Suchergebnisse

We theoretically describe how fast electrons couple to polaritonic modes in uniaxial materials by analyzing the electron energy-loss spectra. We show that in the case of a uniaxial medium with hyperbolic dispersion, bulk and surface modes can be excited by a fast electron traveling through the volume or along an infinite interface between the material and vacuum. Interestingly, and in contrast to excitations in isotropic materials, bulk modes can be excited by fast electrons traveling outside the uniaxial medium. We demonstrate our findings with the representative uniaxial material hexagonal boron nitride. We show that the excitation of bulk and surface phonon polariton modes is strongly related to the electron velocity and highly dependent on the angle between the electron beam trajectory and the optical axis of the material. Our work provides a systematic study for understanding bulk and surface polaritons excited by a fast electron beam in hyperbolic materials and sets a way to steer and control the propagation of the polaritonic waves by changing the electron velocity and its direction. ; J.A. and R.H. acknowledge Grant No. IT1164-19 for research groups of the Basque University system from the Department of Education of the Basque Government. R.H. further acknowledges financial support from the Spanish Ministry of Science and Innovation (national Project No. RTI2018-094830-B-100 and the Project No. MDM-2016-0618 of the María de Maeztu Units of Excellence Program). J.A. acknowledges financial support from the Spanish Ministry of Science and Innovation (national Project No. PID2019-107432GB-I00). ; Peer reviewed

5 páginas, 4 figuras.-- El pdf del artículo es la versión pre-print.-- et al. ; We describe transmission-mode scattering-type near-field optical microscopy (s-SNOM) with interferometric detection. Using this technique, we map the near-field modes of infrared antennas in both amplitude and phase. The use of dielectric probing tips, higher-harmonic demodulation, and a complex-valued subtraction of residual background yields accurate near-field images of the antenna modes. We map metallic nanorods, disks, and triangles, designed for antenna resonance at mid-infrared frequencies, in good agreement with numerical calculations of the modal field distribution. Furthermore, we show that transmission-mode s-SNOM can map the z-component of the antenna near fields. Our results establish a basis for future near-field characterization of complex antenna structures for molecular sensing and spectroscopy. ; Supported by the Etortek program of the Department of Industry of the Basque Government and the European FP7 project "Nanoantenna" (Health-F5-2009- 241818). ; Peer reviewed

We identify weak and strong coupling regimes between a near-field probing tip and a plasmonic sample by imaging plasmon-resonant gold nanodisks with scattering-type scanning near-field optical microscopy (s-SNOM). By means of rigorous electrodynamical calculations based on a model system, we find that in the weak coupling regime, s-SNOM can be applied for direct mapping of plasmonic nanoantenna modes, while in the strong coupling regime, the near-field probe allows for high-precision opto-mechanical control of the antenna response. © 2009 The American Physical Society. ; We acknowledge P. Hanarp and D. S. Sutherland for provision of the disk samples, and financial support from the project ETORTEK-2008 of the Department of Industry of the Basque Government and project FIS2007-66711-C01-01 of the Spanish Ministry of Education and Science. ; Peer Reviewed

Mid-infrared (mid-IR) optical spectroscopy of molecules is of large interest in physics, chemistry, and biology. However, probing nanometric volumes of molecules is challenging because of the strong mismatch of their mid-infrared absorption and scattering cross-sections with the free-space wavelength. We suggest overcoming this difficulty by nanofocusing acoustic graphene plasmon polaritons (AGPs) – oscillations of Dirac charge carriers coupled to electromagnetic fields with extremely small wavelengths – using a taper formed by a graphene sheet above a metallic surface. We demonstrate that due to the appreciable field enhancement and mode volume reduction, the nanofocused AGPs can efficiently sense molecular fingerprints in nanometric volumes. We illustrate a possible realistic sensing sсenario based on AGP interferometry performed with a near-field microscope. Our results can open new avenues for designing tiny sensors based on graphene and other 2D polaritonic materials. ; A.Y.N. acknowledges the Spanish Ministry of Science, Innovation and Universities (national project MAT2017-88358-C3-3-R). K.V.V. and V.S.V. acknowledge the Russian Science Foundation, grant number 18-79-10208. P.AG. acknowledges support from the European Research Council under Starting Grant 715496, 2DNANOPTICA. R.H. acknowledges support from the Spanish Ministry of Economy, Industry, and Competitiveness (National Project RTI2018-094830-B-100 and the ProjectMDM-2016-0618 of theMarie de Maeztu Units of Excellence Program) and the Basque Government (Grant No. IT1164-19). ; Peer reviewed

