Two- and Three-Dimensional Photonic Crystals Built with VLSI Tools
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Two- and Three-
Dimensional Photonic Crystals Built with VLSI Tools
Shawn-Yu Lin, J.G. Fleming, and E. Chow Introduction The drive toward miniature photonic devices has been hindered by our inability to tightly control and manipulate light. Moreover, photonics technologies are typically not based on silicon and, until recently, only indirectly benefited from the rapid advances being made in silicon processing technology. In the first part of this article, the successful fabrication of three-dimensional (3D) photonic crystals using silicon processing will be discussed. This advance has been made possible through the use of integrated-circuit (IC) fabrication technologies (e.g., very largescale integration, VLSI) and may enable the penetration of Si processing into photonics. In the second part, we describe the creation of 2D photonic-crystal slabs operating at the 1.55 m communications wavelength. This class of 2D photonic crystals is particularly promising for planar on-chip guiding, trapping, and switching of light.
Fabrication of 3D Photonic Crystals
Design of 3D Photonic Crystals Yablonovitch1 and John2 first proposed the modern photonic lattice concept in 1987. In general, the idea was to modulate photons in a manner similar to the way electrons are modulated in a semiconductor. This is achieved through a periodic variation in the refractive index. A pseudogap was first demonstrated theoretically in a macroscopic fcc structure with spherical airholes.3 A design with a full bandgap based on diamond symmetry was proposed by Ho et al. of Iowa State University.4,5 This design will be considered in the most detail here since it lends itself to microfabrication.6,7 In the inset of Figure 1a, a scanning electron microscopy (SEM) image of such a 3D photonic crys-
MRS BULLETIN/AUGUST 2001
tal, built up of stacked arrays of silicon bars, is shown. The corresponding diamond lattice structure and a calculated photonic density-of-states (DOS) spectrum are shown in Figures 1a and 1b, respectively. In short, the 1D rods each represent the shortest 110 chain of atoms in a diamond lattice and are stacked like a woodpile. The stacking sequence is such that every four layers constitute a unit cell. For this structure, there is a frequency (f) range in which the photonic DOS vanishes (a photonic bandgap), centered at f 0.5 c0/a (see Figure 1b), where c0 and a are the freespace speed of light and the lattice spacing, respectively. Similar structures consisting of stacked rods have been generically described as “woodpile” structures.8 A different design, which uses alternating rectangular rods and more closely mimics the arrangement of atoms in the diamond structure but which is more difficult to fabricate, has also been proposed.9 Another structure with a full bandgap has been proposed by workers at the Massachusetts Institute of Technology (MIT).10 The simple cubic structure is also predicted to have a full gap.11,12 An interesting “triple-hole” structure has also been demonstrated.13 We at Sandia National Laborator
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