1 Abstract—Very strong resonant polarization conversion is observed in the reflectivity properties of one-dimensional lattices of air trenches deeply etched in AlGaAs surface waveguides. The symmetry properties and the magnitudes of the observed effects are found to be in very good agreement with the results of scattering matrix calculations. Index Terms—Photonic Crystals, Photonic Band Structure, Reflectivity, Polarization, Waveguide, Electromagnetic Coupling. I. INTRODUCTION HOTONIC crystal waveguides constitute a class of photonic crystal structures which exhibit band structure effects for guided modes [1] and in addition several novel external coupling properties for leaky modes [2-8]. Surface external coupling occurs under phase matching conditions when both the energy and the in-plane wave vector of incident light match those of the folded band structure of the photonic crystal modes. Such external coupling to leaky modes above the light line is manifested in external reflectivity spectra by the appearance of sharp resonant features and polarization mixing [4,7-11] and anticrossing between bands; by contrast in two dimensional (2D) photonic crystals of infinite vertical extent, without vertical waveguide confinement, such bands are pure transverse electric (TE) or transverse magnetic (TM) eigenstates of the electromagnetic field [12]. The polarization mixing of bands, leading to TE modes of the 2D crystal appearing in TM polarized reflectivity spectra [4,7] or vice versa, arises from the effects of zone folding and the vertical confinement. Band states which would otherwise lie outside the light cone, are folded back into the first Brillouin zone by the effects of the lattice potential. Since the lattice potential contains off diagonal components which mix pure TE and TM states, bands become observable in 'oppositely' polarized reflectivity spectra [4,7,13]. Two of us have also predicted that polarization conversion of incident TE in (TM in ) polarized light into outgoing TM out (TE out ) may This work is funded by the EPSRC. A. D. Bristow, V. N. Astratov, R. Shimada, I. S. Culshaw, D.M. Whittaker and M. S. Skolnick, are with the Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, U.K. (e-mail: a.d.bristow@shef.ac.uk) V. N. Astratov is also with the Department of Physics, University of North Carolina at Charlotte, Charlotte, North Carolina 28223, U.S.A. R. Shimada is also with the Department of Physics, Japan Women’s University, Tokyo 112-8681, Japan A. Tahraoui and T. F. Krauss are with the School of Physics & Astronomy, University of St. Andrews, St. Andrews, Fife, KY16 9SS, U.K. occur when light is incident away from the main symmetry directions of the photonic lattice [7]. Such polarization conversion was recently observed for 2D triangular lattices of air cylinders [13], with relatively modest polarization of the outgoing light, β=TM out /TE out of order of ~1. In the present paper we report very strong polarization conversion effects in the reflectivity of a one-dimensional lattice of air trenches etched into an AlGaAs surface waveguide structure, of the type employed in refs [5,6]. We observe very strong polarizations of the outgoing beam of order β = 4, for directions of incidence away from the main symmetry directions of the lattice, with high efficiencies (TM out /TM in ) of order 30%. We also show theoretically that the strength of the polarization conversion is a strong function of the depth of the lattice, and is negligibly small for e.g. shallow etched Bragg gratings. The paper is organized as follows. After description of the samples and the measurement techniques in Section II, we present the determination of the photonic band structure in Sec. III. The polarization conversion effects are presented in Sec. IV. Finally we summarise the main conclusions in Sec. V. II. EXPERIMENTAL A. Samples The samples studied in this work are surface waveguides with a 1D lattice of air trenches deep etched through the waveguide, as shown in Fig 1, to give rise to photonic band structure effects for the guided modes. The planar waveguides were grown using molecular beam epitaxy (MBE) and had Al 0.08 Ga 0.92 As core thickness of d = 240nm, and were clad by air on the top side and Al 0.9 Ga 0.1 As, of thickness d cl = 790nm on the lower side. 1D lattices of air stripes of width w = 45nm were fabricated by electron beam lithography and deep reactive ion etching (RIE), see Fig.1 (c). The air trenches were deep etched through the core into the cladding to depths d e ~450nm (aspect ratio ~10) to reduce scattering [5,6]. In this paper we focus on a lattice period of a = 295nm and air fill factor f = 22% as determined by SEM (Fig.1 (b)), although lattices of several other periods and fill fractions were also investigated. The size of the lattices was 80μm x 80μm. Polarization Conversion in the Reflectivity Properties of Photonic Crystal Waveguides Alan D. Bristow, Vasily N. Astratov, Ryoko Shimada, Ian S. Culshaw, Maurice S. Skolnick, David M. Whittaker, Abbes Tahraoui, and Thomas F. Krauss P