Electron Beam Spectroscopy for Nanophotonics

Abstract: Progress in electron-beam spectroscopies has recently enabled the study of optical excitations with combined space, energy and time resolution in the nanometer, millielectronvolt and femtosecond domain, thus providing unique access into nanophotonic structures and their detailed optical responses. These techniques rely on $\approx$ 1-300 keV electron beams focused at the sample down to sub-nanometer spots, temporally compressed in wavepackets a few femtoseconds long, and in some cases controlled by ultrafast light pulses. The electrons undergo energy losses and gains, also giving rise to cathodoluminescence light emission, which are recorded to reveal the optical landscape along the beam path. This review portraits these advances, with a focus on coherent excitations, emphasizing the increasing level of control over the electron wave functions and ensuing applications in the study and technological use of optically resonant modes and polaritons in nanoparticles, 2D materials and engineered nanostructures.

• The paper demonstrates that electron-beam spectroscopies (EELS and CL) enable high-resolution investigation of optical excitations in nanostructured materials.
• It employs advanced numerical simulations like BEM and FDTD to correlate spectral data with precise morphological details.
• The review highlights emerging ultrafast and coherent techniques that pave the way for quantum control and next-generation photonic device innovation.

Electron Beam Spectroscopy for Nanophotonics: A Formal Review

The paper "Electron Beam Spectroscopy for Nanophotonics" by Albert Polman, Mathieu Kociak, and F. Javier García de Abajo provides an extensive review of advancements in electron-beam spectroscopies, specifically focusing on their application in nanophotonics. It critically outlines the significant progress made in achieving unprecedented space, energy, and time resolution that is vital for investigating optical excitations and their complex interactions within nanophotonic structures.

Overview of Techniques and Applications

Electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) are the primary electron-beam spectroscopies discussed in this paper. They exploit high-energy electron beams, ranging from 1 to 300 keV, scanning over specimens in either transmission electron microscopes (TEM) or scanning electron microscopes (SEM). These methods have developed to allow correlations between the spectral data and morphological details obtained from complementary techniques such as secondary electron imaging and energy-dispersive X-ray spectroscopy.

The paper emphasizes the capability of electron-beam spectroscopies to probe optical resonances and polaritons in an array of materials, including nanoparticles, 2D materials, and engineered nanostructures, thereby enhancing the understanding of spatially-localized optical phenomena down to the atomic and nanometer scale. Hyperspectral imaging is highlighted as a key advancement, which involves mapping two-dimensional CL or EELS spectral data across different emitted light wavelengths or electron energy losses.

Numerical Results and Contradictory Claims

One compelling feature of CL and EELS is their ability to simultaneously probe both radiative and non-radiative processes, providing valuable insights into modes that are otherwise inaccessible via far-field optical techniques. The authors demonstrate that these methods can map higher-order modes that dissipate energy non-radiatively. This ability is consolidated by analytical and numerical simulations, employing approaches such as the boundary-element method (BEM) and finite-difference time-domain (FDTD) techniques, which enhance the accurate modeling of complex nanostructures.

Recent Advances and Future Directions

Significant strides in coherent EELS and CL have been achieved, including the integration of ultrafast electron microscopy (UEM) for dynamic studies of material responses under pulsed light and electron conditions. Such developments facilitate innovative pump-probe spectroscopy, where the timing between laser and electron pulses can be precisely controlled, providing deeper insights into the ultrafast dynamics of plasmonic and phononic interactions.

The review discusses the intriguing frontier of shaping electron wave functions, hinting at new frontiers in electron-matter interaction control. These capabilities open avenues for coherent optical manipulation at the nanoscale, allowing experiments to be conducted that exploit the quantum properties of electrons for advanced probing of material states.

Implications and Speculations

The implications of these advancements are significant for both theoretical studies and practical applications. Coherent control over electron beams may lead to new methodologies in quantum information and solid-state physics. For practical applications, the ability to inspect nanoscale features with high spatial and temporal resolution is invaluable for the development of next-generation photonic devices.

Looking forward, the paper prompts the exploration of new experimental configurations that harness structured electron beams, potentially enabling novel interferometric measurements and the probing of quantum states in solid-state systems. Additionally, enhancements in electron source brightness and energy resolution promise to further refine EELS and CL capabilities.

In conclusion, this extensive review underscores the transformative role of electron beam spectroscopy in advancing nanophotonics. As technology progresses, these techniques are poised to unveil new possibilities in coherent electron-light interactions, contributing to both fundamental science and technological innovation in photonics.