Deep ultra-violet plasmonics: exploiting momentum-resolved electron energy loss spectroscopy to probe germanium

Abstract

Germanium is typically used for solid-state electronics, fiber-optics, and infrared applications, due to its semiconducting behavior at optical and infrared wavelengths. In contrast, here we show that the germanium displays metallic nature and supports propagating surface plasmons in the deep ultraviolet (DUV) wavelengths, that is typically not possible to achieve with conventional plasmonic metals such as gold, silver, and aluminum. We measure the photonic band spectrum and distinguish the plasmonic excitation modes: bulk plasmons, surface plasmons, and Cherenkov radiation using a momentum-resolved electron energy loss spectroscopy. The observed spectrum is validated through the macroscopic electrodynamic electron energy loss theory and first-principles density functional theory calculations. In the DUV regime, intraband transitions of valence electrons dominate over the interband transitions, resulting in the observed highly dispersive surface plasmons. We further employ these surface plasmons in germanium to design a DUV radiation source based on the Smith-Purcell effect. Our work opens a new frontier of DUV plasmonics to enable the development of DUV devices such as metasurfaces, detectors, and light sources based on plasmonic germanium thin films.

Electron Microscopy Probing Electron-Photon Interactions in SiC Nanowires with Ultrawide Energy and Momentum Match

“…The recent advances of monochromators in STEM enable an atom-wide kilo-electronvolt electron probe with about sub-10 meV energy resolution, − allowing atomically resolved EELS analysis of many physical excitations such as vibrational spectroscopy, phonon polaritons, and plasmons in an extremely wide continuous spectral range with subnanometer spatial resolution. ,,− In addition, the high energy electrons can usually excite more multipole modes, even including the optical inactive dark modes. , However, research on the optical resonating spectroscopy of dielectric nanostructures via STEM-EELS has been rarely reported, which is mainly due to the optical frequency windows of the resonating modes usually overlap with plasmon/interband transitions of dielectric materials in EELS. In fact, for the dielectric microcavities with suitable dimensions (few tens of nanometers to micrometers), the intrinsically optical resonating spectra can be obtained without the influence of plasmon and interband transitions via aloof geometries (the electron beam positioned in the vacuum near the sample). ,, …”

“…In fact, for the dielectric microcavities with suitable dimensions (few tens of nanometers to micrometers), the intrinsically optical resonating spectra can be obtained without the influence of plasmon and interband transitions via aloof geometries (the electron beam positioned in the vacuum near the sample). 27,39,40 In this work, we characterize the interactions of fast electrons and optical resonating modes in individual silicon carbide (SiC) nanowires with different diameters using STEM-EELS in the aloof mode where the electron beam is positioned several nanometers away from the sample. As a natural wide-bandgap material, SiC has important applications both in electronics and photonics.…”