1041-1135 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LPT.2015.2405611, IEEE Photonics Technology Letters > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract— Germanium-on-silicon is a highly promising platform for planar photonics for the mid-infrared, due to germanium’s wide transparency range. In this paper we report Ge-on-Si waveguides with record low losses of only 0.6dB/cm, which is achieved by using a 2.9 μm thick germanium layer, thus minimizing mode interaction with dislocations at the germanium/silicon interface. Using these waveguides multimode interferometers with insertion losses of only 0.21 ± 0.02 dB are also demonstrated. Index Terms—Mid-infrared, germanium, waveguides. I. INTRODUCTION HERE is a need for waveguide platforms that can be used throughout the mid-infrared (MIR) wavelength range for a broad range of sensing applications, which include environmental monitoring, gas sensing, and toxic chemical detection for homeland security. Planar photonics promises, through integration, to shrink the cost and size of existing MIR sensing systems. In addition, there is growing interest in using the 2-3 μm wavelength region to increase the bandwidth available in future telecommunications systems [1]. Group- IV materials are attractive for mid-infrared photonics because of their wide transparency ranges, high refractive indexes, mechanical robustness, and high nonlinear coefficients [2]. Silicon-on-insulator (SOI) is the most popular near-infrared group-IV material platform, but since silicon dioxide has high absorption throughout most of the MIR, alternatives are required. Recent years have brought rapid investigation of alternative material platforms, including: silicon membrane (or suspended silicon) [3, 4], silicon-on-sapphire [5-7], silicon on silicon nitride [8], and germanium-on-silicon (Ge-on-Si). Ge-on-Si has by far the widest transparency range because Ge has absorption <1 dB/cm from 2-15 μm [9], and while silicon’s high absorption for λ > 8µm prevents its use as a waveguide core material, Ge-on-Si waveguides may have acceptable losses all the way to 15 μm if the mode overlap with the substrate is minimized. Ge-on-Si waveguides were first demonstrated with losses of 2.5-3.0 dB/cm at λ=5.8 μm by Chang et.al [10]. Since then Ge-on-Si multiplexers [11, 12], Mach-Zehnder interferometers [13], and thermo-optic modulators [14] have also been investigated. In this paper we present Ge-on-Si waveguides with greatly improved propagation losses at the 3.8 μm wavelength, and we report for the first time insertion losses of Ge-on-Si 1×2 and 2×2 multimode interferometers (MMI), which are based on those waveguides. It Submitted for review on 05/12/14. The authors acknowledge EPSRC for financial support under the project MIGRATION (EP/L01162X/1). G. Z. Mashanovich would like to acknowledge support from the Royal Society through his University Research Fellowship. should also be noted that grating couplers in Ge have been used for the first time. II. WAVEGUIDES The waveguides were fabricated using commercially available 6” Ge-on-Si wafers with a 3 μm Ge thickness. The Ge film was grown on the Si substrate by reduced pressure chemical vapor deposition (RPCVD). The resulting film exhibits a threading dislocation density (TDD), arising from the lattice constant difference between Si and Ge, of 2×10 7 - 5×10 7 cm -2 , which was determined using a wet selective defect etch. In such an etch, the chemical reaction is sensitive to the presence of defects, which therefore reveals the defects so that they are observable using microscopy techniques (i.e. SEM or optical microscopy) [15] . Waveguide modelling was carried out using the FMM solver in the Photon Design Fimmwave commercial software package [16]. Rib waveguides were designed for single mode propagation at λ=3.8 μm, with the dimensions waveguide height (H) = 2.9 μm, width (W) = 2.7 μm, and etch depth (D) = 1.7 μm. The simulated TE mode profile is shown in fig. 1. Fig. 1. Simulated TE mode profile at λ=3.8 μm, for a Ge-on-Si waveguide with H = 2.9 μm, D = 1.7 μm, and W = 2.7 μm. Waveguide patterns were defined on ZEP-520A positive resist by e-beam lithography. The ZEP was deposited by spinning at 1000rpm, with ellipsometer measurements confirming that the resulting layer had a thickness of 710nm and acceptable uniformity. Following lithography the sample was dry etched in an Oxford Instruments ICP 380 plasma system, using SF6 and C 4 F 8 gases, to transfer the pattern into the Ge layer. The Ge:ZEP etch selectivity was 3.0:1, so no hard mask was required. The ZEP e-beam resist was removed by treatment in an O 2 plasma asher. All authors are with the Optoelectronics Research Centre, University of Southampton, Southampton, Hampshire, SO17 1BJ, United Kingdom. The corresponding author is M. Nedeljkovic (e-mail: m.nedeljkovic@soton.ac.uk). Copyright © 2015 IEEE. Surface Grating Coupled Low Loss Ge-on-Si Rib Waveguides and Multimode Interferometers M. Nedeljkovic, J. Soler Penadés, C. J. Mitchell, A. Z. Khokhar, S. Stanković, T. Dominguez Bucio, C. G. Littlejohns, F. Y. Gardes, and G. Z. Mashanovich T