Suchergebnisse

Trabajo presentado al: "4th International Conference on Metamaterials, Photonic Crystals and Plasmonics" celebrado en Sharjah (UAE) del 18 al 22 de Marzo de 2013. ; A review of the recent capabilities of strongly-coupled optical gap-nanoantennas as building blocks to provide near-field coherent control, ultrafast all-optical switching and addressing high-frequency transport, among others, will be provided. As separation distances between metallic nanoantennas reach subnanometric dimensions, strong nonlocal effects are triggered out and coherent electron tunneling between the nanoantenna blocks is produced. A quantum-corrected model revealing the quantum regime in tunneling plasmonics is described. ; Financial support from the ETORTEK project "nanoiker" from the Dept. of Industry of the Government of the Basque Country is acknowledged ; Peer reviewed

Molecular emitters located in an optical cavity are known to experience a dramatic modification of the energy and dynamics of their light emission, establishing novel routes for the generation of non-classical states of light. Under monochromatic illumination, spectral asymmetries in cavity-enhanced molecular fluorescence often emerge due to the formation of hybrid polaritonic states (upper and lower polaritons). By applying the theory of open-quantum systems, we show that under strong-coupling conditions, it is essential to account for the interaction of the molecular electronic states with their vibrational environment (dephasing reservoir) to address the complex dynamics of light emission. The interaction with the dephasing reservoir yields a transfer of energy between the polariton states, favoring the transition toward the lower polariton. As a result, we show that the inelastic light emission originates mainly from the lower polariton state regardless of the pumping laser frequency, thus producing asymmetric light emission spectra. Furthermore, we show that, when several molecules are considered, intermolecular coupling can break the symmetry of the system, enabling originally dark polaritons to emit light, as revealed in the fluorescence spectrum by the emergence of new emission peaks. These results stress that accounting for the interaction with dephasing reservoirs is key to interpret molecular light emission in cavities, consistent with experimental observations. ; Spanish Ministry of Science, Innovation and Universities (MICIN) (FIS2016-80174-P); Physical Measurement Laboratory (PML) of NIST (70NANB15H32); Department of Education of the Basque Government (IT-756-13). ; Peer reviewed

6 páginas, 5 figuras.-- El pdf del artículo es la versión pre-print.-- et al. ; We propose and explore theoretically a new concept of ultrafast optical switches based on nonlinear plasmonic nanoantennas. The antenna nanoswitch operates on the transition from the capacitive to conductive coupling regimes between two closely spaced metal nanorods. By filling the antenna gap with amorphous silicon, progressive antenna-gap loading is achieved due to variations in the free-carrier density in the semiconductor. Strong modification of the antenna response is observed both in the far-field response and in the local near-field intensity. The large modulation depth, low switching threshold, and potentially ultrafast time response of antenna switches holds promise for applications ranging from integrated nanophotonic circuits to quantum information devices. ; O.L.M. acknowledges support from EPSRC through Grant EP/H019669/1 and from HEFCE through a lectureship funded by SEPnet. M.A. is supported by HEFCE though a PhD studentship funded by SEPnet. J.A. acknowledges funding from the ETORTEK project inanoGUNE from the Basque Government and project FIS2007-66711-C01-01 of the Spanish Ministry of Innovation and Science. ; Peer reviewed

