Matthias Wuttig

Dielectric nanoparticles and microparticles show rich optical responses due to their electric and magnetic multipolar Mie modes and low losses. These optical resonances can be modified via the particle size, shape, and dielectric environment. Here we tune the infrared multipolar Mie resonances of dielectric polystyrene microspheres using the plasmonic phase-change material In3SbTe2 (IST). IST exhibits a phase change between a dielectric amorphous phase and a crystalline phase that is metallic in the infrared. We show that a reversibly switchable particle-on-mirror geometry can be created by locally switching the IST under the polystyrene microspheres. Measuring the light-scattering spectra of single polystyrene microspheres before and after crystallization and reamorphization, we observe huge shifts of the resonance wavelength, e.g., from 5.2 to 8.9 μm and back for 6 μm diameter spheres. Our work is a first step toward a controlled tuning of multipolar Mie modes of various particles using reprogrammable IST mirror structures.

Phase‑change materials exhibit pronounced changes in optical and electronic properties between their amorphous and crystalline phases, which are closely related to differences in chemical bonding. Here, we investigate the ultrafast lattice dynamics of GeTe and GeSe thin films using femtosecond pump‐probe transient reflectivity combined with Raman spectroscopy and structural characterization. By comparing amorphous and crystalline phases of both materials, we explore how different bonding mechanisms influence coherent phonon generation following optical excitation. Crystalline GeTe exhibits strong coherent phonon oscillations associated with the A ₁ optical mode, pronounced fluence‑dependent phonon softening, and very short coherent phonon dephasing times, indicating a highly anharmonic and excitation‐sensitive lattice potential. In contrast, amorphous GeTe shows weaker oscillations and only minor fluence dependence. For GeSe, coherent phonons are weak in the crystalline phase and absent in the amorphous phase. Structural relaxation of the amorphous films below the glass transition temperature does not significantly affect the ultrafast response. These observations can be understood in terms of their distinct bonding mechanisms. While GeSe remains covalently bonded in both phases, crystalline GeTe exhibits metavalent bonding that is highly susceptible to electronic excitation. Our findings demonstrate that coherent phonon spectroscopy provides a sensitive probe of bonding in chalcogenide materials.

Optical metasurfaces composed of metallic or dielectric scatterers (meta‐atoms) promise a powerful way of tailoring light‐matter interactions. Phase‐change materials (PCMs) are prime candidates for non‐volatile resonance tuning of metasurfaces based on a refractive index change. Precise resonance control can be achieved by locally applying laser pulses to crystallize a PCM, modifying the dielectric surrounding of meta‐atoms. However, the complex crystallization kinetics of PCMs in the vicinity of metallic meta‐atoms have not been studied yet. Here, we experimentally investigate metallic dimer antennas on top of the PCM Ge ₃ Sb ₂ Te ₆ and address these nanoantennas with laser pulses. Our study reveals inhomogeneous crystallization caused by the absorption and heat conduction of the metallic nanoantennas. A self‐consistent multiphysics model, including electromagnetic, thermal, and phase‐transition processes, is employed to simulate the crystallization and understand the resulting resonance shift of the antennas. Our work paves the way toward complex antenna geometries optimized for local addressing of PCMs to achieve sophisticated crystallization patterns, enabling on‐demand programming of individual nanoantennas within metasurfaces.

Phase Change Materials (PCMs) can be rapidly and reversibly switched between their amorphous and crystalline state; a transition that is accompanied by a pronounced change of optoelectronic properties. Here, progress is reviewed to explain these property changes, focusing on advances by atom probe tomography (APT). This technique classifies bonding by providing two crucial bonding descriptors. Most important is the Probability of Multiple Events (PME), which is related to the likelihood that more than one ion is dislodged per successful laser pulse in laser‐assisted field evaporation. Crystalline PCMs are characterized by a PME above 55%, not found for metals or iono‐covalent solids. This confirms that crystalline PCMs employ a unique bonding mechanism coined metavalent bonding (MVB). While crystalline PCMs employ MVB, amorphous PCMs behave as covalent solids characterized by a much lower PME. PCMs thus change their bonding upon crystallization, consistent with quantum‐chemical calculations of bonding. Crystalline solids with a high PME lie in a narrow conductivity range between metals and iono‐covalent solids, indicative for a competition between electron localization and delocalization. A map quantifying chemical bonding locates metavalent solids in a region where approximately one electron is shared between adjacent atoms and bonding is not too ionic. This quantum chemical bonding map is now used to find and explain property trends relevant for PCMs in various application domains.

