In this paper, we investigated the nonlinear excitation of tuneable, resonant, and thickness-dependent terahertz surface plasmons in gold nanofilm by an optical mixing of two laser beams. Two laser beams have different frequencies ( \({\omega }_{1}\) and \({\omega }_{2}\) ) are incident obliquely (nearly normal incidence) on the of a gold nanofilm. The film electrons get oscillatory velocities by laser electric fields. The pondermotive force exerted by laser beams on electrons, modified the oscillatory velocity of electrons. A nonlinear current is generated through the coupling between the oscillatory velocity and electron density. This nonlinear current is treated as the sources of resonant terahertz surface plasmon generation. The amplitude and frequency of these terahertz surface plasmons can be tuned resonantly with the thickness of gold film. A novel feature of this study is that the thickness-dependent surface plasmons resonance enhanced the efficiency of this nonlinear processes. The theoretical results predicted in this work can be verified experimentally in the future, and it will be useful to design the compact and tuneable terahertz radiation sources.
