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In this paper, we investigate performance of a self-sustained ON-OFF switch incorporating a new phase change material Ge2Sb2Se4Te1 with Silicon rib waveguide at the telecommunication wavelength 1.55 µm. A full-vectorial H-field finite element method is used to find effective index and modal loss of the quasi-TE modes in GSST-Si waveguide. Both the electro-refraction and electro-absorption type design are studied and the effect of GSST thickness and Si slab thickness on the device performances are presented. These results suggest that a GSST-Si rib waveguide with a 500 nm wide Si core, 60-90 nm Si slab thickness, and 40-60 nm GSST layer could be a realistic switch design which can yield a very compact, 5 μm long device with only 0.135 dB total insertion loss and more than 20 dB extinction ratio.

We propose the design of a nonvolatile, low-loss optical phase shifter based on optical phase change material (O-PCM). The optical phase change material Ge 2 Sb 2 Se 4 Te 1 (GSST), which exhibits low loss at telecommunication wavelength 1.55 µm as compared to other commonly used O-PCMs, is used in this work as the active material. Instead of direct interaction of the waveguide mode with the O-PCM, the design utilizes coupling between the primary SiN strip waveguide and a waveguide formed by O-PCM, in its amorphous state. The phase matching in the amorphous state inhibits the interaction of the waveguide mode with GSST in its highly lossy crystalline state resulting in low loss operation. Due to a high differential refractive index between the two states of GSST, the design requires a very small length of the phase shifter to accumulate the desired phase difference. The overall response of the Mach-Zehnder Interferometer (MZI) configuration using the designed phase shifter shows that the design can be used to obtain optical switching with a very small insertion loss and crosstalk over the entire C-band. Index Terms-Coupled mode analysis, Optical Switches, Phase change materials. I. INTRODUCTION hase shift for optical switching in silicon photonic devices is mainly achieved by free carrier injection [1-3] and thermo-optic effects [4]. These effects give rise to a small change in the refractive index, which results in large device length to obtain the required phase change. The alternative is to use the resonant structures to obtain devices with a small footprint, however, at the expense of low bandwidth and high sensitivity [5,6]. The compact hybrid plasmonic-photonic switches based on 3-waveguide directional couplers have also been reported, but the associated insertion loss is high [7,8]. In recent years, optical phase change materials (O-PCMs) have

This is the accepted version of the paper.This version of the publication may differ from the final published version.Permanent repository link: http://openaccess.city.ac.uk/20694/ Link to published version: http://dx. ABSTRACTDesign of electro-optic ON-OFF switches based on well-known phase change material Ge 2 Sb 2 Te 5 (GST) is presented. The electro-optic switch is achieved by implementing by co-directional coupling between a 220 nm thick silicon nanowire and a silicon waveguide topped with ITO-GST-ITO layers at the 1.55μm wavelength. By introducing the electric field via the ITO electrodes, the GST layer can be changed between the amorphous and crystalline states. As the modal loss in the crystalline state is much higher than the amorphous state, through a rigorous modal analysis of the coupled silicon nanowire and GST waveguide by using the finite element method, the optimal ITO spacing is obtained at 75nm which is less sensitive to device parameter variations and thus offering better tolerances. The GST thickness is also optimized for the phase matching point at 25 nm in order to efficiently transfer power from silicon nanowire to GST waveguide to attain the OFF state. Once the device is phase matched in crystalline state, the power in the amorphous state will pass with very little interaction with the GST waveguide resulting in an ON state. The Eigenmode Expansion Method of Fimmprop is used as a junction analysis approach to calculate the optical power coupling efficiencies to the output silicon nanowire. The extinction ratio of the electro-optic switch and insertion loss in ON state at phase matching can be obtained as a function of the device length. A compact 1.75 μm long device shows a high extinction ratio of 22 dB with an insertion loss of only 0.56 dB.