The high directionality of hyperbolic phonon polaritons (HPhPs) has opened radically new ways to route and steer the flow of energy at the nanoscale. However, launching HPhPs requires fabricating efficient and precisely aligned polariton launching structures, what remains time‐consuming and expensive with conventional nanofabrication approaches. Recently, using visible laser pulses, polariton launching structures have been programmed into the plasmonic phase‐change material In ₃ SbTe ₂ . Here, we leverage this approach to tailor in‐plane hyperbolic polaritons by programming launching and confining nanostructures through α‐MoO ₃ flakes deposited onto In ₃ SbTe ₂ . Importantly, optical programming after flake deposition enables alignment of launching stripes to the flake's [001]‐axis, essential to control the directional polariton propagation. We showcase i) an optically programmed focusing disk, showing similar tuning ranges and confinement as focusing by gold disks; and ii) a first experimental realization toward a nanocavity for in‐plane HPhPs created by reconfiguring the single disk to a double disk structure, tailoring the confinement by simply reprogramming the disk distance. Our fabrication scheme offers fast turnaround times, flexible alignment, and the opportunity to reversibly reconfigure structures within 30 s. Thus, it is a fast, efficient, and versatile way to tailor propagation and confinement of highly directional polaritons on demand.

We demonstrate an approach for the selective modulation of the A 1 g and 1 E g optical phonon amplitudes in bismuth thin films. Combining polarization-sensitive excitation with controlled pump-pulse delays, this is shown to enable the targeted manipulation of the reflectivity signals associated with distinct phonon modes. Employing an electro-optic detection scheme reveals that the observed changes arise from selective amplification or suppression of specific symmetry components within the detected signal. This combined excitation and detection strategy is broadly applicable to other material systems and may facilitate pathways for ultrafast phononic information processing and nanoscale thermal management, critical for next-generation optoelectronic technologies.

Tailoring chemical bonds offers an innovative way to design materials for a wide range of applications. Metavalent bonding is conducive to excellent thermoelectric performance in p‐bonded chalcogenides with octahedral coordination. However, the requirement to form a bond through only a single p‐electron between adjacent atoms (half of an electron pair), such as in PbTe and Bi ₂ Te ₃ , limits the number of possible materials. Here, it is shown that the essence of metavalent bonding is a half‐filled single‐electron σ‐bond, which can also be formed with a significant s‐orbital contribution. This is illustrated for AgBiSe ₂ , which crystallizes in three different phases: hexagonal, rhombohedral, and cubic. Quantum chemical calculations and bond‐breaking behavior reveal that all three octahedrally coordinated AgBiSe ₂ phases utilize metavalent bonding. In addition, PbTe alloying is used to tune the chemical bonding and Br doping to optimize the carrier concentration. With these modifications, a record‐high zT _max value of 1.1 is achieved in n‐type cubic (AgBiSe ₂ ) _0.75 (PbTe) _0.25 −0.01BiBr ₃ at 798 K. The understanding and tailoring of chemical bonds achieved in AgBiSe ₂ can be easily extended to other AgVVI ₂ compounds.

This perspective explores how film thickness influences the optical and vibrational properties of crystalline chalcogenide and pnictogen thin films. Measurements of the dielectric function reveal systematic property trends within these families of materials. A distinct class, including crystalline bismuth (Bi), germanium telluride (GeTe), and antimony telluride (Sb ₂ Te ₃ ), exhibits strong responses to electromagnetic wave excitation. These materials are characterized by large values of the optical dielectric constant ɛ _∞ and high maximum values of ɛ ₂ (ω). Crystalline GeTe, Bi and Sb ₂ Te ₃ furthermore display an unconventional bond rupture observed via atom probe tomography. They utilize a bonding mechanism (metavalent bonding) that balances electron localization typical in iono‐covalent solids with electron delocalization found in metals. In contrast, crystalline Sb ₂ Se ₃ , In ₂ Te ₃ , and amorphous GeTe neither exhibit these distinctive characteristics nor the bond rupture pattern. Significant changes in optical and vibrational properties occur with varying film thicknesses for the metavalent solids, i.e., crystalline Bi, GeTe, and Sb ₂ Te ₃ . For instance, substantial shifts in phonon frequencies are observed in these solids as thickness decreases, reaching frequency shifts up to 30%. This unusual dependence contrasts with covalent counterparts (In ₂ Te ₃ , amorphous GeTe), which show minimal effects. These findings underscore the potential of metavalent bonding for tailoring materials in optoelectronic applications.