal cavity with 185 nm holes and two input/output reduced-size and graded holes. The measured Q-value for the cavity with a single reduce size input/output hole is about 150, at λ ~ 1545.9 nm. The measure Qvalue for the cavity with 2 graded-size input/output holes is about 800 at λ ~ 1542.6 nm. The sharp transmission peak of the cavity with Q ~ 800 demonstrates the enhanced confinement effect produced by increasing the number of size-graded holes. 1
a.u.
0.8 0.6 0.4
Cavity with single reduced size holes
0.2
Cavity with two size-graded holes
0 1500 1510 1520 1530 1540 1550 1560 1570 1580 Wavelength / nm
Figure 2. Transmission measurement for (i) a hexagonal cavity in-filled with 185 nm diameter holes and single reduced-size coupling holes, giving Q ~ 150 and (ii) a cavity in-filled with 185 nm diameter holes and two size-graded coupling holes, giving Q ~ 800.
Such confinement has also been observed by Noda et al. [3] in a microcavity of seven smaller holes together with a larger three rows wide channel waveguide and four periods of photonic crystal as a confinement barrier for the microcavity. However, this three rows wide channel waveguide can exhibit mini stop-band behaviour and is substantially multi-moded [5]. The slight shift in the resonance wavelength between the two configurations is small, but indicates the expected impact on the resonance of using more coupling holes. Conclusions The partially in-filled, directly-coupled cavity gives the ability to tune the resonance frequency by varying the interior hole diameter. A usefully large cavity Q-value of ~ 800 at λ~1542.6 nm has been achieved and the resonance frequency has been shown to depend on the number of size-graded holes used in the input and output waveguide channels.
Acknowledgement This research was supported through the PICCO project under the European Community IST programme. References [1] C. J. M. Smith, R. M. De La Rue, M. Rattier, S. Olivier, H. Benisty, C. Weisbuch, T. F. Krauss, R. Houdre, U. Oesterle, Applied Physics Letters, Vol. 78, No. 11, 14871489, 2001 [2] S. Fan, H. Haus, P. Villeneuve, J. Joanopoulos, Optics Express, Vol. 13, Iss. 1, 1998. [3] S. Noda, A. Chutinan, M. Imada, Nature, Vol. 407, 608-610, 2000. [4] S. Y. Lin, E. Chow, S. G. Johnson, J. D. Joanopolous, Optics Letters. Vol. 26, No. 23, 1903-1905, 2001. [5] C. J. M. Smith, H. Benisty, S. Olivier, M. Rattier, C. Weisbuch, T. F. Krauss, R. M. De La Rue, R. Houdre, U. Oesterle, Applied Physics Letters, Vol. 77, No. 18, 28132815, 2000. [6] H. G. Park, J. K. Hwang, J. Huh, H. Y. Ryu, Y. H. Lee, J. S. Kim, Applied Physics Letters, Vol. 79, No. 19, 3032-3034, 2001. [7] O. Painter, J. Vuckovic, A. Scherer, J. Opt. Soc. Am. B, Vol. 16, No. 2, 1999. [8] M. Palamaru, Ph. Lalanne, Applied Physics Letters, Vol. 78, No. 11, 1466-1468, 2001. [9] J. S. Foresi, P. R. Villeneuve, J. Ferrera, F. R. Thoen, G. Steinmeyer, S. Fan, J. D. Joanopolous, L. C. Kimerling, H. I. Smith, E. P. Ippen, Nature, Vol. 390, 143-145, 1997.
