In this project, we will explore spintronic phenomena such as the spin Hall effect (SHE) and magnetoresistive effects to realize new optoelectronic functionalities in the terahertz (THz) frequency range. We will develop (i) optically driven spintronic emitters for efficient generation of broadband free-space THz pulses and (ii) compact on-chip THz voltage sources, which will be used to directly feed planar waveguides. While free-space THz pulses are of central importance for spectroscopy and characterization of materials, on-chip THz fields can be shaped in time and space. In the long-term, we will use them to switch on-chip magnetic structures. In a closely linked approach, we will (iii) investigate spin valves based on giant magnetoresistance (GMR) and tunneling magnetoresistance (TMR) to push the operation frequency beyond 100 GHz, explore the ultimate speed limits of operation, and in the long run use them as modulators of pulsed THz radiation. The optimization of these devices will rely on gaining new insights into the dynamics of laser-induced spin transport, specifically the spin Hall and magnetoresistive effects on ultrafast time scales.
To realize functionalities (i) and (ii), femtosecond laser pulses will be used to trigger sub-picosecond spin transport in stacks consisting of heavy-metal (HM) and magnetically ordered (FM) thin films. By means of the inverse spin Hall effect, the out-of-plane spin current is converted into an in-plane charge current, which leads to the emission of a sub-picosecond electromagnetic pulse, containing frequency components of up to 30 THz. The analysis of this pulse will be accomplished by far-field electro-optic sampling in the THz range, complemented by electrical measurements in the GHz range. We aim at understanding the origin of the ultrafast spin and charge currents, in particular their dependence on the choice of the FM/HM materials and the FM/HM interface properties. We will use these insights to develop high-performance emitters of THz radiation that will find immediate applications in the field of THz spectroscopy. In order to enhance the amplitude and tailor the bandwidth and emission direction of the THz pulse, we will make use of extensive nanopatterning. This strategy will allow us to optimize spin transport, optical absorption, and THz emission into free space as well as in waveguide structures on a microstructured chip.
To address functionality (iii), we will study the high-frequency electrical response of spin valves with thick spacer layers, using a vector network analyzer at frequencies up to 70 GHz. By analyzing and modelling of the response, we will explore how the Valet-Fert equations need to be modified when describing spin diffusion through the non-magnetic layer on the picosecond timescale. To modulate the GMR/TMR at THz speed, optical pump pulses will be used to quench the magnetization in patterned spin-valve structures on a sub-picosecond time scale. By all-electrical and/or electro-optic probing of the biased spin valve, we will reveal the time scales on which the optically induced demagnetization affects the electronic states and the dynamic spin accumulation that determine spin transport and spin-valve functionality. In the long run, we will combine microstructured spin valves with arrays of resonant THz antennas to realize modulators of amplitude, phase and polarization of pulsed THz radiation.