Understanding non-equilibrium spin-charge transport mechanisms towards high power and antenna-coupled spintronic terahertz devices (HIGH-SPINTERA)

• Applicants: Professor Dr. Markus Meinert; Professor Dr. Sascha Preu; Professor Dr. Dmitry Turchinovich, Ph.D.
• Subject Area: Experimental Condensed Matter Physics ; Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering ; Synthesis and Properties of Functional Materials
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 518575758

The recent demonstration of efficient, spintronic Terahertz (THz, 100 GHz - 10 THz) sources based on inexpensive stacks of non-magnetic heavy metal (HM) and ferromagnetic (FM) films opened a new avenue in THz physics and engineering. Thin film stacks of HM/FM of a few nanometers thickness are illuminated with ultrashort (< 100 fs) laser pulses; the laser pulse generates a normally-directed ultrafast pulse of spin current from the FM into the HM layer, where a transverse, ultrafast charge current pulse is generated via the spin- to charge-current conversion processes by inverse spin Hall effect (ISHE). The ultrafast current flowing in the plane of the emitter in turn becomes the source of the THz emission. The emission efficiency is comparable to many well-established THz emission techniques, such as electro-optic THz generation, and has the potential to become competitive against photoconductive sources. The benefit of spintronic THz emitters (STEs) is: a) The ultrabroadband THz radiation upon excitation with femtosecond laser pulses without any spectral gap caused by the Reststrahlenband, which is otherwise an inherent limitation of standard semiconductor-based THz technology. Excitation with <50 fs laser pulses results in emitted THz bandwidth of >10 THz; and b) The manufacturing process of STEs is relatively inexpensive, as the metallic films can be commercially prepared on large, inexpensive substrates, and no external electric circuit is required for the THz generation. However, the basic physics of spintronic THz emitters is still not fully understood. Contrary to the common expectations, the STE operation in (quasi-)CW regime has not yet been demonstrated. Further, the STEs demonstrated so far do not operate in a reverse mode as THz detectors. In this proposal, we attempt to advance the recent work on the STEs further, by a) better understanding the principles of the underlying charge and heat transport; b) understanding and demonstrating the CW operation with two-color excitation; and c) optimizing the film systems for sustained high-power pulsed and quasi-CW operation. Our proposal has three goals, which in a collaborative manner will be tackled by three established researchers in the field. The first goal is to understand the interplay between electronic and heat transport behavior from the fs to the ns time scale (Dmitry Turchinovich) which plays a key role in CW operation that could not yet be demonstrated. The second goal is to investigate and eliminate degradation mechanisms and improve the film quality and design (Markus Meinert). The third goal is to improve the emission efficiency by implementing engineered antenna structures to shape the THz pulses in space and time, quasi-optical embedding, and optimization for operation with inexpensive, compact fiber laser systems (Sascha Preu). Our collaborative proposal thus leverages the expertise ranging from basic physics and materials science to well-engineered high-power devices.

• DFG Programme: Research Grants