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New 'Mixed-Mode' Optoelectronic Applications Possibilities using Phase-Change Materials and Devices

To date the main applications of phase-change materials and devices have been limited to the provision of non-volatile memories. Recently, however, the potential has been demonstrated for using a phase-change approach for the provision of entirely new concepts in optoelectronics, including phase-change displays, integrated phase-change photonic memories, optical modulation and optical computing [1-3]. Such novel applications are enabled by the ability of phase-change devices to operate in a 'mixed-mode' configuration, where the excitation is provided electrically and the sensing is carried out optically, or vice-versa. Exploitation of this mixed-mode is made possible in phase-change materials due to the large and simultaneous changes that occur in both refractive index and electrical resistivity on transformation between amorphous and crystalline states. In this paper, based on studies part-funded by the NSF Materials World Network, we present recent results of the use of such mixed-mode operation to provide new applications, including a demonstration of phase-change optoelectronics devices that can be used to make ultrathin all-solid-state colour displays of ultrahigh resolution [1], and hybrid integrated phase-change photonic circuits that offer both a low-power, multi-level memory capability and a computing functionality [2,3]. As so often mentioned by the late (and sadly missed) Stanford Ovhinsky at previous MRS meetings [4], phase-change materials have the potential to provide us with so much more than simple digital memory - a potential that we are now beginning to realize and exploit. [1] P Hosseini, C D Wright and H Bhaskaran, Nature 511, 206 (2014) [2] C Rios , P Hosseini , C D Wright , H Bhaskaran and W H P Pernice, Advanced Materials 26, 1372 (2014) [3] C D Wright, Y Liu, K I Kohary, M M Aziz, R J Hicken, Advanced Materials 23, 3408 (2011) [4] S R Ovshinsky and B Pashmakov, MRS Proceedings 803, 49 (2004

Reconfigurable all-dielectric metalens with diffraction limited performance

Active metasurfaces, whose optical properties can be modulated post-fabrication, have emerged as an intensively explored field in recent years. The efforts to date, however, still face major performance limitations in tuning range, optical quality, and efficiency especially for non mechanical actuation mechanisms. In this paper, we introduce an active metasurface platform combining phase tuning covering the full 2$\pi$ range and diffraction-limited performance using an all-dielectric, low-loss architecture based on optical phase change materials (O-PCMs). We present a generic design principle enabling switching of metasurfaces between two arbitrary phase profiles and propose a new figure-of-merit (FOM) tailored for active meta-optics. We implement the approach to realize a high-performance varifocal metalens operating at 5.2 $\mu$m wavelength. The metalens is constructed using Ge2Sb2Se4Te1 (GSST), an O-PCM with a large refractive index contrast ($\Delta$ n > 1) and unique broadband low-loss characteristics in both amorphous and crystalline states. The reconfigurable metalens features focusing efficiencies above 20% at both states for linearly polarized light and a record large switching contrast ratio of 29.5 dB. We further validated aberration-free imaging using the metalens at both optical states, which represents the first experimental demonstration of a non-mechanical active metalens with diffraction-limited performance