arXiv subjects

David Rohrbach

Publications and source records attributed to David Rohrbach.

5 recordsLinked to original sources

arXiv2026

Time-resolved THz Stark spectroscopy of molecules in water

Stark spectroscopy is a powerful method for probing molecular dipole moment changes, charge transfer dynamics, and polarizability under applied electric fields. Time-Resolved Terahertz Stark Spectroscopy (TRTSS), which employs intense single-cycle terahertz (THz) pulses to induce transient Stark shifts, overcomes key limitations of conventional approaches. Unlike static or low-frequency fields, THz pulses oscillate much faster than typical molecular rotation times, effectively preventing dipole reorientation and enabling measurements in solutions at ambient conditions. Here, we extend TRTSS to molecules dissolved in water, the most important polar solvent for chemical and biological systems and report the first demonstration of Stark spectroscopy in water at room temperature. Using Malachite Green and Methyl Orange as model systems, we observe clear THz-induced spectral modulations, demonstrating that TRTSS can successfully reveal THz Stark responses even in highly polar, hydrogen-bonded environments. Measured signals exhibit a combination of linear (dipole-driven) and quadratic (polarizability-driven) Stark effects in both systems, consistent with time-dependent density functional theory (TD-DFT) calculations. Comparison with TD-DFT further suggests that conformational effects can influence the extracted Stark parameters in solvated molecules.

physics.chem-ph↗

arXiv2025

Discovery of magnon self-interaction in a strongly driven antiferromagnet

Nonlinear dynamics govern a wide array of natural phenomena and are essential for understanding nonequilibrium behaviors in condensed matter systems. In magnetically ordered materials, magnons - the quanta of spin waves - exhibit intrinsic nonlinearities that are of great interest in fundamental research and practical applications. Despite progress in the nonlinear control of magnon modes in antiferromagnetic materials, the transition from perturbative to non-perturbative regimes of magnon coherences has remained elusive. Here, we explore the nonlinear dynamics of a magnon mode in an antiferromagnet using two-dimensional terahertz spectroscopy with waveguide-enhanced terahertz fields. By driving the magnon mode far from equilibrium, we demonstrate the emergence of high-order magnon coherences and delineate a distinct transition into non-perturbative magnon nonlinearities. This behavior originates from the intrinsic anharmonicity of the magnetic potential and marks a regime dominated by magnon self-interactions at large spin deflection angles. These findings provide fundamental mechanistic insights that might be exploited for ultrafast switching and other advanced magnonic applications.

cond-mat.mtrl-sci↗

arXiv2022

Time-resolved THz Stark spectroscopy

We introduce THz Stark spectroscopy by using intense single-cycle terahertz pulses as the electric field source and monitoring the induced spectral response of an isotropic molecular ensemble with a coincident femtosecond supercontinuum pulse. THz Stark spectroscopy offers several advantages over conventional Stark spectroscopy and opens previously inaccessible perspectives. Most importantly, THz pulses oscillate faster than typical molecular rotations and consequently eliminate the requirement to freeze the samples to prevent poling effects. Hence, THz Stark spectroscopy allows for time-resolved studies at arbitrary temperatures, specifically ambient conditions more relevant to physiological or operative conditions. Moreover, dynamical field effects, e.g., higher order Stark contributions or hysteresis effects (non-Markovian behavior), can be studied on the time scales of molecular vibrations or rotations. We demonstrate THz Stark spectroscopy for two judiciously selected molecular systems and compare the results to conventional Stark spectroscopy and first principle calculations.

physics.optics↗

arXiv2022

Wideband dispersion-free THz waveguide platform

We present an integrated THz spectroscopy and sensing platform featuring low loss, vacuum-like dispersion, and strong field confinement in the fundamental mode. Its performance was characterized experimentally for frequencies between 0.1 THz and 1.5 THz. While linear THz spectroscopy and sensing gain mostly from low loss and an extended interaction length, nonlinear THz spectroscopy would also profit from the field enhancement associated to strong mode confinement. Moreover, the vacuum-like dispersion allows for a reshaping-free propagation of broadband single- to few-cycle pulses in gas-phase samples or velocity matching between THz pump and visible to infrared probe pulses. Our platform is based on a metallic structure and falls in the category of double ridged waveguides. We characterize essential waveguide properties, for instance, propagation and bending losses, but also demonstrate junctions and interferometers, essentially because those elements are prerequisites for integrated THz waveform synthesis, and hence, for coherently controlled linear and nonlinear interactions.

physics.optics↗

arXiv2022

Structure, Organization and Heterogeneity of Water-Containing Deep Eutectic Solvents

The spectroscopy and structural dynamics of a deep eutectic mixture (KSCN/acetamide) with varying water content is investigated from 2D IR (with the C-N stretch vibration of the SCN$^-$ anions as the reporter) and THz spectroscopy. Molecular dynamics simulations correctly describe the non-trivial dependence of both spectroscopic signatures depending on water content. For the 2D IR spectra, the MD simulations relate the steep increase in the cross relaxation rate at high water content to parallel alignment of packed SCN$^-$ anions. Conversely, the non-linear increase of the THz absorption with increasing water content is mainly attributed to the formation of larger water clusters. The results demonstrate that a combination of structure sensitive spectroscopies and molecular dynamics simulations provides molecular-level insights into emergence of heterogeneity of such mixtures by modulating their composition.

physics.chem-ph↗