From ultrafast photocurrents in anisotropic metals to emitters of broadband terahertz radiation

• Applicant: Professor Dr. Tobias Kampfrath
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 544376034

Terahertz (THz) electromagnetic radiation is an indispensable tool for spectroscopy and characterization of materials. The table-top generation of broadband THz pulses often relies on femtosecond laser pulses that trigger ultrashort photocurrents in semiconductor thin-films. Only very recently, a new class of THz emitters based on anisotropic metal thin-films was demonstrated. In these anisotropic-conductivity THz emitters (ACEs), a normally incident laser pulse first induces an effective transient electric field along the film normal. Second, due to the anisotropic electric conductivity of parts of the film, the transient out-of-plane field drives a photocurrent with a significant in-plane component, which gives rise to the efficient emission of a THz pulse. In this project, we will use ultrabroadband THz time-domain spectroscopy from 0.5 THz up to 40 THz to study ultrafast photocurrents in I|A thin-film stacks consisting of an anisotropic-metal layer A and an isotropic metal layer I. Our first goal is to reveal the origins of the effective transient out-of-plane electric field and to understand its conversion into an in-plane current and, finally, propagating THz electric field. As a result, we will obtain a significantly better understanding of the generation and conversion mechanisms of THz photocurrents in anisotropic metals. Second, we will use the gained insights into the functioning of ACEs to boost the amplitude and bandwidth of the emitted THz pulse. In fact, our preliminary results suggest that optimized ACEs may outperform state-of-the-art THz emitters and, thus, very usefully extend the THz-photonics toolbox.

• DFG Programme: Research Grants