difference in their electrical and optical properties between their amorphous and crystalline phases. Moreover, they can be switched (optically, electrically, or thermally) between phases reversibly (potentially >10 15 cycles) and quickly (nanoseconds or faster). [1][2][3] Both phases (and indeed intermediate phases between fully crystalline and fully amorphous) are also stable at room temperature for a time on the order of years. [4,5] All these properties have made phasechange materials extremely attractive for commercial data storage technologies, in the form of rewritable optical disks and nonvolatile electronic memories. [6][7][8] More recently, as a result of the rather unique properties that phase-change materials possesses, their use has been extended to a number of exciting emerging applications including neuromorphic computing, [9,10] integrated photonic memories [11,12] and, the focus of this work, reconfigurable optical metamaterials/metasurfaces, [13][14][15][16][17][18][19][20][21][22] which we here exploit for the realization of a new form of nonvolatile color display.Optical metasurfaces have great potential to generate color, and several different structures suited to this task have been suggested in the literature. [23][24][25][26][27][28][29][30][31] A common approach is to utilize metallic (or metal-dielectric) nanorods [26,27,30,31] or other lithographically patterned metal-dielectric nanostructures [24,28,29] that generate structural (i.e., noncolorant) color using plasmonic effects. Such approaches are in general though "fixed-by-design," meaning that colors and images are essentially written permanently into the metasurface by the specific nanostructures used. For display and electronic signage applications, however, the ability to change the displayed image or information in real time is required. Here we provide just such a capability by combining a metal-insulator-metal (MIM) resonant absorber type optical metasurface [32,33] with a thin layer of chalcogenide phase-change material (PCM), so providing the key attributes of nonvolatile color generation and dynamic reconfigurability, the latter achieved by turning the MIM resonance "on" and "off" by switching the PCM-layer between its crystalline and amorphous states. Nonvolatility is a particularly attractive feature offered by phase-change based displays, since no power is needed to retain an image once it is written into the phase-change layer/pixels. [34][35][36][37] Moreover, the displays can work using only ambient (natural or artificial) light, which can Chalcogenide phase-change materials, which exhibit a marked difference in their electrical and optical properties when in their amorphous and crystalline phases and can be switched between these phases quickly and repeatedly, are traditionally exploited to deliver nonvolatile data storage in the form of rewritable optical disks and electrical phase-change memories. However, exciting new potential applications are now emerging in areas such as integrated phase-change photonics, phase...

Metasurfaces and nanoantennas are redefining what can be achieved in terms of optical beam manipulation, as they provide a versatile design platform towards moulding the flow of light at will. Yet, once a conventional metasurface is designed and realised, its effect on optical beams is repeatable and stationary, thus its performance is 'locked-in' at the fabrication stage. A much wider range of applications, such as dynamic beam steering, reconfigurable and dynamic lensing, optical modulation and reconfigurable spectral filtering, could be achieved if real-time tuning of metasurface optical properties were possible. Chalcogenide phase-change materials, because of their rather unique ability to undergo abrupt, repeatable and non-volatile changes in optical properties when switched between their amorphous and crystalline phases, have in recent years been combined with metasurface architectures to provide a promising platform for the achievement of dynamic tunability. In this paper, the concept of dynamically tunable phase-change metasurfaces is introduced, and recent results spanning the electromagnetic spectrum from the visible right through to the THz regime are presented and discussed. The progress, potential applications, and possible future perspectives of phase-change metasurface technology are highlighted, and requirements for the successful implementation of real-world applications are discussed.

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We propose a reconfigurable and non-volatile Bragg grating in the telecommunication C-band based on the combination of novel low-loss phase-change materials (specifically Ge2Sb2Se4Te1 and Sb2S3) with a silicon nitride platform. The Bragg grating is formed by arrayed cells of phase-change material, whose crystallisation fraction modifies the Bragg wavelength and extinction ratio. These devices could be used in integrated photonic circuits for optical communications applications in smart filters and Bragg mirrors and could also find use in tuneable ring resonators, Mach–Zehnder interferometers or frequency selectors for future laser on chip applications. In the case of Ge2Sb2Se4Te1, crystallisation produces a Bragg resonance shift up to ∼ 15 nm, accompanied with a large amplitude modulation (insertion loss of 22 dB). Using Sb2S3, low losses are presented in both states of the phase change material, obtaining a ∼ 7 nm red-shift in the Bragg wavelength. The gratings are evaluated for two period numbers, 100 and 200 periods. The number of periods determines the bandwidth and extinction ratio of the filters. Increasing the number of periods increases the extinction ratio and reflected power, also narrowing the bandwidth. This results in a trade-off between device size and performance. Finally, we combine both phase-change materials in a single Bragg grating to provide both frequency and amplitude modulation. A defect is introduced in the Sb2S3 Bragg grating, producing a high quality factor resonance (Q ∼ 104) which can be shifted by 7 nm via crystallisation. A GSST cell is then placed in the defect which can modulate the transmission amplitude from low loss to below -16 dB.

This research aims to understand the fundamental aspects of annealing on the electrochromic performance of tungsten oxides, using as-synthesized W18O49 substoichiometric bundled nanowires benchmarked against commercial WO3 nanoparticles. Linking detailed structural analyses with the electrochromic measurement results, we have investigated the electrochromic performance effects of low temperature annealing, up to 350 °C, on tungsten oxide (WO x ) thin films, trying to establish the fundamental heat treatment–structure–performance loop. We have found that the annealing treatment at low temperature improved the optical modulation and long-term durability of the WO x thin films, without changing the structure and morphology of the as-synthesized samples. The 350 °C annealing was found to have the best stability improvement for the WO3 nanoparticle films during the electrochromic assessments, with a 4% improvement for Li+ intercalation and a 12% improvement for deintercalation, compared with the untreated WO3 samples. Further improvements have been achieved for the W18O49 nanowire thin films, with a stability improvement of 36% for Li+ intercalation and 60% for deintercalation against the as-prepared W18O49 nanowire samples during the electrochromic performance testing.