  We report on generation of pulsed broadband terahertz radiation utilizing the
inverse spin Hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The
emitter was optimized with respect to layer thickness, growth parameters,
substrates and geometrical arrangement. The experimentally determined optimum
layer thicknesses were in qualitative agreement with simulations of the spin
current induced in the ferromagnetic layer. Our model takes into account
generation of spin polarization, spin diffusion and accumulation in Fe and Pt
and electrical as well as optical properties of the bilayer samples. Using the
device in a counterintuitive orientation a Si lens was attached to increase the
collection efficiency of the emitter. The optimized emitter provided a
bandwidth of up to 8 THz which was mainly limited by the low-temperature-grown
GaAs (LT-GaAS) photoconductive antenna used as detector and the pulse length of
the pump laser. The THz pulse length was as short as 220 fs for a sub 100 fs
pulse length of the 800 nm pump laser. Average pump powers as low as 25 mW (at
a repetition rate of 75 MHz) have been used for terahertz generation. This and
the general performance make the spintronic terahertz emitter compatible with
established emitters based on nonlinear generation methods.
