Gudrun Niehues

Long-Range Influence of Carbohydrates on the Solvation Dynamics of Water-Answers from Terahertz Absorption Measurements and Molecular Modeling Simulations

We present new terahertz (THz) spectroscopic measurements of solvated sugars and compare the effect of two disaccharides (trehalose and lactose) and one monosaccharide (glucose) with respect to the solute-induced changes in the sub-picosecond network dynamics of the hydration water. We found that the solute affects the fast collective network motions of the solvent, even beyond the first solvation layer. For all three carbohydrates, we find an increase of 2-4% in the THz absorption coefficient of the hydration water in comparison to bulk water. Concentration-dependent changes in the THz absorption between 2.1 and 2.8 THz of the solute-water mixture were measured with a precision better than 1% and were used to deduce a dynamical hydration shell, which extends from the surface up to 5.7 +/- 0.4 and 6.5 +/- 0.9 A for the disaccharides lactose and trehalose, respectively, and 3.7 +/- 0.9 A for the glucose. This exceeds the values for the static hydration shell as determined, for example, by scattering, where the long-range structure was found to be not significantly affected by the solute beyond the first hydration shell. When comparing all three carbohydrates, we found that the solute-induced change in the THz absorption depends on the product of molar concentration of the solute and the number of hydrogen bonds between the carbohydrate and water molecules. We can conclude that the long-range influence on the sub-picosecond collective water network motions of the hydration water is directly correlated with the average number of hydrogen bonds between the molecule and adjacent water molecules for carbohydrates. This implies that monosaccharides have a smaller influence on the surrounding water molecules than disaccharides. This could explain the bioprotection mechanism of sugar-water mixtures, which has been found to be more effective for disaccharides than for monosaccharides.

Understanding THz Spectra of Aqueous Solutions: Glycine in Light and Heavy Water

THz spectroscopy of aqueous solutions has been established as of recently to be a valuable and complementary experimental tool to provide direct insights into the solute-solvent coupling due to hydrogen-bond dynamics involving interfacial water. Despite much experimental progress, understanding THz spectra in terms of molecular motions, akin to mid-infrared spectra, still remains elusive. Here, using the osmoprotectant glycine as a showcase, we demonstrate how this can be achieved by combining THz absorption spectroscopy and ab initio molecular dynamics. The experimental THz spectrum is characterized by broad yet clearly discernible peaks. Based on substantial extensions of available mode-specific decomposition schemes, the experimental spectrum can be reproduced by theory and assigned on an essentially quantitative level. This joint effort reveals an unexpectedly clear picture of the individual contributions of molecular motion to the THz absorption spectrum in terms of distinct modes stemming from intramolecular vibrations, rigid-body-like hindered rotational and translational motion, and specific couplings to interfacial water molecules. The assignment is confirmed by the peak shifts observed in the THz spectrum of deuterated glycine in heavy water, which allow us to separate the distinct modes experimentally.

A matter of symmetry: terahertz polarization detection properties of a multi-contact photoconductive antenna evaluated by a response matrix analysis

While terahertz time domain spectroscopy (THz-TDS) is a well-established technique, polarization sensitive measurements are challenging due to the need of broadband polarization devices. Here, we characterize our recently introduced multi-contact photoconductive detector antenna with a response matrix analysis. We show that the lead lines attached to electrodes reduce the antenna symmetry and thereby influence the properties of the response matrices. With a wire grid polarizer, we simulate a sample influencing the polarization angle and the intensity of the incident THz pulse. Evaluating the measurements with the response matrix analysis, our results show a well agreement of the adjusted and measured polarization angles and intensities over a frequency range from 0.25 to 0.8 THz.

Utilization of chirped laser pulses to measure stimulated Raman scattering of organic liquids in the terahertz regime

With the present paper, we demonstrate an experimental setup connecting two highly innovative fields of research: stimulated Raman scattering (SRS) spectroscopy and terahertz (THz) spectroscopy of liquids. For this purpose, we use the spectral focusing method allowing us to excite and measure molecular vibrations by chirped laser pulses. With our experimental setup, we apply this method to the THz regime in order to detect the frequency dependence of SRS processes in organic liquids. More specifically, we focus two chirped laser pulses onto the sample to drive molecular vibrations at THz frequencies. Thereby, the time delay between the laser pulses is directly related to the driving frequency. Due to SRS process, an energy transfer between the two laser pulses is established. By detecting the corresponding energy change of only one of the two laser pulses, we are able to observe inverse Raman scattering and stimulated Raman gain scattering as well as the transition between both processes at zero delay time. The peak positions and the line shapes of the measured spectra agree well with literature data obtained by spontaneous Raman scattering spectroscopy and optical heterodyne detection optical Kerr effect spectroscopy. With the present setup, we are able to excite and detect molecular vibrations ranging over two orders of magnitude from 10 THz to ultra-low frequencies at 100 GHz.

Driving of a Small Solvated Peptide in the IR and THz Range—A Comparative Study of Energy Flow

We present a comparative study of energy flow from a vibrationally excited solvated dialanine molecule to the surrounding water in the IR and THz range. We employ the driven molecular dynamics (DMD) approach to investigate the energy flow from the solute molecule to water molecules. As a result, we find a more rapid and efficient energy flow from the solute to the water when exciting THz modes compared to IR modes. Our results show a strong coupling of the low frequency mode of the solute and the water dynamics in the THz regime. In contrast, when exciting the IR modes of the solute, we find much more localized motions.

A polarization-sensitive 4-contact detector for terahertz time-domain spectroscopy

A light polarization angle-sensitive photoconductive detector for terahertz time-domain spectroscopy is computer-modeled, microfabricated, and tested. The experimental results show good agreement with the linear angular response for an ideal detector. The detectors frequency, angular, and crosstalk responses are discussed in the context of theoretical and experimental considerations.

Terahertz surface emission from Cu2ZnSnSe4 thin film photovoltaic material excited by femtosecond laser pulses

We observed efficient terahertz (THz) emission from sol-gel grown Cu2ZnSnSe4 (CZTSe) thin films using THz time domain spectroscopy technique. The THz emission bandwidth exceeds 2 THz with a dynamic range of 20 dB in the amplitude spectrum. The THz emission amplitude from CZTSe is found to be independent of external magnetic fields. Comparing the polarity of THz emission waveforms of CZTSe and GaAs, we suggest that the acceleration of photo-carriers in the surface accumulation layer of CZTSe is the dominant mechanism of radiation emission. Optical excitation fluence dependence measurements show that the saturation fluence of the CZTSe thin film reaches 1.48 μJ/cm2.

Linear Dichroism Detection and Analysis in Terahertz Spectral Range

Two simple experimental techniques of quantitative linear dichroism analysis were developed with Jones calculus for polarization-sensitive terahertz time-domain spectroscopy. Both of them involve the optical measurements at only three angles of rotation for sample or polarizer setup. Such non-destructive evaluation techniques could be a useful addition for quality control of polymer or composite materials. As an example, the production-induced linear dichroism in polytetrafluoroethylene was detected, analyzed, and interpreted as a result of the well-aligned macromolecular arrangement within optical volume probed by terahertz beam.

Polarization-variable emitter for terahertz time-domain spectroscopy

We report on the progress in the development of linear polarization-variable multi-electrode emitters for terahertz time-domain spectroscopy. The results on its microfabrication, the finite element method modeling of appropriate bias distribution between electrodes, the finite-difference time-domain simulated spectral output, and actual experimental testing are presented. The rotation of the emitted terahertz field with linear polarization on an angle multiple of 45° can be achieved by synchronized bias and single polarizer rotations.