Design and Fabrication of Silicon Photonic Crystal Optical Waveguides Marko Loncar,ˇ Theodor Doll, Jelena Vuckovicˇ ´, and Axel Scherer
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1402 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 18, NO. 10, OCTOBER 2000 Design and Fabrication of Silicon Photonic Crystal Optical Waveguides Marko Loncar,ˇ Theodor Doll, Jelena Vuckovicˇ ´, and Axel Scherer Abstract—We have designed and fabricated waveguides that in- there were reports on characterization of both active [11] and corporate two-dimensional (2-D) photonic crystal geometry for lat- passive [12] devices based on this system. eral confinement of light, and total internal reflection for vertical The idea of using line defects in photonic crystals to guide confinement. Both square and triangular photonic crystal lattices were analyzed. A three-dimensional (3-D) finite-difference time- light was proposed by Meade et al. [13]. Mekis et al. [14] have domain (FDTD) analysis was used to find design parameters of the developed the theoretical foundation of understanding 90 photonic crystal and to calculate dispersion relations for the guided bends in the 2-D photonic crystals. Experimental confirmation modes in the waveguide structure. We have developed a new fab- of this theoretical work in the millimeter-wave range using long rication technique to define these waveguides into silicon-on-insu- alumina rods was provided by Lin et al. [15]. The disadvantage lator material. The waveguides are suspended in air in order to improve confinement in the vertical direction and symmetry prop- of using photonic lattices constructed from dielectric rods erties of the structure. High-resolution fabrication allowed us to in- surrounding an air waveguide is that there is no mechanism for clude different types of bends and optical cavities within the waveg- low-loss vertical confinement of the light within the waveguide uides. at optical frequencies. A more favorable geometry is the one Index Terms—Finite-difference time-domain (FDTD) methods, introduced above, a thin dielectric slab perforated with holes, nanooptics, optical device fabrication, photonic bandgap (PBG) within which a line defect of missing holes can be defined. materials, photonic crystals (PCS), photonic crystal waveguides. 3-D theoretical analysis [16], [17] and qualitative experimental characterization [18]–[20] of light guiding within waveguides I. INTRODUCTION made in the photonic crystal slabs have been recently reported. In this paper, we present the theoretical analysis and fabrica- HOTONIC crystals [1], [2] (PCs), two- and three-dimen- tion of waveguides that incorporate 2-D photonic crystal geome- P sionally periodic structures with high refractive index con- tries for lateral confinement of light, and total internal reflection trast, have recently become very interesting for the manipula- for vertical confinement. tion of light. The existence of a photonic bandgap, a frequency range in which propagation of light is prevented in all direc- tions, makes photonic bandgap (PBG) structures very useful in II. THEORETICAL ANALYSIS applications where spatial localization of light is required. For example, by forming defects in a host PBG structure, we can We start with a theoretical analysis of the band structure for form localized electromagnetic states, and “trap the light.” Ide- both square and triangular lattice photonic crystals in order to ally, by making a three-dimensional (3-D) photonic crystal we find design parameters, such as interhole spacing , radius can control propagation of light in all three dimensions [3]–[5]. of holes , and thickness of the slab , that will give us a Since fabrication of 3-D PC structures is still a difficult process, bandgap centered around interesting 1.5- m wavelength range. a more appealing approach is based on the use of lower dimen- Full 3-D analysis is required since the waveguide incorporates sional photonic crystals to achieve confinement of light in three vertical confinement of light. In contrast to the infinite 2-D case, dimensions. A structure that has recently attracted a lot of atten- finite-thickness slabs can support modes with higher order ver- tion is a dielectric slab perforated with a two-dimensional (2-D) tical oscillations, and, if the slab is made too thick, the presence PC lattice [6]–[10]. In such a structure, light is controlled in the of these modes can result in closing of the bandgap [9]. There- vertical direction by the means of total internal reflection (TIR) fore, the thickness of the slab is a critical parameter in this struc- (due to the refractive index contrast of high index core and low ture and needs to be modeled. In addition, 2-D analysis of ver- index cladding), and in the lateral direction by distributed Bragg tically extended