Design and Fabrication of Line-Defect Waveguides in Hexagon-Type SOI Photonic Crystal Slabs Cazimir G. Bostan * a , René M. de Ridder a , Vishwas J. Gadgil a , Henry Kelderman a , Laurens Kuipers a,b , Alfred Driessen a a University of Twente, MESA + Research Institute, P.O. Box 217, 7500 AE Enschede, The Netherlands; b FOM–Institute for Atomic and Molecular Physics (AMOLF), Kruislaan 407, 1098 SJ Amsterdam, The Netherlands. ABSTRACT We present a novel design approach for line-defect waveguides integrated in a photonic crystal slab (PCS) with hexa- gonal holes in a triangular lattice (aka ‘hexagon-type’). Triangular air inclusions are symmetrically added on each side of the waveguide. Size and position of these inclusions are tuning parameters for the band diagram and can be used for minimizing the distributed Bragg reflection (DBR) effect. The waveguides show single-mode behavior with reasonably high group velocity and large transmission window, inside the gap between even-like modes. Qualitative design rules were obtained from 2D calculations based on effective index approximation and full 3D calculations of the band structure were applied for fine-tuning of structural parameters of these high-index contrast systems. Transmission spectra and losses of finite-sized structures were estimated by means of 3D finite-difference time domain (FDTD) calculations. We present a pattern definition technique, which is an integration of optical lithography with focused ion beam (FIB) high-resolution etching. The mask pattern is transferred into the SOI stack by a subsequent reactive ion etching (RIE) process. The combination of moderate resolution optical lithography and FIB etching provides an excellent tool for fast prototyping of PCS-based devices. Keywords: photonic crystal, line defect waveguide, effective index, finite difference time domain, focused ion beam 1. INTRODUCTION In a previous paper 1 we showed that gaps in guided modes (i.e. below the light-cone) may exist in a hexagon-type photonic crystal slab, even in the case of vertical asymmetry (e.g. SOI system). However, these gaps manifest themselves only below the light cone (the lowest dispersion curve of the claddings). As soon as the dispersion curves of guided modes reach the light cone boundary and become guided resonances 2 , they cross the gap; then mode coupling between guided and leaky modes is likely and this leads to propagation losses. The absence of guided modes in a frequency region has potential applications in light extraction enhancement from a point-like cavity, through increasing the in-plane quality factor. The gaps in guided modes obtained for high air filling factors are situated at higher frequencies. This has a number of practical disadvantages when linear defects are introduced: (i) The higher frequencies in the band diagram are more sensitive to small variations in geometry parameters (due to e.g. fabrication imperfections); (ii) Propagation losses are roughly proportional with the area of air holes 3 . Theoretical and experimental studies have almost exclusively concentrated on a triangular lattice of circular holes. The ‘defect-free’ air bridge slab has a large robust gap in even modes between the first two bands. Odd modes did not receive much attention. In practice is very difficult to achieve perfect vertical symmetry and mode coupling due to symmetry breaking is critical 4 . Hexagonal holes provide the same large gap as their circular counterpart and also add some flexibility in design of line defects. An extra degree of freedom is the rotation angle of the hexagons with respect to their symmetry axis, this can be used to tune the waveguide shape and width. In this paper we present a study of linear defect waveguides in hexagon- type SOI slabs and a way of tuning ‘defect’ modes by means of triangular air inclusions (see e.g. Figure 4). These inclusions can be used to tune the waveguide width (i.e. modify the effective index of the waveguide) independently of the lattice constant and in some cases without lattice distortion and also minimize the effect of boundary corrugations. * e-mail: c.g.bostan@el.utwente.nl ; phone: +31-53-4892665 Photonic Crystal Materials and Nanostructures, edited by Richard M. De La Rue, Pierre Viktorovitch, Clivia M. Sotomayor Torres, Michele Midrio, Proceedings of SPIE Vol. 5450 (SPIE, Bellingham, WA, 2004) · 0277-786X/04/$15 · doi: 10.1117/12.545694 323