Tip-enhanced Raman spectroscopy (TERS) of a single molecule is commonly described by considering the change in the polarizability of the molecule with respect to a normal coordinate induced by homogeneous illumination. However, the local fields induced by nanoscale and atomic-scale features at the surface of metallic clusters and nanogaps show strong inhomogeneities in their spatial distribution, which induces breaking of Raman selection rules. In this context, the spatial extension of the molecular electronic states subjected to strongly varying local fields challenges the validity of the point–dipole approximation as an adequate description of TERS in such configurations. Here, we introduce a general treatment to simulate single-molecule TERS spectra and their energy-filtered vibrational fingerprints maps, in which the polarization properties of the single molecule and that of the optical enhancing nanoresonator can be calculated separately and then conveniently combined to obtain the total Raman cross section of the molecule under the strongly inhomogeneous field. We apply the general method to study tip-enhanced scanning Raman picoscopy of a 4,4′-bipyridine and biphenyl molecules in the proximity of a silver icosahedral cluster with a few atoms at the tip apex mimicking an enhancing picocavity. The polarization of the molecules is calculated within density functional theory (DFT), and the optical response of the tip is calculated within a classical atomistic discrete–dipole approximation. The Raman spectra are found to be extremely sensitive to the spatial distribution of the local fields and to the orientation of the molecule. Our calculations show that the spatial mapping of molecular vibrational fingerprints, as probed by a tip with atomic protrusions, is capable to reveal intramolecular features of a single molecule in real space and thus establish a robust basis for scanning Raman picoscopy. ; J. A. acknowledges projects IT1164-19 and KK-2019/00101 from the Government of the Basque Country and project PID2019-107432GB-I00 from the Spanish Ministry of Science and Innovation. Y. Z. acknowledges the National Natural Science Foundation of China (21804125), and Z. C. D. acknowledges the Chinese Academy of Science, the National Key R&D Program of China, and Anhui Initiative in Quantum Information Technologies. ; Peer reviewed

Localised surface plasmons can couple strongly with the electronic transitions of a molecule, inducing new hybridised states of light and matter, the plasmon–exciton polaritons. Furthermore, molecules support vibrational degrees of freedom that interact with the electronic levels, giving rise to inelastic resonant Raman scattering under coherent laser illumination. Here we show the influence of strong plasmon–exciton coupling on resonant Raman processes that populate the vibrational states of the molecule and that lead to the characteristic surface-enhanced Raman scattering spectra. We develop analytical expressions that give insight into these processes for the case of moderate illumination intensity, weak electron–vibration coupling and no dephasing. These expressions help us to elucidate the twofold role of plasmon–exciton polaritons to pump the system efficiently and to enhance the Raman emission. Our results show a close analogy with the optomechanical process described for off-resonant Raman scattering but with a difference in the resonant reservoir. We also use full numerical calculations to study the effects reaching beyond these approximations and discuss the interplay between the fluorescence background and the Raman lines. Our results allow for better understanding and exploitation of the strong coupling regime in vibrational pumping and in the surface-enhanced resonant Raman scattering signal. ; The authors acknowledge project FIS2016-80174-P from the Spanish Ministry of Education, H2020-FETOPEN project "THOR" Nr. 829067 from the European Commission, grant IT1164-19 for consolidated groups of the University from the Basque Government, and project PI2017-30 of the Departamento de Educación, Política Lingüística y Cultura of the Basque Government. ; Peer reviewed

12 páginas, 6 figuras.-- et al. ; In this work we report on the observation of surface plasmon properties of periodic arrays of gold nanoring trimers fabricated by electron beam lithography. It is shown that the localized surface plasmon resonances of such gold ring trimers occur in the infrared spectral region and are strongly influenced by the nanoring geometry and their relative positions. Based on numerical simulations of the optical extinction spectra and of the electric near-field intensity maps, the resonances are assigned to surface plasmon states arising from the strong intra-trimer electromagnetic interaction. We show that the nanoring trimer configuration allows for generating infrared surface plasmon resonances associated with strongly localized electromagnetic energy, thus providing plasmonic nanoresonators well-suited for sensing and surface enhanced near-infrared Raman spectroscopy. ; This work was supported by CALMIP computing center at Paul Sabatier University and the ETORTEK project inano from the Department of Industry of the Basque Government. ; Peer reviewed

