  Spin-based terahertz (THz) emitters, utilizing the inverse spin Hall effect,
are ultra-modern sources for the generation of THz electromagnetic radiation.
To make a powerful emitter having large THz amplitude and bandwidth,
fundamental understanding in terms of microscopic models is essential. This
study reveals important factors to engineer the THz emission amplitude and
bandwidth in epitaxial Pt/Fe emitters grown on MgO and MgAl$_2$O$_4$ (MAO)
substrates, where the choice of the substrate plays an important role. The THz
amplitude and bandwidth are affected by the induced strain in the Fe spin
source layer. On the one hand, the THz electric field amplitude is found to be
larger when Pt/Fe is grown on MgO even though the crystalline quality of the Fe
film is superior when grown on MAO. This is because of the larger defect
density, smaller electron relaxation time, and lower electrical conductivity in
the THz regime when Fe is grown on MgO. On the other hand, the bandwidth is
found to be larger for Pt/Fe grown on MAO and is explained by the
uncoupled/coupled Lorentz oscillator models. This study provides an insightful
pathway to further engineer metallic spintronic THz emitters in terms of the
proper choice of substrate and microscopic properties of the emitter layers.
