Terahertz emission spectroscopy of YIG | topological insulator bilayers*

Building on the demonstration of terahertz (THz) emission spectroscopy (TES) in metallic ferromagnet|normal metal heterostructures, recent works have now observed the THz emission from heterostructure bilayers of both yttrium iron garnet (YIG) | Pt and Co | Bi₂Se₃. We examine the energy dependence of the spin-charge conversion efficiency of THz emission in samples of a few quintuple layers of Bi₂Se₃ and (Bi_.24Sb_.76)₂Te₃ grown on YIG(100) and YIG(111) substrates. We excite the samples with ultrafast femtosecond light pulses of both 1.55 eV and 3.10 eV. At 1.55 eV, no electrons in YIG are excited above the band gap of 2.85 eV. When we double the photon energy to 3.10 eV, electrons in the YIG are excited into the conduction band. Since both photon energies emit THz radiation, we expect that in each case the mechanism of emission will be attributed to different effects. Specifically, with excitations of 1.55 eV, the spin current process should be predominately generated by the spin-Seebeck effect. With excitations of 3.10 eV, the generation of spin current should be dominated by the direct photoexcitation of spin-polarized charge carriers.