Extreme Ultra-Violet Plasmonics and Cherenkov Radiation in Silicon

Silicon is widely used as the material of choice for semiconductor and insulator applications in nano-electronics, MEMS, solar cells and on-chip photonics. In stark contrast, in this paper, we explore silicon's metallic properties and show that it can support propagating surface plasmons, collective charge oscillations, in the extreme ultra-violet (EUV) energy regime not possible with other plasmonic materials such as aluminum, silver, or gold. This is fundamentally different from conventional approaches where doping semiconductors is considered necessary to observe plasmonic behavior. We experimentally map the photonic band structure of extreme ultra-violet (EUV) surface and bulk plasmons in silicon using momentum-resolved electron energy loss spectroscopy. Our experimental observations are validated by macroscopic electrodynamic electron energy loss theory simulations as well as quantum density functional theory calculations. As an example of exploiting these EUV plasmons for applications, we propose a tunable and broadband thresholdless Cherenkov radiation source in the EUV using silicon plasmonic metamaterials. Our work can pave the way for the field of EUV plasmonics.