Select quantum materials can support polaritons, hybrid light matter waves, with sub-diffraction-limited confinement. In this talk I will overview recent progress on polaritons in hyperbolic materials, which propagate as conical rays throughout the bulk of these crystals. I will discuss polaritons in a class of hyperbolic hetero-bicrystals. Our data reveals negative refraction, spectral gaps and wave localization can occur in these systems.
Tip-based nanoscopy techniques have emerged as powerful tools for probing the exceptional optoelectronic properties of van der Waals crystals (vdW) on deeply sub-wavelength scales. Based on two sets of experiments, we demonstrate how bound electron–hole pairs – so-called excitons – can be interrogated with near-field microscopy. First, we build on terahertz nanoscopy with subcycle temporal resolution to access the separation of photo-carriers via interlayer tunneling and their subsequent recombination in transition metal dichalcogenide bilayers. By tracing the local polarizability of electron–hole pairs with evanescent terahertz fields, we reveal pronounced variations of the exciton dynamics on the nanoscale. This approach is uniquely suitable to reveal how ultrafast charge transfer processes shape functionalities in a variety of solid-state systems. Second, we image waveguide modes (WMs) in thin flakes of the biaxial vdW crystal ReS2 across a wide range of near-infrared frequencies. Resolving the dependence of the WM dispersion on the crystallographic direction, polarization of the electric field and sample thickness, enables us to quantify the anisotropic dielectric tensor of ReS2 including the elusive out-of-plane response. The excitonic absorption at ~1.5 eV induces a backbending of the dispersion and increased losses of the WMs as fully supported by numerical calculations. Thus, we provide crucial insights into the optical properties of ReS2 and explore light-matter coupling in layered, anisotropic waveguides. Our findings set the stage for probing ultrafast dynamics in biaxial vdW crystals on the nanoscale.
This Conference Presentation, “Programmable hyperbolic polaritons in van der Waals semiconductors,” was recorded for the Photonics West 2021 Digital Forum.
KEYWORDS: Metals, Transition metals, Vanadium, Near field, Femtosecond phenomena, Nanoimaging, Laser systems engineering, Temporal resolution, Near field optics, Laser sources
We have preformed femtosecond time-resolved and nanometer spatially resolved measurements of the photo-excited insulator to metal transition (IMT) in Vanadium Dioxide (VO2). This work was made possible by several technical advances made by the authors including the development of a nano-imaging technique that is compatible with pulsed laser sources, which is guaranteed to be artifact free [1]. Additionally the authors have combined the Pharos Yb:kGW solid state laser system, which operates at relatively low repetition rates (750 kHz) with a commercial microscope from Neaspecc. This laser system provides the intense pumping that is required to photo-excite the IMT in VO2 and provides extremely broad spectral coverage for probing the IMT (660-20,000 nm). Using these technological advances the authors have obtained femtosecond time-resolved nano-imaging data on VO2, which are guaranteed to be artifact free. our findings expose that the non-equilibrium photo-induced IMT is highly inhomogeneous. The authors are able to extract the length scale of emergent metallic domains as a function of time-delay between the pump and probe channels to provide insight into the mechanisms of growth. Furthermore, by monitoring the monoclinic phonon with nanometer spatial resolution and femtosecond temporal resolution the authors are able to provide insight into the role that the monoclinic to rutile structural transition plays in the IMT. Our advances pave a pathway to study a wide range of systems with nanoscopic spatial, and ultrafast temporal resolution.
[1] A. Sternbach et al., "Artifact free time resolved near-field spectroscopy" Optics Express 25 (23), 28589-28611 (2017)
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