  Spintronic terahertz (THz) emitters (STE) based on the inverse spin Hall
effect in ferromagnetic/heavy metal (FM/HM) heterostructures have become
important sources for THz pulse generation. The design, materials and control
of these interfaces at the nanometer level has become vital to engineer their
THz emission properties.In this work, we present studies of the optimization of
such structures through a multi-pronged approach, taking advantage of material
and interface engineering to enhance the THz spintronic emission. This
includes: the application of multi-stacks of HM/FM junctions and their
application to trilayer structures, the use of spin-sinks to simultaneously
enhance the THz emitted fields and reduce the use of thick Pt layers to reduce
optical absorption, and the use of semi-metals to increase the spin
polarization and thus the THz emission. Through these approaches, significant
enhancements of the THz field can be achieved. Importantly, taking into account
the optical absorption permits to elucidate novel phenomena such as the
relation between the spin diffusion length and the spin-sink using THz
spectroscopy, as well as possibly distinguishing between self and interface
spin-to-charge conversion in semi-metals.
