Concept of terahertz waveguide plasmon amplifier based on a metal groove with active graphene

We propose a concept of terahertz waveguide plasmon amplifier based on a metal groove with active graphene. It is shown that the power amplification factor of the longitudinal-section magnetic (LSM) waveguide plasmon (normalized to its wavelength) near the cutoff frequency of this mode can exceed the amplification factor of the transverse magnetic (TM) plasmon in a layered graphene structure by more than four orders of magnitude for the same frequency. This is caused by the increase of the LSM plasmon wavelength near the cutoff frequency, smaller energy velocity of the LSM mode, and greater energy release from graphene for the LSM plasmon due to stronger lateral confinement of the LSM waveguide plasmon as compared to the TM plasmon in a layered graphene structure. We show that the enhancement of the LSM plasmon amplification factor near the cutoff frequency is a stronger effect than that due to screening of graphene.