Terahertz emission from $\alpha$-W/CoFe epitaxial spintronic emitters

We report efficient terahertz (THz) generation in epitaxial $\alpha$-W/Co$_{60}$Fe$_{40}$ spintronic emitters. Two types of emitters have been investigated; epitaxial $\alpha$-W$(110)$/Co$_{60}$Fe$_{40}(110)$ and $\alpha$-W$(001)$/Co$_{60}$Fe$_{40}(001)$ deposited on single crystalline Al$_{2}$O$_{3}$($11\bar{2}0$) and MgO($001$) substrates, respectively. First principle calculations of the electronic band structure at the W$(001)$ surface reveal Dirac-type surface states, similar to that reported previously for the W$(110)$ surface. The generated THz radiation is about $10\%$ larger for $\alpha$-W$(110)$/Co$_{60}$Fe$_{40}(110)$ grown on single crystalline Al$_{2}$O$_{3}$($11\bar{2}0$), which is explained by the fact that the $\alpha$-W$(110)$/Co$_{60}$Fe$_{40}(110)$ interface for this emitter is more transparent to the spin current due to the presence of \AA ngstr\" om-scale interface intermixing at the W/CoFe interface. Our results also reveal that the generation of THz radiation is larger when pumping with the laser light from the substrate side, which is explained by a larger part of the laser light due to interference effects in the film stack being absorbed in the ferromagnetic Co$_{60}$Fe$_{40}$ layer in this measurement configuration.