Our research program is focused on the following three challenges of THz and ultrafast physics.
1. Reveal ultrafast elementary processes in condensed matter
Ultimately, we want to obtain a kind of movie that shows how electrons, spins and crystal lattice of a solid interact with each other on their natural (typically femtosecond) time scales and at their natural (typically THz) resonance frequencies. This idea is illustrated by the schematic below.
We currently study interactions in complex and application-relevant materials, for instance liquids and magnetic nanostructures. Examples of questions we want to answer are as follows:
- How does the vibrating crystal lattice (phonons) affect the state of the electrons or spins and vice versa (see figure above)?
- How can electrons or spins be transported through a solid by light pulses (see figure below)?
- How do molecules in liquids and soft matter react to strong pulsed electric fields?
2. Development of new spectroscopic tools
To answer the questions posed above, we develop new spectroscopic tools and methods, with a special focus on pulsed THz radiation with frequencies ranging from ~0.3 to 30 THz. We currently work on extending THz spectroscopy to magnetic solids and soft matter (such as liquids). For instance, to reveal the coupling between crystal lattice and electron spins, we make use of the scheme shown above: we use an intense THz pulse to resinantly excite optical phonons and probe the resulting impact on the spin system by a delayed optical probe pulse.
To probe and identify the inital steps of the ultrafast transport of electron spins, we have recently developed the scheme shown below. A femtosecond optical laser pulse is used to launch transport of electron spins (thick red arrows) from a ferromagnetic into a nonmagnetic metal layer. In the nonmagnetic metal, spin-up and spin-down electrons are deflected in opposite directions by an angle γ. This only recently discovered inverse spin Hall effect results in a transverse charge current (blue arrow), acting as a source of a THz electromagnetic pulse. Measurement of the THz electric field allows us to reconstruct of the spin transport dynamics.
Finally, to interpret our experimental results, we work out simple toy models. For example, we have recently modeled the transport experiment above using spin transport equations and Maxwell's equations.
We explore our results for applications in fields such as spintronics (e.g. the ultrafast control of magnetic bits) and THz photonics. Here, the development of new emitters and detectors of THz radiation plays an important role since the THz range is still relatively difficult to access. For example, we have used the spintronic structure of the previous figure to develop a novel emitter of THz radiation that is more efficient and broadband than established emitters.