This work is licensed under a Creative Commons Attribution-NonCommercial-Share Alike 3.0 Unported License.-- et al. ; Light scattering at nanoparticles and molecules can be dramatically enhanced in the 'hot spots' of optical antennas, where the incident light is highly concentrated. Although this effect is widely applied in surface-enhanced optical sensing, spectroscopy and microscopy, the underlying electromagnetic mechanism of the signal enhancement is challenging to trace experimentally. Here we study elastically scattered light from an individual object located in the well-defined hot spot of single antennas, as a new approach to resolve the role of the antenna in the scattering process. We provide experimental evidence that the intensity elastically scattered off the object scales with the fourth power of the local field enhancement provided by the antenna, and that the underlying electromagnetic mechanism is identical to the one commonly accepted in surface-enhanced Raman scattering. We also measure the phase shift of the scattered light, which provides a novel and unambiguous fingerprint of surface-enhanced light scattering. © 2012 Macmillan Publishers Limited. All rights reserved. ; This study was supported by the European FP7 project 'Nanoantenna' (FP7-HEALTH-F5-2009-241818-NANOANTENNA), the ERC Starting grant no. 258461 (TERATOMO), the National Projects MAT2009-08398 and FIS2010-19609-C02-C01 from the Spanish Ministerio de Ciencia e Innovacion and the Etortek-2011 project 'nanoiker' of the Department of Industry of the Basque Government. M.S. and L.A. acknowledges financial support from 'Programa de Formación de Personal Investigador' promoted by the Department of Education, Universities and Research of the Basque Government. A.G.-E. was supported by 'Programa de perfeccionamiento de doctores y doctoras en el extranjero del Departamento de Educación, Universidades e Investigación del Gobierno Vasco'. ; Peer Reviewed

7 páginas, 6 figuras.-- El pdf del artículo es la versión post-print.-- et al. ; The fundamental optical properties of pure nickel nanostructures are studied by far-field extinction spectroscopy and optical near-field microscopy, providing direct experimental evidence of the existence of particle plasmon resonances predicted by theory. Experimental and calculated near-field maps allow for unambiguous identification of dipolar plasmon modes. By comparing calculated near-field and far-field spectra, dramatic shifts are found between the near-field and far-field plasmon resonances, which are much stronger than in gold nanoantennas. Based on a simple damped harmonic oscillator model to describe plasmonic resonances, it is possible to explain these shifts as due to plasmon damping. ; Supported by the European FP7 project 'Nanoantenna' (FP7-HEALTH-F5-2009-241818-NANOANTENNA) and the National Project MAT2009 –08398 from the Spanish Ministerio de Ciencia e Innovacion. J.A. acknowledges fi nancial help by the Department of Industry of the Basque Government through the ETORTEK program NANOPHOT. P.V. acknowledges funding from the Basque Government under Programs No. PI2009–17 as well as the Spanish Ministry of Science and Education under Project No. MAT2009–07980. Z. P. acknowledges support from Swedish Foundation for Strategic Research through RMA08–0109 "Functional Electromagnetic Metamaterials" program. J. N. acknowledges funding from the Generalitat de Catalunya and the Spanish Ministry of Science and Education through No. 2009-SGR-1292 and No. MAT2010–20616-C02 projects. A.D. acknowledges support from the Swedish Research Council. ; Peer reviewed

