Plasmonic excitation and manipulation with an electron beam

When an electron beam passes through or near a metal structure, it will excite surface plasmons, providing a unique way to access surface plasmon behavior at the nanoscale. An electron beam focused to nanometer dimensions thus functions as a point source that is able to probe the local plasmonic mode structure at deep-subwavelength resolution. In this article, we show how well-controlled coupling between an electron beam and surface plasmons, combined with a far-field detection system, allows characterization and manipulation of plasmons on a variety of plasmonic devices. By mapping the spatial profile of inelastic scattering to resonant modes, the dispersion and losses of surface plasmons are resolved. The technique further allows probing of the confinement of plasmons within cavities and measuring the angular emission profile of nanoantennas. The coupling of electrons to surface plasmons allows the use of the electron beam as a dipole emitter that can be positioned at will. The beam position thereby can select between modes with different symmetries. This effect can be used to exert forces on plasmonic structures on the nanometer length scale with great control. © 2012 Materials Research Society.This work is part of the research program of the “Stichting voor Fundamenteel Onderzoek der Materie (FOM),” which is financially supported by the “Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO).” P.E.B. acknowledges financial support from the Basic Energy Sciences Division of the US Department of Energy, Award #DE-SC0005132. J.A. acknowledges financial support from the project FIS2010-19609-C02-01 of the Spanish Ministery of Science and A.R.C. acknowledges Consejo Nacional de Ciencia y Tecnología of Mexico and Benemérita Universidad Autónoma de Puebla.Peer Reviewe

Simultaneous imaging of dopants and free charge carriers by STEM-EELS

Doping inhomogeneities in solids are not uncommon, but their microscopic observation and understanding are limited due to the lack of bulk-sensitive experimental techniques with high-enough spatial and spectral resolution. Here, we demonstrate nanoscale imaging of both dopants and free charge carriers in La-doped BaSnO3 (BLSO) using high-resolution electron energy-loss spectroscopy (EELS). By analyzing both high- and low-energy excitations in EELS, we reveal chemical and electronic inhomogeneities within a single BLSO nanocrystal. The inhomogeneous doping leads to distinctive localized infrared surface plasmons, including a novel plasmon mode that is highly confined between high- and low-doping regions. We further quantify the carrier density, effective mass, and dopant activation percentage from EELS data and transport measurements on the bulk single crystals of BLSO. These results represent a unique way of studying heterogeneities in solids, understanding structure-property relationships at the nanoscale, and opening the way to leveraging nanoscale doping texture in the design of nanophotonic devices

Pines’ demon observed as a 3D acoustic plasmon in Sr₂RuO₄

Sr2RuO4での「パインズの悪魔」の観測 67年前に予言された金属の奇妙な振る舞いの発見. 京都大学プレスリリース. 2023-08-10.Speak of the Demon: Discovery of strange behavior of new plasmons predicted in the 50s. 京都大学プレスリリース. 2023-09-25.The characteristic excitation of a metal is its plasmon, which is a quantized collective oscillation of its electron density. In 1956, David Pines predicted that a distinct type of plasmon, dubbed a ‘demon’, could exist in three-dimensional (3D) metals containing more than one species of charge carrier. Consisting of out-of-phase movement of electrons in different bands, demons are acoustic, electrically neutral and do not couple to light, so have never been detected in an equilibrium, 3D metal. Nevertheless, demons are believed to be critical for diverse phenomena including phase transitions in mixed-valence semimetals, optical properties of metal nanoparticles, soundarons in Weyl semimetals and high-temperature superconductivity in, for example, metal hydrides. Here, we present evidence for a demon in Sr₂RuO₄ from momentum-resolved electron energy-loss spectroscopy. Formed of electrons in the β and γ bands, the demon is gapless with critical momentum qc = 0.08 reciprocal lattice units and room-temperature velocity v = (1.065 ± 0.12) × 10⁵ m s⁻¹ that undergoes a 31% renormalization upon cooling to 30 K because of coupling to the particle–hole continuum. The momentum dependence of the intensity of the demon confirms its neutral character. Our study confirms a 67-year old prediction and indicates that demons may be a pervasive feature of multiband metals

Multipolar and bulk modes: fundamentals of single-particle plasmonics through the advances in electron and photon techniques

Recent developments in the application of plasmonic nanoparticles have showcased the importance of understanding in detail their plasmonic resonances at the single-particle level. These resonances can be excited and probed through various methods, which can be grouped in four categories, depending on whether excitation and detection involve electrons (electron energy loss spectroscopy), photons (e.g., dark-field microscopy), or both (cathodoluminescence and photon-induced near-field electron microscopy). While both photon-based and electron-based methods have made great strides toward deepening our understanding of known plasmonic properties and discovering new ones, they have in general progressed in parallel, without much cross-pollination. This evolution can be primarily attributed to the different theoretical approaches driving these techniques, mainly dictated by the inherent different nature of electrons and photons. The discrepancies that still exist among them have hampered the development of a holistic approach to the characterization of plasmonic materials. In this review therefore, we aim to briefly present those electron-based and photon-based methods fundamental to the study of plasmonic properties at the single-particle level, with an eye to new behaviors involving multipolar, propagating, and bulk modes coexisting in colloidal nanostructures. By exploring the key fundamental discoveries in nanoparticle plasmonics achieved with these techniques, herein we assess how integrating this information could encourage the creation of a unified understanding of the various phenomena occurring in individual nanoparticles, which would benefit the plasmonics and electron microscopy communities alik

