Optical phase change materials (O-PCMs), a unique group of materials featuring exceptional optical property contrast upon a solid-state phase transition, have found widespread adoption in photonic applications such as switches, routers and reconfigurable meta-optics. Current O-PCMs, such as Ge–Sb–Te (GST), exhibit large contrast of both refractive index (Δn) and optical loss (Δk), simultaneously. The coupling of both optical properties fundamentally limits the performance of many applications. Here we introduce a new class of O-PCMs based on Ge–Sb–Se–Te (GSST) which breaks this traditional coupling. The optimized alloy, Ge2Sb2Se4Te1, combines broadband transparency (1–18.5 μm), large optical contrast (Δn = 2.0), and significantly improved glass forming ability, enabling an entirely new range of infrared and thermal photonic devices. We further demonstrate nonvolatile integrated optical switches with record low loss and large contrast ratio and an electrically-addressed spatial light modulator pixel, thereby validating its promise as a material for scalable nonvolatile photonics. Here, the authors introduce optical phase change materials based on Ge-Sb-Se-Te which breaks the coupling between refractive index and optical loss allowing low-loss performance benefits. They demonstrate low losses in nonvolatile photonic circuits and electrical pixelated switching have been demonstrated.

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Broadband transparent optical phase change materials for high-performance nonvolatile photonics.

This work was supported by National Basic Research Program of China
(Project No. 2013CB933301) and National Natural Science Foundation of
China (Project No. 51272038). L.V.B. was supported by China Postdoctoral
Science Foundation. A.O.G. was supported by the Volkswagen
Foundation (Germany) and via the Chang Jiang (Yangtze River)
Chair Professorship (China). G.P.W. acknowledges support from the
Center for Nanoscale Materials, a U.S. Department of Energy Office of
Science User Facility, and support by the U.S. Department of Energy,
Office of Science, under Contract No. DE-AC02-06CH11357. In addition,
the authors acknowledge financial support obtained from the Virtual
Institute for Theoretical Photonics and Energy. This review is part of the
Advanced Optical Materials Hall of Fame article series, which recognizes
the excellent contributions of leading researchers to the field of optical
materials science.

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Broadband Metamaterial Absorbers

We have observed size effects in the structural phase transition of submicron vanadium dioxide precipitates in silica. The VO 2 nanoprecipitates are produced by the stoichiometric coimplantation of vanadium and oxygen andsubsequent thermal processing. The observed size dependence in the transition temperature and hysteresis loops of the semiconductor-to-metal phase transition in VO 2 is described in terms of heterogeneous nucleation statistics with a phenomenological approach in which the density of nucleating defects is a power function of the driving force.

Size effects in the structural phase transition of VO2 nanoparticles

Nanophotonics has garnered intensive attention due to its unique capabilities in molding the flow of light in the subwavelength regime. Metasurfaces (MSs) and photonic integrated circuits (PICs) enable the realization of mass-producible, cost-effective, and highly efficient flat optical components for imaging, sensing, and communications. In order to enable nanophotonics with multi-purpose functionalities, chalcogenide phase-change materials (PCMs) have been introduced as a promising platform for tunable and reconfigurable nanophotonic frameworks. Integration of non-volatile chalcogenide PCMs with unique properties such as drastic optical contrasts, fast switching speeds, and long-term stability grants substantial reconfiguration to the more conventional static nanophotonic platforms. In this review, we discuss state-of-the-art developments as well as emerging trends in tunable MSs and PICs using chalcogenide PCMs. We outline the unique material properties, structural transformation, electro-optic, and thermo-optic effects of well-established classes of chalcogenide PCMs. The emerging deep learning-based approaches for the optimization of reconfigurable MSs and the analysis of light-matter interactions are also discussed. The review is concluded by discussing existing challenges in the realization of adjustable nanophotonics and a perspective on the possible developments in this promising area.

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Tunable nanophotonics enabled by chalcogenide phase-change materials

The ability to control thermal radiation is of fundamental importance for a wide range of applications. Nanophotonic structures, where at least one of the structural features are at a wavelength or sub-wavelength scale, can have thermal radiation properties that are drastically different from conventional thermal emitters, and offer exciting opportunities for energy applications. Here we review recent developments of nanophotonic control of thermal radiation, and highlight some exciting energy application opportunities, such as daytime radiative cooling, thermal textile, and thermophotovoltaic systems that are enabled by nanophotonic structures.

Nanophotonic control of thermal radiation for energy applications [Invited].

Camouflage technology has attracted growing interest for many thermal applications. Previous experimental demonstrations of thermal camouflage technology have not adequately explored the ability to continuously camouflage objects either at varying background temperatures or for wide observation angles. In this study, a thermal camouflage device incorporating the phase-changing material Ge2Sb2Te5 (GST) is experimentally demonstrated. It has been shown that near-perfect thermal camouflage can be continuously achieved for background temperatures ranging from 30 °C to 50 °C by tuning the emissivity of the device, which is attained by controlling the GST phase change. The thermal camouflage is robust when the observation angle is changed from 0° to 60°. This demonstration paves the way toward dynamic thermal emission control both within the scientific field and for practical applications in thermal information.

