Phase-change materials for photonic memories and optoelectronic applications

• Main Author: Ocampo, Carlos Andrés Ríos
• Format: Dissertation
• Language: English
• Published: University of Oxford 2016
• Subjects: Integrated photonics; Non-volatile memories; Phase-change chalcogenide photonics; Phase-change materials; Phase-change nanodisplay; Phase-change photonics; Photonic Materials; Photonic memories

The content of this thesis encompasses the fundamentals, modelling, chip design, nanofabrication process, measurement setup, and experimental results of devices exploiting the optical properties of phase-change chalcogenide materials. Special attention is paid to integrated Si3N4 nanophotonic circuits for optical switching and memory applications, as well as to multilayer stacks for colour modulation. Herein, the implementation of the first robust, non-volatile, phase-change photonic memory is presented. By utilising optical near-field effects for Read, Write and Erase operations, bit storage of up to eight transmission levels is demonstrated in a single device employing Ge2Sb2Te5 as the active material. These on-chip memory cells feature single-shot read-out of the transmission state and switching energies as low as 13.4pJ at speeds approaching 1GHz. The capability to readily switch between intermediate states is also demonstrated, a feature that requires complex iteration-based algorithms in electronic phase-change memories. This photonic memory is not only the first truly non-volatile memory---a long-term elusive goal in integrated photonics---but could also potentially represent the first multi-level memory, including electronic counterparts, that requires no computational post-processing or drift correction. These findings provide a pathway towards solving the throughput limitations of current computer architectures by eliminating the so-called von-Neumann bottleneck and portend a new paradigm in all-photonic memory, non-conventional computing, and tunable photonic devices. Finally, novel capabilities in electro-optic colour modulation using phase-change materials are demonstrated. In particular, this thesis offers the first implementation of Ag3In4Sb76Te17-based optical cavities for colour modulation on low-dimensional multilayer stacks. Moreover, "gray-scale" image writing is demonstrated by establishing intermediate levels of crystallisation via voltage modulation. This finding, in turn, corresponds to the first demonstration of nonvolatile colour-depth modulation in the emerging phase-change materials nanodisplay technology, featuring resolutions down to 50nm. Furthermore, a comprehensive comparison is carried out for two types of materials: growth- (Ag3In4Sb76Te17) and nucleation-dominated (Ge2Sb2Te5) alloys in terms of colour, energy efficiency, and resolution. These results provide new tools for the new generation of bistable and ultra-high-resolution displays and smart glasses while allowing for other potential applications in photonics and optoelectronics.