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Nanostructured In3SbTe2 antennas enable switching from sharp dielectric to broad plasmonic resonances
Phase-change materials (PCMs) allow for non-volatile resonance tuning of nanophotonic components. Upon switching, they offer a large dielectric contrast between their amorphous and crystalline phases. The recently introduced “plasmonic PCM” In3SbTe2 (IST) additionally features in its crystalline phase a sign change of its permittivity over a broad infrared spectral range. While optical resonance switching in unpatterned IST thin films has been investigated before, nanostructured IST antennas have not been studied, yet. Here, we present numerical and experimental investigations of nanostructured IST rod and disk antennas. By crystallizing the IST with microsecond laser pulses, we switched individual antennas from narrow dielectric to broad plasmonic resonances. For the rod antennas, we demonstrated a resonance shift of up to 1.2 µm (twice the resonance width), allowing on/off switching of plasmonic resonances with a contrast ratio of 2.7. With the disk antennas, we realized an increase of the resonance width by more than 800% from 0.24 µm to 1.98 µm while keeping the resonance wavelength constant. Further, we demonstrated intermediate switching states by tuning the crystallization depth within the resonators. Our work empowers future design concepts for nanophotonic applications like active spectral filters, tunable absorbers, and switchable flat optics.
Ordered Peierls distortion prevented at growth onset of GeTe ultra-thin films
Using reflection high-energy electron diffraction (RHEED), the growth onset of molecular beam epitaxy (MBE) deposited germanium telluride (GeTe) film on Si(111)-(√3 × √3)R30°-Sb surfaces is investigated, and a larger than expected in-plane lattice spacing is observed during the deposition of the first two molecular layers. High-resolution transmission electron microscopy (HRTEM) confirms that the growth proceeds via closed layers, and that those are stable after growth. The comparison of the experimental Raman spectra with theoretical calculated ones allows assessing the shift of the phonon modes for a quasi-free-standing ultra-thin GeTe layer with larger in-plane lattice spacing. The manifestation of the latter phenomenon is ascribed to the influence of the interface and the confinement of GeTe within the limited volume of material available at growth onset, either preventing the occurrence of Peierls dimerization or their ordered arrangement to occur normally.
Advanced Optical Programming of Individual Meta-Atoms Beyond the Effective Medium Approach
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. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Revisiting the local structure in Ge-Sb-Te based chalcogenide superlattices
The technological success of phase-change materials in the field of data storage and functional systems stems from their distinctive electronic and structural peculiarities on the nanoscale. Recently, superlattice structures have been demonstrated to dramatically improve the optical and electrical performances of these chalcogenide based phase-change materials. In this perspective, unravelling the atomistic structure that originates the improvements in switching time and switching energy is paramount in order to design nanoscale structures with even enhanced functional properties. This study reveals a high- resolution atomistic insight of the [GeTe/Sb2Te3] interfacial structure by means of Extended X-Ray Absorption Fine Structure spectroscopy and Transmission Electron Microscopy. Based on our results we propose a consistent novel structure for this kind of chalcogenide superlattices.
Formation of resonant bonding during growth of ultrathin GeTe films
A highly unconventional growth scenario is reported upon deposition of GeTe films on the hydrogen passivated Si(111) surface. Initially, an amorphous film forms for growth parameters that should yield a crystalline material. The entire amorphous film then crystallizes once a critical thickness of four GeTe bilayers is reached, subsequently following the GeTe(111)