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Description
Color centers are a solid-state realization of a two-level system (TLS). They are of special technological interest because TLS can be used as single photon emitters [1, 2]. The correlation of EELS and CL may combine the sensitivity of CL with the spatial resolution of EELS [3]. It is therefore particularly appealing for studying color centers at the nanoscale, where their optical properties can be modified by the morphology of the sample, e.g. through strain or quantum confinement [1]. These systems can be excited at controlled energies by photons, what is impossible with free electrons due to the physics of electron-matter scattering.
This limitation can be overcome by the measurement of each electron energy loss in coincidence electron energy loss and photon emission experiments. Drawing inspiration from Photoluminescence Excitation Spectroscopy, this technique allows to target resonant excitation pathways, which motivates the name Cathodoluminescence Excitation Spectroscopy (CLE). It is implemented by measuring temporal coincidences on the nanosecond scale. Time-resolved detection is enabled by a Timepix3 detector for electrons and photomultiplier tubes (PMTs) for photons. This setup, installed on a Nion Hermes STEM, enables us to track which excitation energies lead to photon emission, which is essential for addressing the broadband nature of the electron-matter interaction [3, 4].
So far, the detection of resonant excitation of color centers is hindered by the spectral overlap with another class of signals: the coherent electromagnetic response of the structure [5, 6, 7]. On hBN containing 4.1 eV defects, we observe thickness and acceleration voltage-dependent spectral features, which we identify in concordance with simulations as electromagnetic response of the structure. Based on the different physical nature of the phenomena, we propose approaches for filtering the coherent electromagnetic response from the signal of the sought-after direct excitation of color centers, and assess the theoretical efficacity of different approaches, notably angular and temporal filtering.
[1] M. Kociak, L.F. Zagonel, Ultramicroscopy 176 (2017), 112–131
[2] R Bourellier et al., Nano Letters 16 (2016), 4317-4321
[3] N Varkentina et al., Science Advances 8 (2022), eabq4947
[4] Y Auad et al., Ultramicroscopy 239 (2022), 113539
[5] J Garcia de Abajo et al., Reviews of Modern Physics 82 (2010) 209-275
[6] N Yamamoto et al., Journal of Electron Microscopy 45 (1996), 64-76
[7] N Yamamoto et al., Scanning Microscopy 9 (1995), 669676