Efficient spintronic Terahertz source for lightwave scanning tunnelling microscopy

Terahertz lightwave scanning tunneling microscopy (THz-STM) enables the imaging of ultrafast dynamics of elementary excitations on surfaces with angstrom spatial and THz-sub-cycle temporal resolution. Spintronic THz emitters (STE) are attractive sources for THz-STM because they allow the generation of ultra-broadband single-cycle THz pulses with easy polarity and polarization control by magnetic fields. However, the ultrathin metal film of the STE makes it highly sensitive to laser-induced heating, limiting the THz fields achievable with multi-watt laser systems operating at MHz repetition rates as required for THz-STM. We show that average power heating can be effectively reduced by rotating the STE at sub-kHz speed. The rotating STE can be operated at optimal excitation fluences of ~0.1-1 mJ/cm2 using laser pulses of a few tens of µJ energy and up to 18 W power at MHz repetition rates collimated in a pump spot of a few mm diameter, resulting in power densities that would normally cause immediate thermal damage to the STE. Finally, we highlight the ability of the STE-driven THz-STM to drive plasmonic STM-luminescence and to probe ultrafast electron and phonon dynamics in 1T-TaS2.