Isolated carbon nanotubes as high-impedance transmission lines for microwave through terahertz frequencies

A hybrid approach, combining quantum theory with classical distributed-element transmission-line models, is taken to study the transport of high-frequency energy in isolated metallic carbon nanotubes (CNTs). The characteristic impedance for these transmission lines would be approximately 40 kΩ, which is unusually high because of the kinetic inductance of the CNTs and the lack of capacitive shunting to a conducting ground plane or other objects. The propagation is by TM surface waves, instead of the TEM waves for propagation in nanotubes that are parallel to a planar conducting surface so the phase velocity is much less than the velocity of light in vacuum. The phase velocity would be approximately 3 × 10⁶ m/s, and both the characteristic impedance and phase velocity are essentially independent of the radius of the CNT. However, the numerical results must be regarded as being provisional because corrections have not been made for the effects of transport in multiple channels that are caused by the band structure and spin degeneracy. One application is considered for a new type of device that would generate radiation at microwave through terahertz frequencies by photomixing in laser-assisted field emission.