9 páginas, 8 figuras, 2 tablas.-- PACS number8s): 63.22.-m, 73.21.Cd, 73.22.Lp, 78.30.Fs.-- et al. ; We analyze and interpret resonant Raman-Brillouin scattering by folded acoustic vibrations in short-period GaAs/AlAs superlattices. Analysis of the spectra and their resonance behavior is performed using a Raman-Brillouin electronic density constructed by combining thousands transitions between electronic eigenstates of the system according to their weight in the light-scattering process. We show that plots of this effective electronic density allow for capturing the essential physics of the electron-phonon interaction and of the resonant light-scattering process in a situation where complex effects are simultaneously present: electronic confinement in the quantum wells and wave-function delocalization due to interlayer coupling, folding of acoustic dispersion and symmetry changes in the deformation fields, resonant selection of optical transitions. Comparison between the measured spectra and those simulated using the Raman-Brillouin quantum model and the photoelastic model are presented. Activation and/or deactivation of the scattering by acoustic vibration doublets and changes in their intensity ratio with excitation energy are directly related to the Raman-Brillouin electronic density distribution along the superlattices axis. Limitations of the photoelastic model are discussed by comparing the steplike variation in the photoelastic coefficient to the Raman-Brillouin electronic density profiles. ; The authors thank the Department of Industry of the Basque Government through ETORTEK for funding support. ; Peer reviewed

In the frame of the Self-Energy formalism, we study the interaction between STEM electrons and small particles in the range of the valence electron excitations. We first calculate the energy loss probability for an isolated sphere and study the loss spectrum dependence on the size of the particle and on the relative impact parameter. Then we analyze the loss spectra in more realistic situations: (a) the effect of the coupling between the particle and supporting surface is studied in a simple geometrical model; and (b) we analyze the dependence of the losses on the geometrical shape of the target by considering hemispherical particle. Our results are in a good qualitative (and in simple cases, quantitative too) agreement with several experimental results which show anomalous excitations. We restate the suitability of the dielectric theory to study the surface excitations of these systems. ; They thanks Gipuzkoako Foru Aldundia and the Basque Country Government (Eusko Jaurlaritza) for financial help. ; Peer reviewed

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. ; A single emitter can couple with electromagnetic modes of dielectric cavities or metallic particles. In a similar manner, it can couple with a phononic mode supported by a nearby infrared antenna. We consider an emitter with a sufficiently large dipole moment coupled to a SiC bowtie structure supporting strongly localized phononic modes. We show that vacuum Rabi oscillations and large spectral anticrossing are possible, indicating that the emitter-phononic system is in the strong coupling regime. Pure dephasing degrades the response remarkably little. As expected for a quantum but not for a classical formalism, the frequency of the vacuum Rabi oscillations depends on the initial state. We also discuss the possibility of exciting hybrid modes with contributions from the emitter and from more than one of the phononic modes supported by the antenna. Phononic structures appear attractive to study such complex hybridization, as they can support several strongly confined modes with quality factors larger than one hundred in a relatively small spectral window. © 2014 IOP Publishing and Deutsche Physikalische Gesellschaft. ; RE and JA acknowledge financial support from the project ETORTEK-NanoIker, of the Department of Industry of the Basque Government, from the Spanish National project FIS2010-19609-C02-01E and from the Department of Education of the Basque Government, IT756-13 of consolidated groups. RE acknowledges financial support from the Physics Frontier Center at the Joint Quantum Institute, University of Maryland. ; Peer Reviewed

We present a theoretical study of the optical properties of a strongly coupled metallic dimer when an ensemble of molecules is placed in the inter-particle cavity. The linking molecules are characterized by an excitonic transition which couples to the Bonding Dimer Plasmon (BDP) and the Bonding Quadrupolar Plasmon (BQP) resonances, arising from the hybridization of the dipolar and quadrupolar modes of the individual nanoparticles, respectively. As a consequence, both modes split into two coupled plasmon-exciton modes, so called plexcitons. The Charge Transfer Plasmon (CTP) resonance, involving plasmonic oscillations of the dimer as a whole, arises when the conductance of the excitonic junction is above a threshold value. The possibility of exploiting plexcitonic resonances for sensing is explored in detail. We find high sensitivity to the environment when different dielectric embedding media are considered. Contrary to standard methods, we propose a new framework for effective sensing based on the relative intensity of plexcitonic peaks. © 2013 Optical Society of America. ; This project was supported by the Etortek project nanoiker from the Basque Government (BG), project FIS2010-19609-C02-01 from the Spanish Ministry of Science and Innovation and grant IT-366-07 from BG-UPV/EHU. O.P.G. acknowledges financial support from Vicerrectorado de Investigación of the University of the Basque Country (Ayudas Especialización Doctores). Computational resources were provided by DIPC (UPV/EHU, MICINN, BG, ESF). ; Peer Reviewed

