Optical excitations in electron microscopy

Abstract

This review discusses how low-energy valence excitations created by swift electrons can render information on the optical response of structured materials with unmatched spatial resolution. Electron microscopes are capable of focusing electron beams on subnanometer spots and probing the target response either by analyzing electron energy losses or by detecting emitted radiation. Theoretical frameworks suited to calculate the probability of energy loss and light emission ͑cathodoluminescence͒ are reconsidered and compared with experimental results. More precisely, a quantum-mechanical description of the interaction between the electrons and the sample is discussed, followed by a powerful classical dielectric approach that can be applied in practice to more complex systems. The conditions are assessed under which classical and quantum-mechanical formulations are equivalent. The excitation of collective modes such as plasmons is studied in bulk materials, planar surfaces, and nanoparticles. Light emission induced by the electrons is shown to constitute an excellent probe of plasmons, combining subnanometer resolution in the position of the electron beam with nanometer resolution in the emitted wavelength. Both electron energy-loss and cathodoluminescence spectroscopies performed in a scanning mode of operation yield snapshots of plasmon modes in nanostructures with fine spatial detail as compared to other existing imaging techniques, thus providing an ideal tool for nanophotonics studies.

Gallium Plasmonics: Deep Subwavelength Spectroscopic Imaging of Single and Interacting Gallium Nanoparticles

“…The size tuning observed from D = 40À80 nm is the result of phase retardation across the particle and is reproduced well in the simulations ( Figure 4B). 27 For the largest particles, with D > 100 nm, the spectra from the edge and center are distinct, and we observe the vertical modes (colored lines) shift from the UV into the detection range of our CL system (λ 0 > 350 nm). Rather than a single resonance, this peak is nearly a continuum of resonances resulting from strong interactions between the Ga NPs and the underlying high index Si substrate, which enables the vertical dipolar mode to hybridize with higher-order modes.…”

“…G dielectric interface, could also provide such a spectral feature, 16,27 we have accounted for this background signal by subtracting the CL measured within an interstitial region directly adjacent to the measured NP. This removes both TR and incoherent emission from the Si substrate.…”

“…Different excitation methods will, therefore, produce different emission spectra. 27 On the basis of these calculations, the two observed excitability distributions, annuli and bright centers, can be understood as rotationally averaged emission from these in-plane or out-of-plane dipolar resonances ( Figure 3D,E). 38,42 For an in-plane dipole the largest vertical field components will occur at the edges of the NP.…”

“…27,34À36 A larger induced local field corresponds to an increased transition probability. 27 Within the spatial map ( Figure 2C), six particles are highlighted and assigned the letters aÀf, with corresponding diameters estimated from the survey scan as D = 40, 60, 80, 100, 120, and 140 nm, respectively. This distribution of sizes spans the particle distribution measured in Figure 1C, so analysis of these six NPs can provide a representative understanding of the optical response of this Ga NP ensemble.…”

Fano Resonance in an Electrically Driven Plasmonic Device

“…Finite-difference time-domain (FDTD) calculations reveal that this resonance is due to interference between the nanoparticle and electrodes dipolar fields, and can be conveniently controlled by the structural parameters. Electrically driven plasmonic devices may offer unique opportunities as a research tool and for practical applications [1][2][3][4][5] . In such devices, current that flows across a metallic tunnel junction…”