van der Waals nanomaterials supporting phonon polariton quasiparticles possess unprecedented light confinement capabilities, making them ideal systems for molecular sensing, thermal emission, and ...subwavelength imaging applications, but they require defect-free crystallinity and nanostructured form factors to fully showcase these capabilities. We introduce bottom-up-synthesized {\alpha}-MoO3 structures as nanoscale phonon polaritonic systems that feature tailorable morphologies and crystal qualities consistent with bulk single crystals. {\alpha}-MoO3 nanoribbons serve as low-loss hyperbolic Fabry-Pérot nanoresonators, and we experimentally map hyperbolic resonances over four Reststrahlen bands spanning the far- and mid-infrared spectral range, including resonance modes beyond the tenth order. The measured quality factors are the highest from phonon polaritonic van der Waals structures to date. We anticipate that bottom-up-synthesized polaritonic van der Waals nanostructures will serve as an enabling high-performance and low-loss platform for infrared optical and optoelectronic applications.
In article number 1700982, Juncheng Cao, Hua Li, Tiger H. Tao, and co‐workers report a realtime, multispectral terahertz imager based on a multispectral metamaterial focal plane array. Versatile ...imaging capabilities, including unambiguous identification of concealed substances with intrinsic terahertz characteristics and effective diagnosis of cancerous tissues without notable spectral signatures in the terahertz range, are presented, thus underscoring the utility of applying the multispectral terahertz imager for sensing, identification, and medical imaging.
The underlying physics behind an experimental observation often lacks a simple analytical description. This is especially the case for scanning probe microscopy techniques, where the interaction ...between the probe and the sample is nontrivial. Realistic modeling to include the details of the probe is always exponentially more difficult than its "spherical cow" counterparts. On the other hand, a well-trained artificial neural network based on real data can grasp the hidden correlation between the signal and sample properties. In this work, we show that, via a combination of model calculation and experimental data acquisition, a physics-infused hybrid neural network can predict the tip-sample interaction in the widely used scattering-type scanning near-field optical microscope. This hybrid network provides a long-sought solution for accurate extraction of material properties from tip-specific raw data. The methodology can be extended to other scanning probe microscopy techniques as well as other data-oriented physical problems in general.
Three previously unreported photo-carrier relaxation pathways are presented and discussed in GaAs-based systems. In bulk GaAs, a transient bleach of the spin-split exciton transition 1s→2p± is ...reported following photo-excitation at low temperatures and is likely caused by final state blocking of the 2p±1 exciton level. The bleach of the 1s → 2p−1 transition is delayed with respect to that of the free carriers and 1s → 2p+1, suggesting electronic relaxation occurs through two simultaneous mechanisms: elastic scattering between quantized conduction band states and spin-dependent relaxation through the 2p±1 exciton states. For ErAs:GaAs composites, the response at short time delays is completely dependent on the occupation of the interface trap state between the ErAs nanoparticles and the GaAs matrix. Occupation of the interface state depends on the photo-carrier energy, carrier density, and trap density. Carrier scattering from the interface state plays a large role in the response as it prevents full relaxation of the system on ultrashort timescales. The composite ErAs:GaAs systems also exhibit an oscillatory response highly suggestive of surface plasmon polaritons at the interface between the semi-metallic ErAs and semiconducting GaAs, which couple to the GaAs phonon modes. The oscillation frequencies are observed to follow the same trend with volume fraction as the static absorption resonance peaks, suggesting different nanoparticle size distributions exist with different ErAs incorporation.
Unlike conventional plasmonic media, polaritonic van der Waals (vdW) materials hold promise for active control of light–matter interactions. The dispersion relations of elementary excitations such as ...phonons and plasmons can be tuned in layered vdW systems via stacking using functional substrates. In this work, infrared nanoimaging and nanospectroscopy of hyperbolic phonon polaritons are demonstrated in a novel vdW heterostructure combining hexagonal boron nitride (hBN) and vanadium dioxide (VO2). It is observed that the insulator‐to‐metal transition in VO2 has a profound impact on the polaritons in the proximal hBN layer. In effect, the real‐space propagation of hyperbolic polaritons and their spectroscopic resonances can be actively controlled by temperature. This tunability originates from the effective change in local dielectric properties of the VO2 sublayer in the course of the temperature‐tuned insulator‐to‐metal phase transition. The high susceptibility of polaritons to electronic phase transitions opens new possibilities for applications of vdW materials in combination with strongly correlated quantum materials.
In van der Waals heterostructures comprising hexagonal boron nitride (hBN) and vanadium dioxide (VO2), dynamic and reversible tuning of hyperbolic phonon polaritons is achieved via the insulator‐to‐metal phase transition by controlling the temperature. Using infrared nanospectroscopy, opposite tuning trends for in‐plane and out‐of‐plane phonon resonances are demonstrated during the phase transition.
Ultrafast optical pump - optical probe and optical pump - terahertz probe spectroscopy were performed on vanadium dioxide (VO2) and vanadium sesquioxide (V2O3) thin films over a wide temperature ...range. A comparison of the experimental data from these two different techniques and two different vanadium oxides, in particular a comparison of the electronic oscillations generated by the photoinduced longitudinal acoustic modulation, reveals the strong electron-phonon coupling that exists in the metallic state of both materials. The low energy Drude response of V2O3 appears more susceptible than VO2 to ultrafast strain control. Additionally, our results provide a measurement of the temperature dependence of the sound velocity in both systems, revealing a four- to fivefold increase in VO2 and a three- to fivefold increase in V2O3 across the phase transition. Our data also confirm observations of strong damping and phonon anharmonicity in the metallic phase of VO2, and suggest that a similar phenomenon might be at play in the metallic phase of V2O3. More generally, our simple table-top approach provides relevant and detailed information about dynamical lattice properties of vanadium oxides, opening the way to similar studies in other complex materials.
Unlike conventional plasmonic media, polaritonic van der Waals (vdW) materials hold promise for active control of light-matter interactions. The dispersion relations of elementary excitations such as ...phonons and plasmons can be tuned in layered vdW systems via stacking using functional substrates. In this work, infrared nanoimaging and nanospectroscopy of hyperbolic phonon polaritons are demonstrated in a novel vdW heterostructure combining hexagonal boron nitride (hBN) and vanadium dioxide (VO
). It is observed that the insulator-to-metal transition in VO
has a profound impact on the polaritons in the proximal hBN layer. In effect, the real-space propagation of hyperbolic polaritons and their spectroscopic resonances can be actively controlled by temperature. This tunability originates from the effective change in local dielectric properties of the VO
sublayer in the course of the temperature-tuned insulator-to-metal phase transition. The high susceptibility of polaritons to electronic phase transitions opens new possibilities for applications of vdW materials in combination with strongly correlated quantum materials.
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