We study the modification of the decay rates of a single dipolar emitter positioned in the vicinity of metallic linear nanoantennas when higher-order plasmonic excitations are induced. We show that it is possible to effectively tune the enhancement or suppression of both the radiative and nonradiative decay processes by controlling the position and orientation of the dipole with respect to the antenna. Transverse polarization of a single emitter, with respect to the antenna axis, located at the center of the antenna activates dark antenna modes that modify dramatically both the intensity and the spectral features of the decay rates. © 2012 Optical Society of America. ; Financial support from the WELCOME program "Hybrid nanostructures as a stepping-stone towards efficient artificial photosynthesis" awarded by the Foundation for Polish Science and from the ETORTEK 2011 "nanoiker" of the Department of the Industry of the Basque Country Government are gratefully acknowledged. ; Peer Reviewed

We report a quantum mechanical study of the plasmonic response of bimetallic spherical core/shell nanoparticles. The systems comprise up to 104 electrons and their optical response is addressed with Time Dependent Density Functional Theory calculations. These quantum results are compared with classical electromagnetic calculations for core/shell systems formed by Al/Na, Al/Au and Ag/Na, as representative examples of bimetallic systems. We show that for shell widths in the nanometer range, the system cannot be described as a simple stack of two metals. The finite size effect and the transition layer formed between the core and the shell strongly modify the optical properties of the compound nanoparticle. In particular this configuration leads to a frequency shift of the plasmon resonance with shell character and an increased plasmon decay into electron-hole pairs which eventually quenches this resonance for very thin shells. This effect is difficult to capture with a classical theory even upon adjustment of the parameters of a combination of metallic dielectric functions. ; J.A. acknowledges financial support from the Department of Education of the Basque Government through project IT756/13 for consolidated groups, project FIS2013-41184-P from the Spanish Ministry of Economy and Competitiveness, and grant 70NANB15H321, "PLASMOQUANTUM", from the US Department of Commerce (NIST). ; Peer reviewed

Resumen del trabajo presentado a la Conferencia Española de Nanofotónica (CEN), celebrada en Vigo del 20 al 22 de septiembre de 2021. ; Project PID2019-107432GB-I00 (Spanish MICINN). Predoctoral Fellowship No. PRE2017_1_0267 and Grant IT1164-19 (Department of Education of the Basque Government). Project H2020-FET Open 'POSEIDON' (European Commision). ; Peer reviewed

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

We introduce a design strategy to maximize the Near Field (NF) enhancement near plasmonic antennas. We start by identifying and studying the basic electromagnetic effects that contribute to the electric near field enhancement. Next, we show how the concatenation of a convex and a concave surface allows merging all the effects on a single, continuous nanoantenna. As an example of this NF maximization strategy, we engineer a nanostructure, the indented nanocone. This structure, combines all the studied NF maximization effects with a synergistic boost provided by a Fano-like interference effect activated by the presence of the concave surface. As a result, the antenna exhibits a NF amplitude enhancement of ∼ 800, which transforms into ∼1600 when coupled to a perfect metallic surface. This strong enhancement makes the proposed structure a robust candidate to be used in field enhancement based technologies. Further elaborations of the concept may produce even larger and more effective enhancements. ; This work was supported by the Etortek-2011 project nanoiker of the Department of Industry of the Basque Government, project FIS2010-19609-C02-01 of the Spanish Ministry of Science and Innovation and the Swedish Foundation for Strategic Research through the project RMA08 Functional Electromagnetic Metamaterials. ; Peer Reviewed