Low-loss nanostructured dielectric metasurfaces have emerged as a breakthrough platform for ultrathin optics and cutting-edge photonic applications, including beam shaping, focusing, and holography. However, the static nature of their constituent materials has traditionally limited them to fixed functionalities. Tunable all-dielectric infrared Huygens' metasurfaces consisting of multi-layer Ge disk meta-units with strategically incorporated non-volatile phase change material Ge 3 Sb 2 Te 6 are introduced. Switching the phase-change material between its amorphous and crystalline structural state enables nearly full dynamic light phase control with high transmittance in the mid-IR spectrum. The metasurface is realized experimentally, showing postfabrication tuning of the light phase within a range of 81% of the full 2π phase shift. Additionally, the versatility of the tunable Huygen's metasurfaces is demonstrated by optically programming the spatial light phase distribution of the metasurface with single meta-unit precision and retrieving high-resolution phase-encoded images using hyperspectral measurements. The programmable metasurface concept overcomes the static limitations of previous dielectric metasurfaces, paving the way for "universal" metasurfaces and highly efficient, ultracompact active optical elements like tunable lenses, dynamic holograms, and spatial light modulators.

The high dielectric optical contrast between the amorphous and crystalline structural phases of non-volatile phase-change materials (PCMs) provides a promising route towards tuneable nanophotonic devices. Here, we employ the next-generation PCM In3SbTe2 (IST) whose optical properties change from dielectric to metallic upon crystallization in the whole infrared spectral range. This distinguishes IST as a switchable infrared plasmonic PCM and enables a programmable nanophotonics material platform. We show how resonant metallic nanostructures can be directly written, modified and erased on and below the meta-atom level in an IST thin film by a pulsed switching laser, facilitating direct laser writing lithography without need for cumbersome multi-step nanofabrication. With this technology, we demonstrate large resonance shifts of nanoantennas of more than 4 µm, a tuneable mid-infrared absorber with nearly 90% absorptance as well as screening and nanoscale “soldering” of metallic nanoantennas. Our concepts can empower improved designs of programmable nanophotonic devices for telecommunications, (bio)sensing and infrared optics, e.g. programmable infrared detectors, emitters and reconfigurable holograms.

Nanometer‐thick active metasurfaces (MSs) based on phase‐change materials (PCMs) enable compact photonic components, offering adjustable functionalities for the manipulation of light, such as polarization filtering, lensing, and beam steering. Commonly, they feature multiple operation states by switching the whole PCM fully between two states of drastically different optical properties. Intermediate states of the PCM are also exploited to obtain gradual resonance shifts, which are usually uniform over the whole MS and described by effective medium response. For programmable MSs, however, the ability to selectively address and switch the PCM in individual meta‐atoms is required. Here, simultaneous control of size, position, and crystallization depth of the switched phase‐change material (PCM) volume within each meta‐atom in a proof‐of‐principle MS consisting of a PCM‐covered Al–nanorod antenna array is demonstrated. By modifying optical properties locally, amplitude and light phase can be programmed at the meta‐atom scale. As this goes beyond previous effective medium concepts, it will enable small adaptive corrections to external aberrations and fabrication errors or multiple complex functionalities programmable on the same MS.

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 Ge3Sb2Te6 (GST) deposited on a silicon carbide (SiC) substrate. Scattering-type scanning near-field optical microscopy visualizes the near-field distribution on the samples and confirms directly that the SPhP field is strongly concentrated at the center of the centrosymmetric cavities. The smallest concentrated field size is 220 nm in diameter which corresponds to 1/50 of the wavelength of the incident light that is far below the diffraction limit. The thin GST film enhances the SPhP confinement, and it is used to switch the confinement off by tuning the cavity resonance induced by the phase change from the amorphous to the crystalline phase. This subwavelength and switchable field concentration within a high-Q polariton cavity has the potential to greatly enhance the light-matter interaction for molecular sensing and emission enhancement in MIR systems.

enables applications in optical components such as anti-reflective coatings, [1] optical filters, [2] and optical absorbers. [3] Generally, their optical properties can be tuned by designing the structural geometries and materials involved, which remain fixed after fabrication. Using phase-change materials (PCMs) and phase-transition materials (PTMs) as active optical layers provides the optical components with new features of switchable optical properties. PCMs have at least two (meta-)stable phases, that is, one amorphous phase and one or more crystalline phases with high contrast in their electrical and optical properties. [4] A PCM can be crystallized by a thermal, optical, or electrical stimulus which heats it above its glass transition temperature T g . The phase change is non-volatile, which means the switched phase is maintained even after the switching stimulus ends. [4,5] Common PCMs like GeTe and GeSbTe-derivatives are, for example, applied in rewritable data storage devices. [6][7][8] In contrast, PTMs also have a temperature-stimulated, but volatile phase transition, where the switched phase is only maintained at temperatures above its transition temperature (T c ). [9] These materials automatically change back to the original phases when cooled down below T c . Therefore, their optical and electrical properties change reversibly with the temperature. A commonly used example of PTMs is vanadium dioxide (VO 2 ), which undergoes a semiconductor-metal Phase-change materials (PCMs) and phase-transition materials (PTMs) both show a large contrast in their respective optical properties upon switching, enabling compact optical components with diverse functionalities like sensing, thermal imaging, and data recording. However, their switching properties differ significantly, that is, the switching is non-volatile for PCMs while volatile for PTMs. Here, new-generation smart mid-infrared modulators with switchable transmission, reflection, and absorption are demonstrated conceptually and experimentally, which combine one PCM (Ge 3 Sb 2 Te 6 or In 3 SbTe 2 ) with one PTM (VO 2 ) as two active layers. The bottom VO 2 layer is employed as a thermally regulated (modulated) dynamic mirror, facilitating the switching of transmission between "on" state (using VO 2 in its semiconducting state at temperatures below its phase transition temperature T c ) and "off" state (metallic VO 2 at temperatures above T c ). The PCM layer on top of the metallic VO 2 layer is used either for continuously adjusting the absorption peak spectrally (by up to 1.8 µm using different phases of Ge 3 Sb 2 Te 6 ) or for switching between absorption mode (A = 0.99 with amorphous In 3 SbTe 2 ) and reflection mode (R = 0.85 with crystalline In 3 SbTe 2 ). The presented concept of merging static, non-volatile thermal switching (via PCMs) with dynamic, volatile thermal modulation (via PTMs) empowers a new generation of optical devices for smart optical switching, for example in spectrally tunable safety optical switches.