14 research outputs found
Arrival time and intensity binning at unprecedented repetition rates
Understanding dynamics on ultrafast timescales enables unique and new insights
into important processes in the materials and life sciences. In this respect,
the fundamental pump-probe approach based on ultra-short photon pulses aims at
the creation of stroboscopic movies. Performing such experiments at one of the
many recently established accelerator-based 4th-generation light sources such
as free-electron lasers or superradiant THz sources allows an enormous
widening of the accessible parameter space for the excitation and/or probing
light pulses. Compared to table-top devices, critical issues of this type of
experiment are fluctuations of the timing between the accelerator and external
laser systems and intensity instabilities of the accelerator-based photon
sources. Existing solutions have so far been only demonstrated at low
repetition rates and/or achieved a limited dynamic range in comparison to
table-top experiments, while the 4th generation of accelerator-based light
sources is based on superconducting radio-frequency technology, which enables
operation at MHz or even GHz repetition rates. In this article, we present the
successful demonstration of ultra-fast accelerator-laser pump-probe
experiments performed at an unprecedentedly high repetition rate in the few-
hundred-kHz regime and with a currently achievable optimal time resolution of
13 fs (rms). Our scheme, based on the pulse-resolved detection of multiple
beam parameters relevant for the experiment, allows us to achieve an excellent
sensitivity in real-world ultra-fast experiments, as demonstrated for the
example of THz-field-driven coherent spin precession
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
Driving and Decoding Structural and Polarization Dynamics in the THz Spectral Rangeal Optical Kerr Effect
Unifying Ultrafast Polarization Responses of Lead Halide Perovskites via Two-Dimensional Optical Kerr Effect
2D Optical- and THz-Kerr Effect in Lead Halide Perovskites
Ultrafast optical- or magneto-optical Kerr responses exhibiting THz frequency oscillations are commonly assumed to trace coherent low-energy quasi-particles, such as phonons or magnons. Here, we decode the complex nonlinear polarization response of lead halide perovskites (LHPs) by developing two-dimensional optical Kerr spectroscopy (2D-OKE). In contrast to the quasi-particle interpretation, we unveil a unified origin for fully inorganic and hybrid LHPs, governed by anisotropic and highly-dispersive light propagation in the vicinity of the optical bandgap. Finally, to directly trace the ultrafast lattice or molecular polarization, we extend this investigation to the THz Kerr effect