To answer the questions posed above, we develop new spectroscopic tools and methods, with a special focus on pulsed terahertz radiation with frequencies ranging from ~0.3 to 30 THz. We currently work on extending terahertz spectroscopy to quantum materials with magnetic order or other effects related to angular momentum. For instance, to reveal the coupling between crystal lattice and electron spins, we make use of the scheme in Figure 1: We use an intense THz pulse to resonantly 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 recently developed the scheme shown in Figure 2. 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.

Meanwhile, the approach of Figure 2 was extended to electron orbital angular momentum and to a platform that allows us to measure the spin conductance across a given material.

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.