Stimulated electron energy loss and gain in an electron microscope without a pulsed electron gun

Abstract

We report on a novel way of performing stimulated electron energy-loss and energy-gain spectroscopy (sEELS/sEEGS) experiments that does not require a pulsed gun. In this scheme, a regular scanning transmission electron microscope (STEM) equipped with a conventional continuous electron gun is fitted with a modified EELS detector and a light injector in the object chamber. The modification of the EELS detector allows one to expose the EELS camera during tunable time intervals that can be synchronized with nanosecond laser pulses hitting the sample, therefore allowing us to collect only those electrons that have interacted with the sample under light illumination. Using ∼ 5 ns laser pulses of ∼ 2 eV photon energy on various silver samples, we obtain evidence of sEELS/sEEGS through the emergence of up to two loss and gain peaks in the spectra at ±2 and ±4 eV. Because it does not involve any modification of the gun, our method retains the original performances of the microscope in terms of energy resolution and spectral imaging with and without light injection. Compared to pulsed-gun techniques, our method is mainly limited to a perturbative regime (typically no more that one gain event per incident electron), which allows us to observe resonant effects, in particular when the plasmon energy of a silver nanostructure matches the laser photon energy. In this situation, both EELS and EEGS signals are enhanced in proportion to n + 1 and n, respectively, where n is the average plasmon population due to the external illumination. The n term is associated with stimulated loss and gain processes, and the term of 1 corresponds to conventional (spontaneous) loss. The EELS part of the spectrum is therefore an incoherent superposition of spontaneous and stimulated EEL events. This is confirmed by a proper quantum-mechanical description of the electron/light/plasmon incorporating light-plasmon and plasmon-electron interactions, as well as inelastic plasmon decay.

Near field excited state imaging via stimulated electron energy gain spectroscopy of localized surface plasmon resonances in plasmonic nanorod antennas

“…Table 2 summarizes the M x max numbers estimated from the peak fits of the spectra taken at the spontaneous EELS intensity maximum positions for each mode. Note that while higher light-driven plasmon populations are realized in high-irradiance pulsed experiments 57 , the values realized here are consistent with previous low irradiance cw experiments 59 .…”

“…As noted previously 51 and confirmed in our previous work 59 , the sEELS and sEEGS peaks have nearly the same amplitude and thus while the sEEL peaks are convolved with the LSPR peaks, we can unambiguously fit the sEEG peaks and thus de-convolve the sEEL and LSPR peaks (see supplemental information for peak fitting). Furthermore, Das et al 57 showed that the light-driven population of the plasmon mode (M x max ) can be estimated…”

“…Recently, Das et al demonstrated that by appropriately gating the EEL spectrometer, a high frequency nanosecond pulsed laser can be used to generate characteristic sEEL and sEEG with a continuous current electron source 57 . Furthermore, by coupling to a plasmonic nanostructure with a resonance at the laser frequency they demonstrated so-called resonant sEEL and sEEG.…”

Attosecond metrology in a continuous-beam transmission electron microscope

“…Continuous-wave laser illumination of complex materials is feasible at intensities of 10 4 to 10 6 W/cm 2 (31, 32) and with field strengths of ~10 6 V/m, enough to produce substantial holographic image distortions (9) or spectral changes in photon-induced near-field electron microscopy (6,31). Nanosecond excitation can be used for alleviating thermal load (33). In contrast to femtosecond pump-probe microscopy and diffraction, which aim at tracking the evolution of a material after absorption of a femtosecond laser pulse, the main application of our electron microscopy will be investigations of electromagnetic phenomena that are triggered by the cycles of light (4) and not by the intensity envelope of pulses (1,2).…”

High brightness ultrafast transmission electron microscope based on a laser-driven cold-field emission source: principle and applications

“…To allow excitation of the sample by a laser beam or collection of its cathodoluminescence, a light injection/cathodoluminescence system has been installed in the objective lens. It is composed of a parabolic mirror placed above the sample which can be adjusted from outside the TEM column by micrometer screws [36]. We describe in the following the electron emission process and compute the temporal and spectral properties of the electron pulses.…”