  Spin-to-charge conversion (SCC) involving topological surface states (TSS) is
one of the most promising routes for highly efficient spintronic devices for
terahertz (THz) emission. Here, the THz generation generally occurs mainly via
SCC consisting in efficient dynamical spin injection into spin-locked TSS. In
this work, we demonstrate sizable THz emission from a nanometric thick
topological insultator (TI)/ferromagnetic junction - SnBi$_2$Te$_4$/Co -
specifically designed to avoid bulk band crossing with the TSS at the Fermi
level, unlike its parent material Bi$_2$Te$_3$. THz emission time domain
spectroscopy (TDS) is used to indicate the TSS contribution to the SCC by
investigating the TI thickness and angular dependence of the THz emission. This
work illustrates THz emission TDS as a powerful tool alongside angular resolved
photoemission spectroscopy (ARPES) methods to investigate the interfacial
spintronic properties of TI/ferromagnet bilayers.
