Optimum excitation wavelength and photon energy threshold for spintronic terahertz emission from Fe/Pt bilayer

Terahertz emission from ferromagnetic/non-magnetic spintronic heterostruc-tures had been demonstrated as pump wavelength-independent. We report, however, the pump wavelength dependence of terahertz emission from an opti-mized Fe/Pt spintronic bilayer on MgO substrate. Maximum terahertz generation per total pump power was observed in the 1200-to 1800-nm pump wavelength range, and a marked decrease in the terahertz emission efficiency beyond 2500 nm (pump photon energies <0.5 eV) suggests a similar to 0.35-eV threshold pump photon energy for effective spintronic terahertz emission. The inferred threshold is supported by previous theoretical results on the onset energy of significant spin-filtering at the Fe-Pt interface, and confirmed by Fe/Pt electronic structure calculations in this present work. The results of terahertz time-domain emission spectroscopy show the sensitivity of spintronic terahertz emission to both the op-tical absorptance of the heterostructure and the energy-dependent spin trans-port, as dictated by the properties of the metallic thin films.