Edson P. Bellido

Toward 10 meV electron energy-loss spectroscopy resolution for plasmonics.

Energy resolution is one of the most important parameters in electron energy-loss spectroscopy. This is especially true for measurement of surface plasmon resonances, where high-energy resolution is crucial for resolving individual resonance peaks, in particular close to the zero-loss peak. In this work, we improve the energy resolution of electron energy-loss spectra of surface plasmon resonances, acquired with a monochromated beam in a scanning transmission electron microscope, by the use of the Richardson-Lucy deconvolution algorithm. We test the performance of the algorithm in a simulated spectrum and then apply it to experimental energy-loss spectra of a lithographically patterned silver nanorod. By reduction of the point spread function of the spectrum, we are able to identify low-energy surface plasmon peaks in spectra, more localized features, and higher contrast in surface plasmon energy-filtered maps. Thanks to the combination of a monochromated beam and the Richardson-Lucy algorithm, we improve the effective resolution down to 30 meV, and evidence of success up to 10 meV resolution for losses below 1 eV. We also propose, implement, and test two methods to limit the number of iterations in the algorithm. The first method is based on noise measurement and analysis, while in the second we monitor the change of slope in the deconvolved spectrum.

Self-Similarity of Plasmon Edge Modes on Koch Fractal Antennas

We investigate the plasmonic behavior of Koch snowflake fractal geometries and their possible application as broadband optical antennas. Lithographically defined planar silver Koch fractal antennas were fabricated and characterized with high spatial and spectral resolution using electron energy loss spectroscopy. The experimental data are supported by numerical calculations carried out with a surface integral equation method. Multiple surface plasmon edge modes supported by the fractal structures have been imaged and analyzed. Furthermore, by isolating and reproducing self-similar features in long silver strip antennas, the edge modes present in the Koch snowflake fractals are identified. We demonstrate that the fractal response can be obtained by the sum of basic self-similar segments called characteristic edge units. Interestingly, the plasmon edge modes follow a fractal-scaling rule that depends on these self-similar segments formed in the structure after a fractal iteration. As the size of a fractal structure is reduced, coupling of the modes in the characteristic edge units becomes relevant, and the symmetry of the fractal affects the formation of hybrid modes. This analysis can be utilized not only to understand the edge modes in other planar structures but also in the design and fabrication of fractal structures for nanophotonic applications.

Very High Resolution Energy Loss Spectroscopy: Applications in Plasmonics

The development of monochromators in commercial instruments [1] has lead to a surge in interest in high-energy resolution electron energy loss spectroscopy (EELS). While the early applications were focused on the use of high-resolution EELS for the study of EELS Near-Edge Structures (ELNES) [2], one of the most fruitful applications of such devices has been in the field of plasmonics, spurred by the first results obtained with non-monochromated cold-field emission electron sources, which demonstrated the detection of surface plasmon resonances in Au and Ag nanostructures [3,4]. Energy resolution (measured at the full width-half maximum) of the Zero-Loss Peak (ZLP) in the range of 0.1 to 0.2 eV has been typically considered sufficient for ELNES work because of the intrinsic width of spectral features due to the broadening arising from the lifetime of the core hole and the excited states, even for edges at relatively low energy losses (e.g. the C and B K edges). However, interests in the measurements of bandgaps [e.g. 5] and the recent demonstration of detection of phonons [6, 7] have spurred interest towards much improved energy resolution capabilities. Here we present some examples of applications in high-resolution EELS achieved through a combination of monochromators and numerical deconvolution methods providing in principle an effective resolution in the range between 10 to 30 meV thus making it possible to detect very low energy features arising from surface plasmon resonances (SPR) in metallic nanostructures [8]. Improved resolution is shown to make it possible to detect and subsequently map subtle spectroscopic features due to coupling of excitation modes and very low intrinsic resonance energies in very large size structures.

Correlative electron energy loss spectroscopy and cathodoluminescence spectroscopy on three-dimensional plasmonic split ring resonators

We present the surface plasmon resonance modes in three-dimensional (3D) upright split ring resonators (SRR) as studied by correlative cathodoluminescence (CL) spectroscopy in a scanning electron microscope (SEM) and electron energy loss spectroscopy (EELS) in a transmission electron microscope. We discuss the challenges inherent in studying the plasmon modes of a 3D nanostructure and how meeting these challenges benefits from the complementary use of EELS and SEM-CL. With the use of EELS, we detect a strong first order mode in the SRR; with comparison to simulations, we are able to identify this as the well-known magnetic dipole moment of the SRR. Combining the EELS spectra with SEM-CL on the same structure reveals the higher order modes present in this 3D nanostructure, which we link to the coupling and hybridization of rim modes present in the two upright hollow pillars of the split ring.

