arXiv subjects

Johanna Kölbel

Publications and source records attributed to Johanna Kölbel.

4 recordsLinked to original sources

arXiv2025

Ultrafast Electron Temperature Dynamics in Spintronic Terahertz Emitters Studied by Optical-Pump Terahertz-Probe Spectroscopy

Spintronic terahertz emitters (STEs) pumped by femtosecond lasers have become a widely used source of broadband terahertz radiation. However, the strength of the emitted field is limited in part by the optical damage threshold at the pump wavelength. Thermal management of STEs can be improved by understanding electron temperature relaxation in the spintronic metal layer. Here, we present a measurement of electron temperature dynamics on a picosecond timescale using optical-pump terahertz-probe spectroscopy. We observe that the optical pump induces a change in terahertz transmission through the STE. By analyzing the resulting signal with a two-temperature model, we extract the dynamic electron temperature of the STE. This approach offers an advantage over other methods by avoiding additional heating of the sample by the probe pulse, making it particularly suitable for studying cumulative heating effects, which are believed to contribute to optical damage under MHz repetition rate pumping.

physics.optics↗

arXiv2025

Terahertz-Induced Nonlinear Response in ZnTe

Measuring terahertz waveforms in terahertz spectroscopy often relies on electrooptic sampling employing a ZnTe crystal. Although the nonlinearities in such zincblende semiconductors induced by intense terahertz pulses have been studied at optical frequencies, a quantitative study of nonlinearities in the terahertz regime has not been reported. In this work, we investigate the nonlinear response of ZnTe in the terahertz frequency region utilizing time-resolved terahertz-pump terahertz-probe spectroscopy. We find that the interaction of two co-propagating terahertz pulses in ZnTe leads to a nonlinear polarization change which modifies the electro-optic response of the medium at terahertz frequencies. We present a model for this polarization that showcases the second-order nonlinear behavior. We also determine the magnitude of the third-order susceptibility in ZnTe at terahertz frequencies, $χ^{\mathrm{(3)}}(ω_\text{THz})$. These results clarify the interactions in ZnTe at terahertz frequencies, with implications for measurements of intense terahertz fields using electro-optic sampling.

physics.optics↗

arXiv2023

The protein dynamical transition is independent of hydration

Terahertz time-domain spectroscopy and differential scanning calorimetry were used to study the role of the dynamics of biomolecules decoupled from solvent effects. Lyophilised sucrose exhibited steadily increasing absorption with temperature as anharmonic excitations commence as the system emerges from a deep minimum of the potential energy landscape where harmonic vibrations dominate. The polypeptide bacitracin and two globular proteins, lysozyme and human serum albumin, showed a more complex temperature dependence. Further analysis focused on the spectral signature below and above the boson peak. We found evidence for the onset of anharmonic motions that are characteristic for partial unfolding and molecular jamming in the dry biomolecules. The activation of modes of the protein molecules at temperatures comparable to the protein dynamical transition temperature was observed in the absence of hydration. No evidence for Fröhlich coherence, postulated to facilitate biological function, was found in our experiments.

cond-mat.dis-nn↗

arXiv2022

Terahertz Dynamics in the Glycerol-Water System

The model glass-former glycerol and its aqueous mixtures were investigated with terahertz-time domain spectroscopy (THz-TDS) in the frequency range of 0.3--3.0\,THz at temperatures from 80--305\,K. It was shown that the infrared absorption coefficient measured with THz-TDS can be theoretically related to the reduced Raman intensity ($\propto α/ω^2$) and the reduced density of states ($\propto α/ω^3$) and the agreement with experimental results confirms this. The data were further used to investigate the behaviour of model glasses in the harmonic (below the glass transition temperature $T_{\text{g}}$), anharmonic (above $T_{\text{g}}$), and liquid regime. The onset temperature of the molecular mobility as measured by the infrared active dipoles, $T_{\text{g}}$, was found to correlate with the onset of anharmonic effects, leading to an apparent shift of the boson peak and obscuring it at elevated temperatures. The influence of clustered and unclustered water on the dynamics, the boson peak, and the vibrational dynamics was also investigated. A change in structural dynamics was observed at a water concentration of approximately 5\,wt.\%, corresponding to a transition from isolated water molecules distributed homogeneously throughout the sample to the presence of small water clusters and an increased number of water-water hydrogen bonds which lower the barriers on the potential energy surface.

cond-mat.dis-nn↗