Deep ultra-violet plasmonics in germanium (Invited Paper)

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 a metallic nature and supports propagating surface plasmons in the deep ultraviolet (DUV) wavelengths, which 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 momentum-resolved electron energy loss spectroscopy. The observed spectrum is validated through electrodynamic electron energy loss theory and first-principles 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 detectors, and light sources based on germanium.