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Phase-change materials (PCMs), which are well-established in optical and random-access memories, are increasingly studied for emerging topics such as brain-inspired computing and active photonics. ...

Mid-infrared (MIR) photonics demands highly confined optical fields to obtain efficient interaction between long-wavelength light and nanomaterials. Surface polaritons excited on polar semiconductor and metallic material interfaces exhibit near-fields localized on subwavelength scales. However, realizing a stronger field concentration in a cavity with a high quality ( Q) factor and a small mode volume is still challenging in the MIR region. This study reports MIR field concentration of surface phonon polaritons (SPhPs) using planar circular cavities with a high Q factor of ∼150. The cavities are fabricated on a thin film of the phase change material Ge

We introduce the Optical Materials Express feature issue on Phase Change Materials for Optics and Photonics. This issue comprises a collection of seventeen manuscripts on the development, characterization, control, and applications of optical Phase Change Materials.

For manipulation of light’s amplitude, phase and polarization, metasurfaces arose as promising candidates with capabilities beyond those of classical optical components. Once fabricated, however, metasurfaces have set optical properties and are thus passive . Active metasurfaces are made out of nanostructures whose optical properties can be externally manipulated. 1 To tune e.g . the optical environment of the nanostructures, phase-change materials (PCMs) became popular due to their high optical contrast and non-volatility. PCMs have been employed as surrounding medium for plasmonic resonators and studies employing PCMs as all-dielectric metasurfaces are promising. The switching of PCMs by optical means lead to successful patterning in the µm-regime. 2 Bypassing the limits of diffraction imposed by optical switching, we employ a powerful thermal scanning probe lithography (t-SPL) technique to control position, depth and size of the switched PCM. First theoretical studies and metasurface designs for metasurfaces enabled by t-SPL could have exciting applications in the near infrared spectral range. 3

Metasurfaces allow for the spatiotemporal variation of amplitude, phase, and polarization of optical wavefronts. Implementation of active tunability of metasurfaces promises compact flat optics capable of reconfigurable wavefront shaping. Phase-change materials (PCMs), such as germanium telluride or germanium antimony telluride, are a prominent material class enabling reconfigurable metasurfaces due to their large refractive index change upon structural transition. However, commonly employed laser-induced switching of PCMs limits the achievable feature sizes and thus, restricts device miniaturization. Here, we propose thermal scanning-probe-induced local switching of germanium telluride to realize near-infrared metasurfaces with feature sizes far below what is achievable with diffraction-limited optical switching. Our design is based on a planar multilayer stack and does not require fabrication of protruding dielectric or metallic resonators as commonly applied in the literature. Instead, we numerically demonstrate that a broad-band tuning of perfect absorption could be realized by the localized and controlled tip-induced crystallization of the PCM layer. The spectral response of the metasurface is explained using simple resonance mode analysis and numerical simulations. To facilitate experimental realization, we provide a detailed theoretical description of the tip-induced crystallization employing multiphysics simulations to demonstrate the great potential for fabricating compact reconfigurable metasurfaces. Our concept allows for tunable perfect absorption and can be applied not only for thermal imaging or sensing, but also for spatial frequency filtering., Comment: 7 pages, 4 figures

Germanium telluride (GeTe), a phase-change material, is known to exhibit four different structural phases: three at room temperature (one amorphous and two crystalline, $\alpha$ and $\gamma$) and one at high temperature (crystalline $\beta$). Because transitions between the amorphous and crystalline phases lead to significant changes in material properties (e.g., refractive index and resistivity), GeTe has been investigated as a phase-change material for photonics, thermoelectrics, ferroelectrics, and spintronics. Consequently, the temperature-dependent phase transitions in GeTe have been studied for bulk and thin-film GeTe, both fabricated by sputtering. Colloidal synthesis of nanoparticles offers a more flexible fabrication approach for amorphous and crystalline GeTe. These nanoparticles are known to exhibit size-dependent properties, such as an increased crystallization temperature for the amorphous-to-$\alpha$ transition in sub-10\,nm GeTe particles. The $\alpha$-to-$\beta$ phase transition is also expected to vary with size, but this effect has not yet been investigated for GeTe. Here, we report time-resolved X-ray diffraction of GeTe nanoparticles with different diameters and from different synthetic protocols. We observe a non-volatile amorphous-to-$\alpha$ transition between 210$^{\circ}$C and 240$^{\circ}$C and a volatile $\alpha$-to-$\beta$ transition between 370$^{\circ}$C and 420$^{\circ}$C. The latter transition was reversible and repeatable. While the transition temperatures are shifted relative to the values known for bulk GeTe, the nanoparticle-based samples still exhibit the same structural phases reported for sputtered GeTe. Thus, colloidal GeTe maintains the same general phase behavior as bulk GeTe while allowing for more flexible and accessible fabrication. Therefore, nanoparticle-based GeTe films show great potential for applications, such as in active photonics.

Programmed broadband fine-tuning of hybrid metasurface resonances in the infrared spectral range is realized by applying nanosecond laser pulses to a phase-change material thin film covering metallic nanostructures and taking advantage of the local resonator’s field.

Recently, phase-change materials (PCMs) have gained a lot of interest in the field of active metamaterials and plasmonics due to their switchable optical properties. In the infrared spectral range the huge contrast in the refractive index between an amorphous and a crystalline phase can be employed for nonvolatile tuning of nanoantenna or metasurface resonances. To make use of such concepts in devices, the reversible switching of the active material has to be realized. Here we demonstrate such reversible cycling by applying femtosecond pulses from a Ti:sapphire laser. These optical pulses trigger the phase transitions of the PCM thin film, which is covering infrared nanoantennas. Ge3Sb2Te6 is chosen as the PCM, since it offers very low losses in the infrared spectral range. The layer geometry presented is exceptionally thin (∼1/50 of the operating wavelength) and the design intentionally avoids lossy capping layers. Infrared reflectivity measurements verify the laser-induced resonance shifts of the plasmo...

