  Femtosecond laser excitation of a Co/Pt bilayer results in the efficient
emission of picosecond THz pulses. Two known mechanisms for generating THz
emission are spin-polarized currents through a Co/Pt interface, resulting in
helicity-independent electric currents in the Pt layer due to the inverse
spin-Hall effect and helicity-dependent electric currents at the Co/Pt
interface due to the inverse spin-orbit torque effect. Here we explore how
roughness, crystal structure and intermixing at the Co/Pt interface affect the
efficiency of the THz emission. In particular, we varied the roughness of the
interface, in the range of 0.1-0.4 nm, by tuning the deposition pressure
conditions during the fabrication of the Co/Pt bilayers. To control the
intermixing at the Co/Pt interface a 1-2 nm thick CoxPt1-x alloy spacer layer
was introduced with various compositions of Co and Pt. Finally, the crystal
structure of Co was varied from face centered cubic to hexagonal close packed.
Our study shows that the roughness of the interface is of crucial importance
for the efficiency of helicity-dependent THz emission induced by femtosecond
laser pulses. However, it is puzzling that intermixing while strongly enhancing
the helicity-independent THz emission had no effect on the helicity-dependent
THz emission which is suppressed and similar to the smooth interfaces.
