Sebastian Mährlein

Terahertz spin current pulses controlled by magnetic heterostructures

1. Department of Physical Chemistry, Fritz Haber Institute, Berlin, Germany. 2. Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden. 3. I. Physikalisches Institut, Georg-August-Universität Göttingen, Göttingen, Germany. 4. Helmholtz-Zentrum Berlin fϋr Materialien und Energie, Berlin, Germany. 5. Peter Grünberg Institute and Institute for Advanced Simulation, Forschungszentrum Jülich and JARA, Jülich, Germany.In spin-based electronics, information is encoded by the spin state of electron bunches. Processing this information requires the controlled transport of spin angular momentum through a solid, preferably at frequencies reaching the so far unexplored terahertz regime. Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is used to drive spins from a ferromagnetic iron thin film into a non-magnetic cap layer that has either low (ruthenium) or high (gold) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter based on the inverse spin Hall effect, which converts the spin flow into a terahertz electromagnetic pulse. We find that the ruthenium cap layer yields a considerably longer spin current pulse because electrons are injected into ruthenium d states, which have a much lower mobility than gold sp states. Thus, spin current pulses and the resulting terahertz transients can be shaped by tailoring magnetic heterostructures, which opens the door to engineering high-speed spintronic devices and, potentially, broadband terahertz emitters.

Nonperturbative Interband Response of a Bulk InSb Semiconductor Driven Off Resonantly by Terahertz Electromagnetic Few-Cycle Pulses

Intense multi-THz pulses are used to study the coherent nonlinear response of bulk InSb by means of field-resolved four-wave mixing spectroscopy. At amplitudes above 5 MV/cm the signals show a clear temporal substructure which is unexpected in perturbative nonlinear optics. Simulations based on a two-level quantum system demonstrate that in spite of the strongly off-resonant character of the excitation the high-field pulses drive the interband resonances into a non-perturbative regime of Rabi flopping.

High-Field High-Repetition-Rate Sources for the Coherent THz Control of Matter

Ultrashort flashes of THz light with low photon energies of a few meV, but strong electric or magnetic field transients have recently been employed to prepare various fascinating nonequilibrium states in matter. Here we present a new class of sources based on superradiant enhancement of radiation from relativistic electron bunches in a compact electron accelerator that we believe will revolutionize experiments in this field. Our prototype source generates high-field THz pulses at unprecedented quasi-continuous-wave repetition rates up to the MHz regime. We demonstrate parameters that exceed state-of-the-art laser-based sources by more than 2 orders of magnitude. The peak fields and the repetition rates are highly scalable and once fully operational this type of sources will routinely provide 1 MV/cm electric fields and 0.3 T magnetic fields at repetition rates of few 100 kHz. We benchmark the unique properties by performing a resonant coherent THz control experiment with few 10 fs resolution.

Femtosecond formation dynamics of the spin Seebeck effect revealed by terahertz spectroscopy

Understanding the transfer of spin angular momentum is essential in modern magnetism research. A model case is the generation of magnons in magnetic insulators by heating an adjacent metal film. Here, we reveal the initial steps of this spin Seebeck effect with <27 fs time resolution using terahertz spectroscopy on bilayers of ferrimagnetic yttrium iron garnet and platinum. Upon exciting the metal with an infrared laser pulse, a spin Seebeck current js arises on the same ~100 fs time scale on which the metal electrons thermalize. This observation highlights that efficient spin transfer critically relies on carrier multiplication and is driven by conduction electrons scattering off the metal–insulator interface. Analytical modeling shows that the electrons’ dynamics are almost instantaneously imprinted onto js because their spins have a correlation time of only ~4 fs and deflect the ferrimagnetic moments without inertia. Applications in material characterization, interface probing, spin-noise spectroscopy and terahertz spin pumping emerge.Probing spin pumping in the terahertz regime allows one to reveal its initial elementary steps. Here, the authors show that the formation of the spin Seebeck current in YIG/Pt critically relies on hot thermalized metal electrons because they impinge on the metal-insulator interface with maximum noise.

Nonlinear response of semiconductors driven by intense THz pulses

We present a review of our recent nonlinear spectroscopy experiments on bulk semiconductors performed using a novel source of ultra-intense multi-THz transients. The field-induced interband optical absorption in InP is studied on subcycle timescales. Our simulations corroborate the Franz-Keldysh effect as the main reason for the observed optical anomalies. The time-resolved four-wave mixing signals generated in InSb demonstrate clear signatures of a nonperturbative excitation regime and can be qualitatively reproduced by a simplified model of a two-level system driven far from the resonance.

Depletion of the superconducting condensate in Bi 2 Sr 2 CaCu 2 O 8+δ induced by intense multi-THz pulses

The superconducting state of the high-T_c cuprate Bi₂Sr₂CaCu₂O_8+δ is resonantly excited by intense multi-THz pulses and the following quasiparticle dynamics is probed. The dependence of the pump-probe signal on excitation fluence shows a super-linear behavior.

Multi-THz nonlinear optics and sub-cycle control of charge and spin

Strong THz electric and magnetic fields are harnessed to coherently control charge and spin in solids with sub-cycle resolution. Exploiting coherent THz phonons we transiently induce and destroy spin density wave order in pnictides.

Field-resolved four-wave mixing with multi-THz transients in InSb

Two-dimensional-spectroscopy with near-infrared (NIR) or mid-infrared pulses has given insight into ultrafast dynamics and spectral correlations in many systems [1]. Terahertz (THz) four-wave-mixing (FWM) has attracted growing interest as a potential way to access important low-energy excitations such as inter- and intra-molecular vibrations or energy gaps in superconductors. However, lead-off experiments [2] have been limited to model systems due to moderate THz fields. Here we employ a recently developed ultraintense multi-THz source [3,4] to pursue THz four-wave mixing in a bulk semiconductor even under two-photon resonant conditions. The temporal profile of the strong FWM signal shows first indications of a THz photon echo. A non-collinear geometry allows for spatial selection of nonlinear signals and background-free measurements.

Interband transitions in InP biased with THz fields of 4 MV/cm

The influence of strong electric fields on electronic properties of semiconductors is of particular interest both for fundamental science and applications in high speed electronics. Investigations using large quasi-static [1] and THz [2] fields have been performed. However, the accessible field amplitudes have been limited to values typically below 1 MV/cm due to the dielectric breakdown under stationary bias and the availability of intense phase-stable THz sources, respectively.

Ultrafast dynamics of semiconductor interband transitions in THz fields up to 4 MV/cm

Phase-locked multi-THz transients bias semiconductors far above the usual threshold for dielectric breakdown. Few-cycle NIR pulses synchronized to the THz transients on an as-timescale, probe interband transitions under electric fields of 4 MV/cm.