Multipolar and bulk modes: fundamentals of single-particle plasmonics through the advances in electron and photon techniques

Recent developments in the application of plasmonic nanoparticles have showcased the importance of understanding in detail their plasmonic resonances at the single-particle level. These resonances can be excited and probed through various methods, which can be grouped in four categories, depending on whether excitation and detection involve electrons (electron energy loss spectroscopy), photons (e.g., dark-field microscopy), or both (cathodoluminescence and photon-induced near-field electron microscopy). While both photon-based and electron-based methods have made great strides toward deepening our understanding of known plasmonic properties and discovering new ones, they have in general progressed in parallel, without much cross-pollination. This evolution can be primarily attributed to the different theoretical approaches driving these techniques, mainly dictated by the inherent different nature of electrons and photons. The discrepancies that still exist among them have hampered the development of a holistic approach to the characterization of plasmonic materials. In this review therefore, we aim to briefly present those electron-based and photon-based methods fundamental to the study of plasmonic properties at the single-particle level, with an eye to new behaviors involving multipolar, propagating, and bulk modes coexisting in colloidal nanostructures. By exploring the key fundamental discoveries in nanoparticle plasmonics achieved with these techniques, herein we assess how integrating this information could encourage the creation of a unified understanding of the various phenomena occurring in individual nanoparticles, which would benefit the plasmonics and electron microscopy communities alike

Electromagnetic forces on plasmonic nanoparticles induced by fast electron beams

19 páginas, 12 figuras.-- PACS number(s): 73.20.Mf, 68.37.Ma, 42.50.Wk, 41.75.FrThe total momentum transfer from fast electron beams, like those employed in scanning transmission electron microscopy (STEM), to plasmonic nanoparticles is calculated. The momentum transfer is obtained by integrating the electromagnetic forces acting on the particles over time. Numerical results for single and dimer metallic nanoparticles are presented, for sizes ranging between 2 and 80 nm. We analyze the momentum transfer in the case of metallic dimers where the different relevant parameters such as particle size, interparticle distance, and electron beam impact parameter are modified. It is shown that depending on the specific values of the parameters, the total momentum transfer yields a force that can be either attractive or repulsive. The time-average forces calculated for electron beams commonly employed in STEM are on the order of piconewtons, comparable in magnitude to optical forces and are thus capable of producing movement in the nanoparticles. This effect can be exploited in mechanical control of nanoparticle induced motion.Financial support from the Department of Industry of the Basque Government through the ETORTEK project inano, from the Spanish Ministerio de Ciencia e Innovación through Project No. FIS207-066711-C02-00, from the Consejo Nacional de Ciencia y tecnología (Mexico) through Project No. 82073, and from the U.S. NSF under Grant No. 0959905 are acknowledged.Peer reviewe

Time-dependent excited state response in nanostructures

Resumen del trabajo presentado al 8th International Workshop on Electron Energy Loss Spectroscopy and Related Techniques, celebrado en Okuma, Okinawa (Japan) del 14 al 19 de mayo de 2017.Recently, we calculated the lateral forces imposed by a keV electron on a metal nanoparticle, using the time- and spatially-dependent EM fields for the swift electron – nanoparticle system. We can also evaluate, in space and time, the parallel forces which drive electron energy loss. The result describes a dielectric response in time and space. In principle, this quantity is accessible by Fourier analysis of the complex frequency dependent specimen response to a driving force having a known amplitude and phase. For a Causal process, we should be able to obtain the complex experimental result from EELS spectra by a Kramers-Kronig (KK) transform, provided we can obtain complete, error-free spectra. Usually, we also restrict this treatment to uniform, thin materials, correcting for surface scattering to remove non-material dependent behavior. Nanoscale objects are very hard to treat using this approach. Surfaces and interfaces, as mentioned above, shift resonant frequencies, introduce damping mechanisms that do not exist in the bulk material, and create coupling between bulk and aloof behavior in nearby free space. Controlling this behavior would be valuable for the design of better photovoltaics, photocatalysis, and other devices that depend on light matter interaction. We are looking at theoretical details of the microscopic, time dependent behavior of materials, nanoparticles, surfaces and interfaces to better understand the nature of spatially resolved inelastic scattering. We hope to develop tools that allow a more direct spatial probe of behavior on a point by point level in nanostructures. And we think that understanding the time dependence of these processes may be important for better understanding of energy transfer among resonant modes of nanostructures, in the same way that understanding the detailed time dependence of the lateral forces revealed many unforeseen behaviors. We will review the work on the time dependence of lateral forces, showing how, for instance, expected attractive dielectric forces become repulsive diamagnetic forces during the very close approach of a relativistic electron to a 2nm diameter Au sphere. We will also describe the prediction of a “mini-wake” that occurs within 10-20 attoseconds of the close approach. This remarkable excitation describes a ~ 1nm wavelength fluctuation in charge density that likely packs hundreds of eV energy into the nm sized particle during the close approach of the electron. Later, this fluctuation decays into surface plasmons during femtosecond times. In MgO similar calculations show the decay processes continuing into picosecond times with phonon modes. We will also discuss progress on the project to create a time resolved picture of excited states of a nanoparticle, derived from experimental energy loss data.PEB and MJL acknowledge support from the Department of Energy, Office of Science, Basic Energy Sciences under Award No. DE-SC0005132. JA and AK acknowledge support from the Spanish Ministry of Economy and Competitiveness MINECO under the Project FIS2013-41184-P.Peer reviewe