Electron Spectroscopy at Cryogenic Temperatures for Nano-Optics

<br/>Fast electrons spectroscopies have had huge success for nano-optics [1]. For phase-locked excitations (e. g. surface plasmons) electron energy loss spectroscopy (EELS) is an optical extinction analogue and cathodoluminescence (CL) that of optical scattering [2]. For "incoherent" excitations, EELS also measures optical extinction for atomically thin materials [3,4,5], while CL measures spectra similar to off-resonance CL [3]. For many technologically and fundamentally interesting materials, including semiconductors, experiments at cyrogenic temperatures is a requirement. For example, many quasi-particles (such as excitons and trions) are only stable at low temperatures, due to their low binding energy. This will be discussed in this contribution, including variable temperature measurements down to about 100 K. This will be used as a motivation for still colder experiments (to be realized).<br/><br/><br/>Despite clear benefits (link to structural and chemical information, atomic-scale spatial resolution and broadband excitation), electron spectroscopies have some penalties which limit applications to nano-optics: lack of resonant excitation and polarization degrees of freedom and still limited spectral resolution (EELS). In this seminar, we will discuss how temporally resolved spectroscopies can mitigate some of these issues.<br/><br/>The lack of excitation energy control can be circumvented by measuring the energy lost by each electron in time coincidence EELS-CL experiments. This has been achieved using a nanosecond-resolved direct electron detector (Timepix3) [6], correlation electronics and a PMT. The information retrieved here is analogous to that of photoluminescence excitation spectroscopy (PLE), hence we name it cathodoluminescence excitation spectroscopy (CLE) [7]. With it, we explored the relative quantum efficiency of different excitation energies and decay pathways towards 4.1 eV defect photon emission in h-BN flakes [7].<br/><br/>The current impressive energy resolution achieved with electron beams (~meV range) is still orders of magnitude away from the necessary one to access the physics of lifetime-limited high quality factor optical modes, such as cavity modes in dielectric spheres [8]. As proposed more than a decade ago [9], electron energy gain spectroscopy (EEGS) should easily overcome this limitation. Here, we will discuss EEGS experiments using fast electron blankers (~ns) in a continuous-gun electron microscope, allowing energy resolution delivered by laser beams compatible with a state-of-the-art electron microscope performance. As an example, we will show measurements of whispering gallery modes in dielectric spheres separated by a few hundreds µeV with energy sampling in the µeV scale [10].<br/><br/>[1] F. J. García de Abajo, Rev. Mod. Phys. 82, 209 (2010).<br/>[2] A. Losquin, et al., Nano Lett. 15, 1229 (2015).<br/>[3] N. Bonnet, et al., Nano Lett. 21, 10178 (2021).<br/>[4] F. Shao, et al., Phys. Rev. Mater. 6, 074005 (2022).<br/>[5] S. Y. Woo, et al., in preparation (2022).<br/>[6] Y. Auad, et al., Ultramicroscopy 239, 113539 (2021).<br/>[7] N. Varkentina, et al., 8, eabq4947 (2022) (2022).<br/>[8] Y. Auad, et al., Nano Lett. 22, 319 (2022).<br/>[9] F. J. García de Abajo and M. Kociak, New J. Phys. 10, 073035 (2008).<br/>[10] Y. Auad et al., in preparation (2022).