Melanie Müller
Spintronic THz emitters (STE) are ideal and versatile THz sources to generate broadband, ultrashort single-cycle THz pulses with straightforward polarity and polarization control [1]. This makes them ideal candidates for THz-field-driven applications such as THz scanning tunneling microscopy (THz-STM) [2] and field-resolved THz scanning near-field optical microscopy (THz-SNOM) and spectroscopy [3,4]. To achieve measurable signal amplitudes, those applications require high repetition rates in the MHz-range at sufficiently high incident THz field strength of few kV/cm, which implies STE excitation at >10 W average powers and ~1 mJ/cm2 incident fluence [1]. However, the enhanced sensitivity of ultrathin metallic films such as the STE to average power heating [5], and the requirement of well-defined broadband THz propagation to the scanning probe tip pose significant challenges to the use of the STE for field-driven applications at high repetition rates.
In this talk I discuss the successful and reliable operation of spintronic THz emitters at high pump powers up to ~18 Watt and MHz repetition rates and its application for THz-STM. A rotating emitter design allows us to operate the STE at fluences close to ~1 mJ/cm2 using 10’s μJ pulse energies at MHz repetition rates. This enables STE operation at average power densities of ~1 kW/cm2, 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 ~105-106. 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 our rotating STE design opens up new possibilities for the integration of broadband STE sources in various applications that require high THz fields or THz power at high repetition rates.
[1] T.S. Seifert, L. Cheng, Z. Wei, T. Kampfrath, and J. Qi, “Spintronic sources of ultrashort terahertz electromagnetic pulses,” Appl. Phys. Lett., 2022.[2] M. Müller, N. Martín Sabanés, T, Kampfrath, and M. Wolf, “Phase-Resolved Detection of Ultrabroadband THz Pulses inside a Scanning Tunneling Microscope Junction,” ACS Photonics 7, 8, 2046-2055, 2020. [3] N. Martín Sabanés, F. Krecinic, T. Kumagai, F. Schulz, M. Wolf, and M. Müller, “Femtosecond Thermal and Nonthermal Hot Electron Tunneling Inside a Photoexcited Tunnel Junction,” ACS Nano 16, 9, 14479–14489, 2022.
[4] M. Eisele, T.L. Cocker, M.A. Huber, M. Plankl, L. Viti, D. Ercolani, L. Sorba, M.S. Vitiello and R. Huber, “Ultrafast multi-terahertz nano- spectroscopy with sub-cycle temporal resolution,” Nature Photonics 8, 841–845, 2014. [5]. T. Vogel, A. Omar, S. Mansourzadeh, F. Wulf, N. Martín Sabanés, M. Müller, T.S. Seifert, A. Weigel, G. Jakob, M. Kläui, I. Pupeza, T. Kampfrath, and C.J. Saraceno, “Average power scaling of THz spintronic emitters efficiently cooled in reflection geometry,” Opt. Express 30, 20451-20468, 2022.