Graphene on nanoscale gratings for THz electron-beam radiation and plasmonics
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Terahertz (THz) technologies have numerous applications such as biological and medical imaging, security screening, remote sensing, and industrial process control. However, the lack of practical THz sources and detectors is still a significant problem limiting the impact of these applications. In this Thesis work, three novel THz radiation mechanisms are proposed and investigated, based on the distinctive electronic properties of charge carriers in 2D single-layer graphene and related 1D conductors (i.e., graphene nanoribbons and carbon nanotubes), combined with the use of nanoscale dielectric gratings. Numerical simulations as well as fabrication and characterization activities are carried out. The first proposed radiation mechanism is based on the mechanical corrugation of a single-layer sheet of graphene or 1D carbon conductor, deposited on a lithographically-defined sinusoidal grating. In the presence of a dc voltage, carriers will therefore undergo periodic angular motion and correspondingly radiate (similar to cyclotron emission but without the need for any external magnetic field). My numerical simulations indicate that technologically significant output power levels can correspondingly be obtained at geometrically tunable THz frequencies. Initial graphene samples on sinusoidal gratings were fabricated and found to undergo significant strain redistribution, which affects their structural quality. Charge carriers moving in a flat sheet of graphene or linear 1D carbon conductor parallel to a nanoscale grating can also produce THz radiation based on the Smith-Purcell effect. The role of the grating in this case is to diffract the evanescent electromagnetic fields produced by the moving electrons and holes so that THz light can be radiated. Once again, numerical simulations indicate that this approach is promising for the realization of ultra-compact THz sources capable of room-temperature operation. Initial experimental results with ultra-high-mobility graphene samples embedded in boron nitride films show promising THz electroluminescence spectra. The last approach considered in this Thesis involves graphene plasmons at THz frequencies, which can be excited through the decay of hot electrons injected with an applied bias voltage. A nearby grating can then be used to outcouple the guided electromagnetic fields associated with these collective charge oscillations into radiation. The excitation of these THz plasmonic resonances at geometrically tunable frequencies has been demonstrated experimentally via transmission spectroscopy measurements.