We directly visualize and identify the capacitive coupling of infrared dimer antennas in the near field by employing scattering-type scanning near-field optical microscopy (s-SNOM). The coupling is identified by (i) resolving the strongly enhanced nano-localized near fields in the antenna gap and by (ii) tracing the red shift of the dimer resonance when compared to the resonance of the single antenna constituents. Furthermore, by modifying the illumination geometry we break the symmetry, providing a means to excite both the bonding and the >dark> anti-bonding modes. By spectrally matching both modes, their interference yields an enhancement or suppression of the near fields at specific locations, which could be useful in nanoscale coherent control applications. © 2013 Optical Society of America. ; We acknowledge support from the European FP7 projects "Nanoantenna" (FP7-HEALTHF5-2009-241818-NANOANTENNA), NMP (NMP3-SL-2011-263104- HINTS), and "SPINTROS" (257654), and the National Projects MAT2009-08398, MAT2009-08494 and FIS2010-19609-C02-C01 from the Spanish Ministerio de Ciencia e Innovacion. We also acknowledge the Marie Curie Actions PIRG06-GA-2009-25647, ITAMOSCINOM, as well as the Basque Government Program PI2011-1 and Etortek-2011 (nanoiker). ; Peer Reviewed

15 páginas, 8 figuras. ; Interaction between micrometer-long nanoantennas within an array considerably modifies the plasmonic resonant behaviour; for fundamental resonances in the infrared already at micrometer distances. In order to get systematic knowledge on the relationship between infrared plasmonic resonances and separation distances dx and dy in longitudinal and transverse direction, respectively, we experimentally studied the optical extinction spectra for rectangularly ordered lithographic gold nanorod arrays on silicon wafers. For small dy, strong broadening of resonances and strongly decreased values of far-field extinction are detected which come along with a decreased near-field intensity, as indicated by near-field amplitude maps of the interacting nanoantennas. In contrast, near-field interaction over small dx does only marginally broaden the resonance. Our findings set a path for optimum design of rectangular nanorod lattices for surface enhanced infrared spectroscopy. ; Financial support by the European project NANOANTENNA (HEALTH-F5-2009-241818), by the Strategic International Cooperative Program through the Japan Science and Technology Agency and the German Science Foundation (DFG PU193/9-1), by the Heidelberg Graduate School of Fundamental Physics, by the National project FIS-2010-19609-C02-02 from the Spanish Ministry of Science and Innovation, and by the ETORTEK project inanoGUNE from the Department of Industry of the Basque Government is gratefully acknowledged. ; Peer reviewed

Resumen del trabajo presentado a la Conferencia Española de Nanofotónica (CEN), celebrada en Vigo del 20 al 22 de septiembre de 2021. ; Department of Education of the Basque Government (Project PI2017-30 and Grant IT1164-19); Spanish Ministry of Science and Innovation (PID2019-107432GB-I00, RTI2018-094861-B-100, MDM-2016-0618); European Union H2020 (796817-ARTEMIS). ; Peer reviewed

Strong coupling between molecular vibrations and microcavity modes has been demonstrated to modify physical and chemical properties of the molecular material. Here, we study the less explored coupling between lattice vibrations (phonons) and microcavity modes. Embedding thin layers of hexagonal boron nitride (hBN) into classical microcavities, we demonstrate the evolution from weak to ultrastrong phonon-photon coupling when the hBN thickness is increased from a few nanometers to a fully filled cavity. Remarkably, strong coupling is achieved for hBN layers as thin as 10 nm. Further, the ultrastrong coupling in fully filled cavities yields a polariton dispersion matching that of phonon polaritons in bulk hBN, highlighting that the maximum light-matter coupling in microcavities is limited to the coupling strength between photons and the bulk material. Tunable cavity phonon polaritons could become a versatile platform for studying how the coupling strength between photons and phonons may modify the properties of polar crystals. ; This work is supported by the Spanish Ministry of Science and Innovation under the María de Maeztu Units of Excellence Program (MDM-2016-0618), and Projects PID2019-107432GB-I00 and RTI2018-094861-B-100; by the European Union H2020 under the Marie Curie Actions (796817-ARTEMIS); and by Project PI2017-30 and Grant IT1164-19 for the research groups of the Basque University System from the Department of Education of the Basque Government. ; Peer reviewed