High-Power Operation of Spintronic Terahertz Emitters for THz-Field-Driven Scanning Probe Microscopy at MHz Repetition Rates

We discuss the successful and reliable operation of spintronic Terahertz (THz) emitters at high pump powers up to $\sim 18$ Watt and MHz repetition rates for THz-field-driven scanning tunneling microscopy (THz-STM). A rotating design of the spintronic emitter (STE) allows us to operate the STE at fluences close to $\sim 1$ mJ/cm
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using 10’s ${\mu} \mathrm{J}$ pulse energies at MHz repetition rates. This enables STE operation at average power densities of $\sim 1 kW/\mathrm {cm}^{2}$, well above the laser damage threshold of thin metal films, with minimized thermal heating and no material degradation. With this new STE design, we reach incident THz field strength of several kV/cm at the tip-sample junction of the STM, resulting in THz bias voltages of more than 10 Volts using standard tungsten tips with THz field enhancement of $(\sim 10)^{5}-10^{{6}}$. We discuss the importance of well-optimized THz beam propagation, which due to limited mirror size and long beam paths is a crucial aspect for STE-driven THz-STM operation. The scalability of the rotating STE design opens up new possibilities for the integration of broadband STE sources in applications that require high THz fields or THz power at high repetition rates.