Nonlinear current-driven terahertz surface plasmons in metallic nanotubes via optical beating

We develop a theory of terahertz surface-plasmon generation in hollow metallic nanotubes driven by a beat-induced nonlinear current. Two co-propagating femtosecond laser beams produce a second-order ponderomotive force at the difference frequency, generating an axial surface current confined within the metal skin depth. This nonlinear current acts as a boundary source that excites cylindrical surface-plasmon modes when an axial phase-synchronism condition is satisfied. We derive the dispersion relation and obtain an analytical expression for the terahertz amplitude, demonstrating that the emission frequency and efficiency are governed predominantly by geometric confinement rather than the intrinsic material response. The radiated power scales with the nanotube radius and interaction length, enabling tunable and scalable terahertz output. The results identify a geometry-controlled nonlinear coupling mechanism and establish metallic nanotubes as compact plasmonic platforms for coherent terahertz generation.