Suchergebnisse

The linear optical response of the ultrawide bandgap h‐BN is investigated by spectroscopic ellipsometry. The ordinary dielectric function of h‐BN is determined up to 25 eV on a high‐quality single‐crystal platelet. The direct bandgap and the high‐energy transitions are characterized with the aid of ab initio self‐consistent GW calculations and the optical properties are calculated using the Bethe–Salpeter equation. The dispersion of the ordinary refractive index in the visible and UV part of the spectrum below the bandgap is in excellent agreement with previous transmission experiments. A sharp excitonic peak at 6.1 eV associated with the direct bandgap dominates the absorption spectrum, and a second peak is observed at 6.8 eV. At higher energies, a strong absorption peak emerges at 15 eV associated with higher σ → σ* transitions. As a consequence of the unique electronic band structure of h‐BN, a transparency window is observed in the far‐UV region between 7 and 13 eV, where the light penetration depth reaches a value of 38 nm, as opposed to the value of 0.8 nm at the absorption peak of the fundamental bandgap. ; This work was supported by the Spanish MINECO/FEDER under grant no. MAT2015‐71035‐R. The authors thank Norbert Esser (Leibniz Institut für Analytische Wissenschaften – ISAS‐e.V.) for access to the synchrotron ellipsometer and the PTB (Physikalisch Technische Bundesanstalt) for expert assistance during the beamtime at the MLS synchrotron. The authors gratefully acknowledge Michael Winkler (OvGU Magdeburg) for data recording at the synchrotron. C.A. acknowledges funding from the European Union Seventh Framework Program under grant agreement no. 785219 Graphene Core 2 and COST Action TUMIEE CA17126. C.A. acknowledges A. Saul for the management of the computer cluster Rosa. Support for the h‐BN crystal growth from the Office of Naval Research award number N00014‐20‐1‐2474 is appreciated. The authors thank Dr. Neelam Khan (Georgia Gwinnett College) for the AFM analysis. ; Peer reviewed

Phonon polaritons in van der Waals materials can strongly enhance light–matter interactions at mid-infrared frequencies, owing to their extreme field confinement and long lifetimes1,2,3,4,5,6,7. Phonon polaritons thus bear potential for vibrational strong coupling with molecules. Although the onset of vibrational strong coupling was observed spectroscopically with phonon-polariton nanoresonators8, no experiments have resolved vibrational strong coupling in real space and with propagating modes. Here we demonstrate by nanoimaging that vibrational strong coupling can be achieved between propagating phonon polaritons in thin van der Waals crystals (hexagonal boron nitride) and molecular vibrations in adjacent thin molecular layers. We performed near-field polariton interferometry, showing that vibrational strong coupling leads to the formation of a propagating hybrid mode with a pronounced anti-crossing region in its dispersion, in which propagation with negative group velocity is found. Numerical calculations predict vibrational strong coupling for nanometre-thin molecular layers and phonon polaritons in few-layer van der Waals materials, which could make propagating phonon polaritons a promising platform for ultrasensitive on-chip spectroscopy and strong-coupling experiments. ; We acknowledge financial support from the Spanish Ministry of Science, Innovation and Universities (national projects MAT2017-88358-C3, RTI2018-094830-B-100, RTI2018-094861-B-100, and the project MDM-2016-0618 of the Maria de Maeztu Units of Excellence Program), the Basque Government (grant numbers IT1164-19 and PIBA-2020-1-0014) and the European Union's Horizon 2020 research and innovation programme under the Graphene Flagship (grant agreement numbers 785219 and 881603, GrapheneCore2 and GrapheneCore3). F. Calavelle acknowledges support from the European Union H2020 under the Marie Skłodowska-Curie Actions (766025-QuESTech). J.T.-G. acknowledges support through the Severo Ochoa Program from the Government of the Principality of Asturias (number PA-18-PF-BP17-126). P.A.-G. acknowledges support from the European Research Council under starting grant number 715496, 2DNANOPTICA. Further, support from the Materials Engineering and Processing program of the National Science Foundation, award number CMMI 1538127 for h-BN crystal growth is greatly appreciated. ; Peer reviewed

Refraction between isotropic media is characterized by light bending towards the normal to the boundary when passing from a low- to a high-refractive-index medium. However, refraction between anisotropic media is a more exotic phenomenon which remains barely investigated, particularly at the nanoscale. Here, we visualize and comprehensively study the general case of refraction of electromagnetic waves between two strongly anisotropic (hyperbolic) media, and we do it with the use of nanoscale-confined polaritons in a natural medium: α-MoO3. The refracted polaritons exhibit non-intuitive directions of propagation as they traverse planar nanoprisms, enabling to unveil an exotic optical effect: bending-free refraction. Furthermore, we develop an in-plane refractive hyperlens, yielding foci as small as λp/6, being λp the polariton wavelength (λ0/50 compared to the wavelength of free-space light). Our results set the grounds for planar nano-optics in strongly anisotropic media, with potential for effective control of the flow of energy at the nanoscale. ; G.Á.-P. and J.T.-G. acknowledge support through the Severo Ochoa Program from the Government of the Principality of Asturias (nos. PA-20-PF-BP19-053 and PA-18-PF-BP17-126, respectively). S.X. acknowledges the support from Independent Research Fund Denmark (Project No. 9041-00333B). B.C. acknowledges the support from VILLUM FONDEN (No. 00027987). The Center for Nanostructured Graphene is sponsored by the Danish National Research Foundation (Project No. DNRF103.) K.V.V. and V.S.V. gratefully acknowledge the financial support from the Ministry of Science and Higher Education of the Russian Federation (Agreement No. 075-15-2021-606). J.M.-S. acknowledges financial support through the Ramón y Cajal Program from the Government of Spain (RYC2018-026196-I). A.Y.N. and J.I.M. acknowledge the Spanish Ministry of Science, Innovation and Universities (national projects MAT201788358-C3-3-R and PID2019-104604RB/AEI/10.13039/501100011033). R.H. acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities (national project RTI2018-094830-B-100 and the project MDM-2016-0618 of the Marie de Maeztu Units of Excellence Program) and the Basque Government (grant No. IT1164-19). A.Y.N. also acknowledges the Basque Department of Education (grant no. PIBA-2020-1-0014). P.A.-G. acknowledges support from the European Research Council under starting grant no. 715496, 2DNANOPTICA and the Spanish Ministry of Science and Innovation (State Plan for Scientific and Technical Research and Innovation grant number PID2019-111156GB-I00). ; Peer reviewed