[Paper Review] Spectrally resolved free electron-light coupling strength in a transition metal dichalcogenide

Niklas Müller, Soufiane el Kabil+4 more|arXiv (Cornell University)|May 20, 2024

2D Materials and ApplicationsMaterials Science3 citations

TL;DR

This study demonstrates spectrally resolved free electron-light coupling in a MoS₂ thin film edge using a strongly chirped broadband optical pulse in a ultrafast electron microscope. By mapping the coupling strength across position and photon energy, it reveals spectral and spatial modulations caused by interference between incident/reflected fields and guided optical modes, validated by FDTD simulations, enabling high-resolution mapping of near-field optical properties in 2D materials.

ABSTRACT

Recent advancements in electron microscopy have introduced innovative techniques enabling the inelastic interaction of fast electrons with tightly confined and intense light fields. These techniques, commonly summarized under the term photon-induced nearfield electron microscopy now offer unprecedented capabilities for a precise mapping of the characteristics of optical near-fields with remarkable spatial resolution but their spectral resolution were only scarcely investigated. In this study, we employ a strongly chirped and temporally broadband light pulse to investigate the interaction between free electrons and light at the edge of a MoS2 thin film. Our approach unveils the details of electron-light coupling, revealing a pronounced dependence of the coupling strength on both the position and photon energy. Employing numerical simulations of a simplified model system we identify these modulations to be caused by optical interferences between the incident and reflected field as well as an optical mode guided within the transition metal dichalcogenide film.

Motivation & Objective

• To achieve spectrally resolved mapping of electron-light coupling in transition metal dichalcogenides (TMDCs), overcoming limitations of conventional EELS and PINEM.
• To investigate the dependence of coupling strength on both spatial position and photon energy at a MoS₂ flake edge.
• To identify the origin of spectral and spatial modulations in coupling strength using experimental and numerical approaches.
• To demonstrate the feasibility of broadband, spectrally resolved near-field probing in 2D materials using chirped optical pulses in ultrafast electron microscopy.

Proposed method

• A 200-kV ultrafast transmission electron microscope (UTEM) with a laser-driven cold-field emitter source generates 200-fs electron pulses with 1 e⁻ per pulse.
• A non-collinear optical parametric amplifier (NOPA) produces a broadband, strongly chirped optical pulse (650–800 nm, up to 25 ps duration) focused at 57° to the surface.
• Electron energy spectra are recorded using a CEOS CEFID spectrometer and CMOS camera, with spatial scans performed via beam deflection coils and a de-scanning routine.
• The coupling strength is calculated from the electric field distribution using the formula $ g = \frac{e}{\hbar\omega} \int E_z(z) e^{-i\Delta kz} dz $, where $ \Delta k = \omega / v_e $.
• Numerical simulations use Lumerical’s FDTD solver to model the electric field around a MoS₂ flake (120×11 µm²), with permittivity tensors from literature and a Gaussian spectral pulse (725 nm center, 150 nm FWHM).
• Spatial and spectral coupling strength maps are compared between experiment and simulation to validate the origin of observed modulations.

Experimental results

Research questions

• RQ1 How does the free electron-light coupling strength vary spectrally and spatially at the edge of a MoS₂ thin film?
• RQ2 What physical mechanisms underlie the observed spectral and spatial modulations in the coupling strength?
• RQ3 To what extent do optical interference between incident and reflected fields, and guided modes in the flake, contribute to the coupling strength variations?
• RQ4 Can a chirped broadband pulse enable spectrally resolved mapping of near-field optical properties in 2D materials with sub-wavelength resolution?
• RQ5 How well do FDTD simulations reproduce the experimentally observed coupling strength patterns?

Key findings

• The coupling strength exhibits strong spectral and spatial modulations, with distinct peaks and dips correlated to interference between incident and reflected optical fields.
• The observed spectral dependence is attributed to interference effects and the excitation of a guided optical mode within the MoS₂ flake, confirmed by FDTD simulations.
• The coupling strength varies significantly across the flake edge, with maximum values reaching up to 0.3 eV for specific photon energies and positions.
• Simulations reproduce the experimental coupling strength patterns, validating that the modulations arise from interference and guided mode excitation rather than sample inhomogeneities.
• The use of a chirped broadband pulse enables spectrally resolved mapping without sacrificing spatial resolution, overcoming limitations of narrowband excitation.
• The method achieves spectral resolution limited by the optical pulse bandwidth, enabling detection of closely spaced optical modes in TMDCs.