structure (infinitely thick slab) would result in reflection (DBR), due to the presence of the 2-D PC. Recently, band diagrams that are shifted toward the lower frequencies. The reason for that is the fact that in the case of the slab waveguide Manuscript received December 23, 1999; revised June 28, 2000. This work guided modes are not completely confined in the slab, but also was supported in part by the AFOSR under Contract AFS-49620-98-1-01447. extend into the air. Therefore, the energy of the guided mode M. Loncar,ˇ J. Vuckovicˇ ´, and A. Scherer are with the Department of Elec- will be higher and the bands will be at higher frequencies than trical Engineering, California Institute of Technology, MS 136-93, Pasadena, CA 91125 USA (e-mail: [email protected]). in the case of 2-D waveguide with infinitely long holes. T. Doll was with the Department of Physics, California Institute of Tech- The structure that we analyze first is a silicon slab patterned nology, MS 136-93, Pasadena, CA 91125 USA. He is now with the Faculty with square or triangular lattice of holes and surrounded by air of Electrical Engineering, Ilmenau Technical University, Ilmenau 98693, Ger- many. (Fig. 1). Three-dimensional final-difference time-domain (3-D Publisher Item Identifier S 0733-8724(00)09101-5. FDTD) scheme [21], [22] was used to calculate band diagrams 0733–8724/00$10.00 © 2000 IEEE LONCARˇ et al.: DESIGN AND FABRICATION OF SILICON PHOTONIC CRYSTAL OPTICAL WAVEGUIDES 1403 (a) (a) (b) Fig. 1. (a) Si slab perforated with 2-D square lattice of holes (top view). Lattice vectors are a @IY HY HA and a @HY IY HA. Reciprocal lattice vectors are a@P%aA @IY HY HA and a@P%aA @HY IY HA. Without loss of generality, interhole spacing in the figure is chosen to be a %a (dimensionless units), in order to conveniently show reciprocal lattice vectors. The square represents the first Brillouin zone (BZ), and YY and w are the high symmetry points with coordinates (in reciprocal space): a@HY HY HAY a@HY %aY HAYw a@%aY %aY HA. Empty circles represent location where the waveguide will be defined by removing the entire (b) row of holes. (b) Si slab perforatedp with 2-D triangular lattice of holesp (top view). Fig. 2. Band diagrams of eigen modes of Si slab perforated with 2-D lattice a @ QaPY IaPY HAY a @ QaPY IaPY HA Lattice vectors are pand p . of holes of (a) square and (b) triangular symmetry. The thickness of the slab is a@R%a QA @IaPY QaPY HAY Reciprocal latticep vectorsp are and chosen to be d aHXSS and the hole radius is r aHXR, where is the interhole a@R%a QA @IaPY QaPY HA. As explained in (a), we chose aR%aQ spacing. Even modes are represented with circles and odd with stars. Insets show in order to represent reciprocal and real-space lattice vectors in the same the SEM micrographs of the fabricated photonic crystal structures (scale bar is figure. The hexagon represents the first BZ. Coordinatesp of highp symmetry 500 nm), and first Brillouin zones with characteristic high symmetry points. points are: a@HY HY HAY a@P%a QA @IaPY QaPY HAY and t a@R%aQA @HY IY HA. Band diagrams for the even (TE-like) and odd (TM-like) eigenmodes of the PC structure with square symmetry are for square and triangular [23] lattices. In both cases, we ana- shown in Fig. 2(a), and with triangular symmetry in Fig. 2(b). lyzed a lattice unit cell by applying appropriate boundary condi- The -axis represents different directions in the reciprocal lat- tions to its sides: Bloch boundary conditions to all four sides per- tice (see Fig. 1), and the -axis normalized frequency in units of pendicular to the plane of the slab, Mur’s absorbing boundary , where is the wavelength in the air. The solid condition [24] (ABC) to the top boundary, and mirror boundary line shown in the plots represents the light line—dispersion condition at the middle of the slab. In that way we reduce the relation for the photons in the air having momentum as one on computation time by analyzing only one half of the unit cell. By the -axis. Modes with frequencies above the light line will changing the type of the mirror symmetry we could select even leak energy into the air, and thus represent the loss mechanism (TE-like) or odd (TM-like) eigenmodes of the photonic crystal of the waveguide. structure. The spatial resolution that we used to discretize our As can be seen, in both structures the first-order bandgap is structures was 20 computational points per interhole spacing open for the guided modes of the slab (modes below the light . The thickness of the Si slab was equal to line) that have an even symmetry (TE-like). However, it is im- and the hole radius was . The index of refraction of Si is portant to notice that the Si slab perforated with 2-D photonic assumed to be . Mur’s ABC was applied at a distance crystal does not have a full 3-D bandgap.