B. Ögüt

Toroidal Plasmonic Eigenmodes in Oligomer Nanocavities for the Visible

Plasmonics has become one of the most vibrant areas in research with technological innovations impacting fields from telecommunications to medicine. Many fascinating applications of plasmonic nanostructures employ electric dipole and higher-order multipole resonances. Also magnetic multipole resonances are recognized for their unique properties. Besides these multipolar modes that easily radiate into free space, other types of electromagnetic resonances exist, so-called toroidal eigenmodes, which have been largely overlooked historically. They are strongly bound to material structures and their peculiar spatial structure renders them practically invisible to conventional optical microscopy techniques. In this Letter, we demonstrate toroidal modes in a metal ring formed by an oligomer of holes. Combined energy-filtering transmission electron microscopy and three-dimensional finite difference time domain analysis reveal their distinct features. For the study of these modes that cannot be excited by optical far-field spectroscopy, energy-filtering transmission electron microscopy emerges as the method of choice. Toroidal moments bear great potential for novel applications, for example, in the engineering of Purcell factors of quantum-optical emitters inside toroidal cavities.

Hybridized metal slit eigenmodes as an illustration of Babinet's principle

By energy-filtering transmission electron microscopy (EFTEM), we observe Fabry-Pérot-like surface plasmon resonances (SPRs) along the length of rectangular single and double slits drilled into free-standing thin silver films. These eigenmodes hybridize in closely situated slits. The nature of their lateral coupling is uncovered from finite-element simulations, which show that the symmetry and energy sequence of hybrid modes is governed by Babinet complementarity principles. Interestingly, the modes of a double slit system, being proto-self-complementary, may alternatively be explained by magnetic interactions between slit fields or by electrostatic interactions across the metallic bridge separating the slits.

Low-loss EFTEM Imaging of Surface Plasmon Resonances in Ag Nanostructures

Understanding how light interacts with matter at the nanometer scale is a fundamental issue in optoelectronics, nanophotonics and nanoplasmonics. The optical properties of metallic nanoparticles are entirely dependent on collective excitations of their valence electrons, known as surface plasmon resonances (SPR), under electromagnetic illumination. Measuring these properties locally at the level of the individual nanoobject in combination with spectral information over the entire visible range constitutes a challenging issue for linking of the global response of the nanoparticles and the underlying structure and morphology. The visualization of localized SPRs on the nanometer scale in combination with spectral information over the entire visible range is of prime importance in the field of biosensors, surface-enhanced Raman spectroscopy, and for the design of metamaterials. But also the explanation of abnormal transmission of light through sub-wavelength holes relies on such information.

Plasmons of Hexamer and Pentamer Nanocavities Probed with Swift Electrons

Electron energy-loss spectroscopy (EELS) is an efficient tool for investigating the local density of optical states in single and coupled nano-systems in a transmission electron microscope (TEM) [1]. In EELS, a relativistic electron inelastically interacts with a sample, and hence loses energy by pumping the sample to a higher photonic state. The amount of energy loss of the electron is detected, providing us with information about the resonant energies of the sample. Mapping EELS by parallel acquisition, energy-filtered transmission electron microscopy (EFTEM) is a fast and efficient detection tool for mapping the optical modes in two spatial dimensions. Here, this is made possible by the in-column MANDOLINE energy filter in the Zeiss SESAM microscope [2].

The Stuttgart Center for Electron Microscopy at the Max Planck Institute for Metals Research

The Stuttgart Center for Electron Microscopy at the Max Planck Institute for Metals Research was founded in 2005, fo- cusing a long tradition of transmission electron microscopy research within the former department for internal bound- aries, headed by Prof. Dr. Manfred Ruhle, into a modern cen- tral scientific facility. The Stuttgart Center for Electron Mi- croscopy pursues advanced research activities aiming at the atomic-scale characterization of novel materials by making use of the latest technological and instrumental innovations in transmission electron microscopy, such as monochroma- tors and energy filters. New electron microscopy techniques are developed and applied in order to experimentally reveal the local structural, physical, and chemical properties at high spatial resolution, with a special focus on the atomic and elec- tronic structure properties within and between metals, ceram- ics, and bio-materials, as well as the role of interface-con- trolled, at the nanometer-scale confined materials.

Study of surface plasmon resonances on assemblies of slits in thin Ag films by low-loss EFTEM imaging

With the ongoing developments in nanotechnology, surface plasmon resonances (SPRs) have started to play a crucial role in many different areas of science. Surface Plasmon resonances are described as collective oscillations in the valence electron density at the surface of a conductor. They have especially received attention in the areas of biosensing in cancer diagnostics [1], near-field Raman spectroscopy [2] and different applications in optoelectronics. In this study, the optical response of a specially perforated thin Ag film is investigated with EFTEM. The experiments were carried out in the 200 kV FEG-TEM Sub-Electron-VoltSub-Angstrom-Microscope (Zeiss SESAM) equipped with an electrostatic monochromator and the in-column MANDOLINE filter [3]. The superior properties of this instrument enable EFTEM imaging in the ultraviolet–near-infrared domain with very high energy resolution and spatial sampling [4]. The Ag specimen was prepared as follows: Using physical vapour deposition, a Ag film with about 100 nm thickness was deposited onto a C film on a standard TEM Cu grid (the dimensions of each mesh is 100 × 100 μm). Focused ion beam (FIB) technique was used to drill different slit structures into the Ag film. The EFTEM series were acquired in the energy loss range from 0.4 eV to 5 eV by using a 0.19 eV energy slit and a step size of 0.2 eV. The EFTEM images were recorded on a 2k × 2k CCD camera with 8 times binning and an acquisition time of 30 sec / image (at each energy loss 3 images were recorded with an exposure time of 10 s and then aligned and averaged). Figure 1(a) shows a zero loss bright-field image of a double-slit structure with dimensions of 200 nm × 1 μm and a separation of 100 nm. A sample image taken from the drift-corrected EFTEM series at an energy loss of 0.6 eV is shown in figure 1(b). The intensity distribution is attributed to a localized plasmon resonance. Such resonances will be discussed and compared with numerical simulations [5].

Surface plasmon resonance effects in a perforated Ag film studied by energy-filtering TEM

The visualization of localized surface plasmon resonances (LSPR) on the nanometer scale in combination with spectral information over the entire visible range is of prime importance in the field of biosensors, surface-enhanced Raman spectroscopy (SERS), aperture-less scanning near-field optical microscopy (SNOM), and for the design of metamaterials. But also the understanding of the abnormal transmission of light through sub- wavelength holes may gain by this technique. With the advent of monochromators and highly dispersive energy filters, energy- filtering TEM has now become available for the study of the optical response of materials. This technique was applied to the detection of band gaps (1) as well as to the study of surface plasmons on metal particles, like Ag nanoprisms (2-4) or Au nanorods (5). Here, the dielectric response of holes in a 100 nm thick Ag film, drilled by using a focused ion beam, is studied by acquiring EFTEM series in the energy range between 0.4 and 4 eV using the Zeiss SESAM microscope (Fig.1). The energy-slit width was 0.2 eV. Apart from multipolar ring-shaped resonances, visible particularly at the isolated holes in the upper row, a number of LSPRs are found which are due to the strong coupling effects between adjacent holes. They sensitively depend on the hole arrangement (6).