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 kp > 70k0, 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.

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.

Low-loss surface phonon polariton (SPhP) modes supported within polar dielectric crystals are a promising alternative to conventional, metal-based plasmonic systems for the realization of nanophotonic components. Here we show that monopolar excitations in 4H-silicon carbide nanopillar arrays exhibit an unprecedented stable efficiency even when the resonator filling fraction is varied by an order of magnitude. This provides a powerful mid-IR platform with excellent spectral tunability and strong field confinement. Combining IR spectroscopy measurements with full electrodynamic calculations, we elucidate the nature of the optical modes in these elongated subwavelength nanostructures by investigating their spectral behavior and local field dependence on the size and periodicity. The present study also gives a clear understanding and practical guidelines for the spectral tuning of localized SPhP and the coupling mechanisms at play. This work is integral with the development of phonon-polariton based applications for surface-enhanced infrared absorption spectroscopy (SEIRA), polychromatic detectors, and thermal imaging.

Resonant metasurfaces are an attractive platform for enhancing the non-linear optical processes, such as second harmonic generation (SHG), since they can generate very large local electromagnetic fields while relaxing the phase-matching requirements. Here, we take this platform a step closer to the practical applications by demonstrating visible range, continuous wave (CW) SHG. We do so by combining the attractive material properties of gallium phosphide with engineered, high quality-factor photonic modes enabled by bound states in the continuum. For the optimum case, we obtain efficiencies around 5e-5 % W −1 when the system is pumped at 1200 nm wavelength

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.

We use low-cost colloidal lithography with micrometer-sized polystyrene spheres to fabricate arrays of triangular gold microstructures on different infrared-transparent substrates while varying the structures' lateral size. The refractive index n of the substrate in the infrared spectral range can be varied strongly, e.g., from n = 1.4 (calcium fluoride) to n = 4.0 (germanium) for the used materials. Variation of antenna size and substrate material allows us to tune the spectral resonance position of the fabricated antennas from 3 to 13 μm and therefore to cover the absorption bands of the infrared fingerprint region and functional groups. The easy handling and good tunability is demonstrated with surface enhanced infrared absorption (SEIRA) spectroscopy measurements on poly(methyl methacrylate) (PMMA) covered antennas on two different substrate materials (calcium fluoride and silicon) but equal spectral resonance positions of the antennas to ensure the comparability. Additional near-field measurements show that large antennas on a low-index substrate yield stronger local field enhancement compared to smaller antennas on a higher-index substrate, despite the same far-field response.

Nanoantenna arrays with resonances in the mid-infrared spectral range enable a high sensitivity in surface-enhanced infrared absorption spectroscopy. Commonly, multiple antenna arrays with different geometries or surrounding materials have to be fabricated in order to match and enhance different absorption bands of interest. Here, we demonstrate that, by simply changing the angle of incidence, the near-field enhancement of the antenna arrays can be spectrally tuned for maximizing sensitivity for different vibrational modes of surface molecules. Varying the incident angle spectrally shifts the rayleigh anomalies and thus the wavelengths at which collective excitation and the peak field enhancement of the antennas occur. This allows us to tune the antenna array resonance to two adjacent molecular absorption bands without changing the geometry or surrounding material of the antennas. Characteristic Fano lineshapes that alter upon changing the incident angle are observed, and the angle-dependent signal enhancement is analyzed. We gain an improvement of the absorption enhancement by a factor of up to 1.75 compared to the usual angle-averaged measurements.