Advanced materials 31(29), 1901033 (2019). doi:10.1002/adma.201901033, Published by Wiley-VCH, Weinheim

We show tuning of the resonance frequency of aluminum nanoantennas via variation of the refractive index n of a layer of phase-change material. Three configurations have been considered, namely, with the antennas on top of, inside, and below the layer. Phase-change materials offer a huge index change upon the structural transition from the amorphous to the crystalline state, both stable at room temperature. Since the imaginary part of their permittivity is negligibly small in the mid-infrared spectral range, resonance damping is avoided. We present resonance shifting to lower as well as to higher wavenumbers with a maximum shift of 19.3% and a tuning figure of merit, defined as the resonance shift divided by the full-width at half-maximum (FWHM) of the resonance peak, of 1.03.

Surface phonon-polaritons (SPhPs), collective excitations of photons coupled with phonons in polar crystals, enable strong light-matter interaction and numerous infrared nanophotonic applications. However, as the lattice vibrations are determined by the crystal structure, the dynamical control of SPhPs remains challenging. Here, we realize the all-optical, non-volatile, and reversible switching of SPhPs by controlling the structural phase of a phase-change material (PCM) employed as a switchable dielectric environment. We experimentally demonstrate optical switching of an ultrathin PCM film (down to 7 nm,λ/1,200) with single laser pulses and detect ultra-confined SPhPs (polariton wavevector kp70k0, k0 = 2π/λ) in quartz. Our proof of concept allows the preparation of all-dielectric, rewritable SPhP resonators without the need for complex fabrication methods. With optimized materials and parallelized optical addressing we foresee application potential for switchable infrared nanophotonic elements, for example, imaging elements such as superlenses and hyperlenses, as well as reconfigurable metasurfaces and sensors.

Active control over the handedness of a chiral metamaterial has the potential to serve as key element for highly integrated polarization engineering approaches, polarization sensitive imaging devices, and stereo display technologies. However, this is hard to achieve as it seemingly involves the reconfiguration of the metamolecule from a left-handed into a right-handed enantiomer and vice versa. This type of mechanical actuation is intricate and usually neither monolithically realizable nor viable for high-speed applications. Here, enabled by the phase change material Ge3Sb2Te6 (GST-326), we demonstrate a tunable and switchable mid-infrared plasmonic chiral metamaterial in a proof-of-concept experiment. A large tunability range of the circular dichroism response from λ = 4.15 to 4.90 μm is achieved, and we experimentally demonstrate that the combination of a passive bias-type chiral layer with the active chiral metamaterial allows for switchable chirality, that is, the reversal of the circular dichroism sign, in a fully planar, layered design without the need for geometrical reconfiguration. Because phase change materials can be electrically and optically switched, our designs may open up a path for highly integrated mid-IR polarization engineering devices that can be modulated on ultrafast time scales.

A switchable perfect absorber with multispectral thermal imaging capability is presented. Aluminum nanoantenna arrays above a germanium antimony telluride (GST) spacer layer and aluminum mirror provide efficient wavelength-tunable absorption in the mid-infrared. Utilizing the amorphous-to-crystalline phase transition in GST, this device offers switchable absorption with strong reflectance contrast at resonance and large phase-change-induced spectral shifts.

Phase-change materials (PCMs) are a promising platform for programmable metamaterials in the infrared (IR) spectral range. This is based on the large refractive index change between the amorphous and crystalline phases of PCMs accompanied by their low dielectric losses in the IR. These PCMs can exist in various stoichiometries, leading to distinct changes in dielectric properties which can be exploited for resonance tuning. To investigate these differences, nanoantennas with thin films of three (GeTe)x–(Sb2Te3)1−x compounds such as Ge2Sb2Te5 (GST-225), Ge3Sb2Te6 (GST-326), and Ge8Sb2Te11 (GST-8211) are combined. Different PCM stoichiometries regarding their suitability for resonance tuning are systematically compared for the first time and the PCM GST-8211 is introduced into the field of photonics. An increase of the antenna resonance frequency shift based on an increasing GeTe content in the compound is experimentally demonstrated. This leads to the largest figure of merit for antenna resonance tuning in the IR using PCMs reported so far. Based on the insights obtained, the changes in the real part of the permittivity Δe1, the band gap EG, and the charge carrier concentration N are identified as design parameters for resonance tuning with PCMs.

Polymers 8(12), 413 - (2016). doi:10.3390/polym8120413, Published by MDPI, Basel

Phase Change Materials (PCM) show two stable states in the solid phase with significantly different optical and electronic properties. They can be switched reversibly between those two states and are promising candidates for future non-volatile memory applications. The development of phase change devices demands characterization tools, yielding information about the switching process at high spatial resolution. Scattering-type Scanning Near-field Optical Microscopy (s-SNOM) allows for spectroscopic analyses of the different optical properties of the PCMs on the nm-scale. By correlating the optical s-SNOM images with transmission electron microscopy images of the same sample, we unambiguously demonstrate the correlation of the infrared optical contrast with the structural state of the phase change material. The investigated sample consists of sandwiched amorphous and crystalline regions of Ag4In3Sb67Te26 below a 100 nm thick (ZnS)80−(SiO2)20 capping layer. Our results demonstrate the sensitivity of s-SNOM ...

Optics express 23(20), 25487-25495 (2015). doi:10.1364/OE.23.025487, Published by Optical Society of America, Washington, DC