Sub-wavelength plasmonic-enhanced phase-change memory Emanuele Gemo* a , Santiago García-Cuevas Carrillo a , Joaquin Faneca a , Carlota Ruíz de Galarreta a , Hasan Hayat a,b , Nathan Youngblood c , A. Baldycheva a , Wolfram H.P. Pernice d , Harish Bhaskaran c , C. David Wright a a Department of Engineering, University of Exeter, North Road, Exeter EX4 4QF, UK; b College of Engineering, Swansea University, Swansea SA1 8EN, UK; c Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK; d Institute of Physics, University of Muenster, Heisenbergstrasse 11, 48149 Muenster, Germany ABSTRACT The Ge2Sb2Te5 phase-change alloy (GST) is known for its dramatic complex refractive index (and electrical) contrast between its amorphous and crystalline phases. Switching between such phases is also non-volatile and can be achieved on the nanosecond timescale. The combination of GST with the widespread SiN integrated optical waveguide platform led to the proposal of the all-optical integrated phase-change memory, which exploits the interaction of the guided mode evanescent field with a thin layer of GST on the waveguide top surface. The relative simplicity of the architecture allows for its flexible application for data storage, logic gating, arithmetic and neuromorphic computing. Read operation relies on the transmitted signal optical attenuation, due to the GST extinction coefficient. Write/erase operations are performed via the same optical path, with a higher power ad-hoc pulsing scheme, which locally increases the temperature and triggers either the melt-quench process (write) or recrystallization (erase), encoding the information into the GST crystal fraction. Here we investigate the physical mechanisms involved in the write/erase and read processes via computational methods, with the view to explore novel architecture concepts that improve memory speed, energy efficiency and density. We show the achievements of the development of a 3D simulation framework, performing self-consistent calculations for wave- propagation, heat diffusion and phase-transition processes. We illustrate a viable memory optimization route, which adopts sub-wavelength plasmonic dimer nanoantenna structures to harvest the optical energy and maximize light-matter interaction. We calculate both a speed and energy efficiency improvement of around one order of magnitude, with respect to the conventional (non-plasmonic) device architecture. 1. INTRODUCTION Phase-change materials (PCMs) exhibit strong variation in their physical properties between amorphous and crystalline phase, and can be switched between such phases quickly (nanosecond or less) and repeatedly (many billions of times), characteristics that are very desirable for many optical and electronic applications 1–10 , in particular for non-volatile memories 1–3,10–14 . Indeed, both optical (e.g. rewritable DVD and Blu-Ray discs) and electrical (e.g. Intel® Optane) commercially available memory devices exploit PCMs. One of the archetypal PCMs is the Ge2Sb2Te5 alloy (GST), which shows a significant variation of both the complex refractive index and the electrical conductivity between phases 15– 17 . Added to the characteristic transition time on the nanosecond scale and high endurance (i.e. large number of switching cycles), these material properties are of extreme interest for fast and non-volatile photonic applications. Indeed, the combination of GST with the low-loss SiN integrated optical platform led to the proposal of the all-optical integrated phase-change memory 18,19 , which exploits the propagating wave evanescent field interaction with a thin layer of GST deposited on the top surface of the integrated waveguide. In such memory architecture, the phase-state of the GST layer (referred to as optical unit cell) is responsible for the modulation of the waveguide transmission T, which is encoded within the optical cell crystal fraction Χ. The concept is illustrated in Fig. 1(a). The read operation is carried out with a low-power optical probe, whose interaction with the optical cell drives the transmission modulation 20 . Information encoding operations use the same optical path, by delivering relatively high-power pulses. The energy absorbed by the optical cell gives rise to the necessary temperature increase to either trigger GST melting (which is followed by rapid cooling to freeze in the amorphous state), or to induced rapid crystallization (of a previously amorphous cell). More specifically, the amorphization (or write) operation is typically performed by use of a single high-power pulse of short duration (i.e. 1-100 ns) whereas the crystallization (or erase) operation is carried out by