Toroidal dipole plasmon resonance modes in upright split ring resonators

Nanoscale split ring resonators (SRRs) have been a popular topic of study due to their surface plasmon resonance (SPR) modes and their many interesting interactions with light. They can be used as components in metamaterials exhibiting, among other properties, a negative refractive index. The surface plasmon properties of these structures are strongly dependent on their size and spatial arrangement. Most studies so far have focussed on the horizontal SRR due to the ease of fabrication. However, there are some advantages to be gained in the design of materials using upright SRRs. We are studying a structure composed of four upright SRRs as shown in Figure 1. The coupling of these four upright SRRs produces a magnetic dipole moment and a toroidal dipole moment. The toroidal dipole moment, when compared to electric and magnetic dipole moments, shows a higher quality factor and lower gain threshold for a nanoscale laser analogue, the spaser (surface plasmon amplification by stimulated emission of radiation) [1]. The presence of a strong toroidal dipole moment isolated from magnetic and electric dipole moments makes the structure under study a promising candidate for a spaser for use in on-chip telecommunications. A similar structure was first realized experimentally in the microwave regime of the electromagnetic spectrum [2]. Scaling the geometry down to nanoscale dimensions has been shown by simulation to shift the toroidal dipole energies into the near infra-red regime [1]. In this work we demonstrate the experimental fabrication (Figure 2) and characterization of this structure using electron energy loss spectroscopy (EELS), with confirmation of the modes provided by finite element method (FEM) simulations. We have fabricated this structure using a double patterning process in electron beam lithography, with precise alignment of the second lithography layer to the first. The structures are made from gold deposited on a 50 nm thick silicon nitride membrane. We probe the plasmon modes using EELS on a monochromated scanning transmission electron microscope, collecting spectrum images with nanometer spatial resolution and 60 meV energy resolution. We extract site-specific spectra (Figure 3a) and energy-resolved maps of the SPR modes (Figure 3b, c). We apply the Richardson-Lucy algorithm to further increase the effective energy resolution and identify the magnetic and toroidal dipole modes at energies of 0.52 eV and 0.72 eV, with SPR maps as shown in Figure 3b and 3c, respectively. We are able to correlate our EELS results with COMSOL Multiphysics FEM simulations. The simulated SPR response is given in Figure 3a, d, and e, showing close agreement in the peaks with our experimental data. Simulations confirm the low energy magnetic dipole mode (0.56 eV) and reveal two closely spaced toroidal dipole modes (0.61 eV, 0.66 eV) which are not perfectly resolved in the EELS data. We are able to tune the energy and strength of the toroidal dipole moment through tuning of the fabrication parameters; with careful design this structure is a promising spaser design for a range of applications near telecommunications frequencies. Keywords: surface plasmon resonance; EELS; split ring resonator; toroidal dipole

High Resolution Characterization of Plasmonic Hybridization in Silver Nanostructures

Surface plasmon resonances (SPR) in metallic nanostructures arise from the collective oscillation of conduction electrons, which create strong confined electric fields around the nanostructures. This confinement of electromagnetic (EM) energy at nanoscale dimensions holds potential towards the miniaturization of photonic devices [1]. Tremendous effort has been devoted towards optimization and design of nanostructures for several applications [2,3,4]. Most of these applications involve arrays of closely-spaced nanostructures: the plasmonic properties of the array differ from those of its isolated parts due to the interaction of evanescent fields. The study of SPR in these arrays, in particular the coupling of resonant modes, requires a characterization technique with both high spatial and energy resolution. Electron energy loss spectroscopy (EELS) meets these requirements, but has previously been limited to energies in the range of visible light or higher, mainly because of the relative intensities of the zero loss peak (ZLP) and low energy loss signal.

High resolution characterization of plasmon resonances in silver nanostructures

In this work, we use the iterative Richardson-Lucy (RL) deconvolution to further increase the energy resolution of electron energy loss spectra of surface plasmon resonances (SPR) in silver nanostructures. We obtain a record e_ective energy resolution of 10 meV after 500 iterations for spectral features below 1 eV. We extract energy- _ltered maps of SPR of a nanorod at energies down to 0.25 eV, corresponding to the mid-infrared region on the electromagnetic spectrum. And we are able to identify hydrid-SPR peaks separated by only 70 meV from two nano-squares with a gap of 100 nm between them, demonstrating that the RL deconvolution applied to spectra acquired with a monochromator is a useful tool to characterize plasmonic structures at low energies with high energy resolution.