Thermal camouflage based on the phase-changing material GST

Dynamic thermal emission control has attracted growing interest in a broad range of fields, including radiative cooling, thermophotovoltaics and adaptive camouflage. Previous demonstrations of dynamic thermal emission control present disadvantages of either large thickness or requiring sustained electrical or thermal excitations. In this paper, an ultrathin (∼0.023λ, λ is the emission peak wavelength) metal-insulator-metal plasmonic metamaterial-based zero-static-power mid-infrared thermal emitter incorporating phase-changing material GST is experimentally demonstrated to dynamically control the thermal emission. The electromagnetic modes can be continuously tuned through the intermediate phases determined by controlling the temperature. A typical resonance mode, which involves the coupling between the high-order magnetic resonance and anti-reflection resonance, shifts from 6.51 to 9.33 μm while GST is tuned from amorphous to crystalline phase. This demonstration will pave the way towards the dynamical thermal emission control in both the fundamental science field and a number of energy-harvesting applications.

Dynamic Thermal Emission Control Based on Ultrathin Plasmonic Metamaterials Including Phase‐Changing Material GST

Controlling the emissivity of a thermal emitter has attracted growing interest, with a view toward a new generation of thermal emission devices. To date, all demonstrations have involved using sustained external electric or thermal consumption to maintain a desired emissivity. In the present study, we demonstrated control over the emissivity of a thermal emitter consisting of a film of phase-changing material Ge2Sb2Te5 (GST) on top of a metal film. This thermal emitter achieves broad wavelength-selective spectral emissivity in the mid-infrared. The peak emissivity approaches the ideal blackbody maximum, and a maximum extinction ratio of >10 dB is attainable by switching the GST between the crystalline and amorphous phases. By controlling the intermediate phases, the emissivity can be continuously tuned. This switchable, tunable, wavelength-selective and thermally stable thermal emitter will pave the way toward the ultimate control of thermal emissivity in the field of fundamental science as well as for energy harvesting and thermal control applications, including thermophotovoltaics, light sources, infrared imaging and radiative coolers. The use of phase-change materials can provide tunable control over the emissivity of a thermal emitter. This discovery, made by Kaikai Du and co-workers at Zhejiang University in China, could be useful for applications in thermophotovoltaics, infrared imaging and radiative cooling. The team coated gold substrates with thin layers of the phase-change material Ge2Sb2Te5 (GST). They found that the emissivity of these samples changed with the thickness of the GST layer and the sample temperature. Specifically, the wavelength of the emissivity peak shifted from around 9 to 13 micrometres as the thickness of the GST layer increased from 360 to 540 nanometres. Furthermore, gradually changing the temperature of the GST layer to switch it between its amorphous and crystalline states provided continuous control over the emissivity.

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Control over emissivity of zero-static-power thermal emitters based on phase-changing material GST.

We propose a broadband, efficient, ultra-thin metal-insulator-metal (MIM) absorber with a simple single-sized disk configuration by utilizing metals with high imaginary part of permittivity (e″). The physics behind this is that field dissipation is remarkably enhanced in MIM absorbers with high-e″ metals, significantly extending the absorption bandwidths, which are conventionally limited by magnetic resonances of MIM absorbers with low-e″ metals. The experimentally demonstrated MIM absorber based on tungsten with high-e″ yields broadband absorption from visible to near-infrared range (400–1700 nm) with an average measured absorption of 84%. The ultra-thin and single-sized nanostructure with broadband efficient absorption facilitates the scalability to large-area photonic applications.

Broadband optical absorption based on single-sized metal-dielectric-metal plasmonic nanostructures with high-ε″ metals

Two spatially and spectrally resolved narrowband absorbers based on 2D grating nanostructures (polymethylmethacrylate (PMMA) grating and gold grating) on metallic films are designed, fabricated, and characterized. For PMMA grating on a metallic film, the measured absorption bandwidth is 12 nm with a nearly 80% absorption at normal incidence. The transverse electric (TE) localized mode shows a calculated 3.5° angular width. The transverse magnetic (TM) resonant mode supports both a localized mode and a lattice mode. The TM lattice mode shows a calculated angular width of 0.75° and exhibits a large wavelength-angle sensitivity of nearly 15 nm per degree and a large absorption-angle sensitivity of 77.5% per degree. For gold grating on a metallic film, the measured absorption bandwidth is 40 nm with a nearly 70% absorption. Such spatially and spectrally resolved 2D nanostructure arrays with large angle sensitivities have potential applications in angle measurement, thermal emitters, optical filters, and biosensors.

Kaikai Du

Papers

Thermal camouflage based on the phase-changing material GST

Dynamic Thermal Emission Control Based on Ultrathin Plasmonic Metamaterials Including Phase‐Changing Material GST

Control over emissivity of zero-static-power thermal emitters based on phase-changing material GST.

Broadband optical absorption based on single-sized metal-dielectric-metal plasmonic nanostructures with high-ε″ metals

Spatially and Spectrally Resolved Narrowband Optical Absorber Based on 2D Grating Nanostructures on Metallic Films