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- ItemApplying Quantum Cascade Laser Spectroscopy in Plasma Diagnostics(Basel : MDPI, 2016) Röpcke, Jürgen; Davies, Paul; Hamann, Stephan; Hannemann, Mario; Lang, Norbert; van Helden, Jean-PierreThe considerably higher power and wider frequency coverage available from quantum cascade lasers (QCLs) in comparison to lead salt diode lasers has led to substantial advances when QCLs are used in pure and applied infrared spectroscopy. Furthermore, they can be used in both pulsed and continuous wave (cw) operation, opening up new possibilities in quantitative time resolved applications in plasmas both in the laboratory and in industry as shown in this article. However, in order to determine absolute concentrations accurately using pulsed QCLs, careful attention has to be paid to features like power saturation phenomena. Hence, we begin with a discussion of the non-linear effects which must be considered when using short or long pulse mode operation. More recently, cw QCLs have been introduced which have the advantage of higher power, better spectral resolution and lower fluctuations in light intensity compared to pulsed devices. They have proved particularly useful in sensing applications in plasmas when very low concentrations have to be monitored. Finally, the use of cw external cavity QCLs (EC-QCLs) for multi species detection is described, using a diagnostics study of a methane/nitrogen plasma as an example. The wide frequency coverage of this type of QCL laser, which is significantly broader than from a distributed feedback QCL (DFB-QCL), is a substantial advantage for multi species detection. Therefore, cw EC-QCLs are state of the art devices and have enormous potential for future plasma diagnostic studies.
- ItemElectron Population Dynamics in Optically Pumped Asymmetric Coupled Ge/SiGe Quantum Wells: Experiment and Models(Basel : MDPI, 2020) Ciano, Chiara; Virgilio, Michele; Bagolini, Luigi; Baldassarre, Leonetta; Rossetti, Andrea; Pashkin, Alexej; Helm, Manfred; Montanari, Michele; Persichetti, Luca; Di Gaspare, Luciana; Capellini, Giovanni; Paul, Douglas J.; Scalari, Giacomo; Faist, Jèrome; De Seta, Monica; Ortolani, Michelen-type doped Ge quantum wells with SiGe barriers represent a promising heterostructure system for the development of radiation emitters in the terahertz range such as electrically pumped quantum cascade lasers and optically pumped quantum fountain lasers. The nonpolar lattice of Ge and SiGe provides electron-phonon scattering rates that are one order of magnitude lower than polar GaAs. We have developed a self-consistent numerical energy-balance model based on a rate equation approach which includes inelastic and elastic inter-and intra-subband scattering events and takes into account a realistic two-dimensional electron gas distribution in all the subband states of the Ge/SiGe quantum wells by considering subband-dependent electronic temperatures and chemical potentials. This full-subband model is compared here to the standard discrete-energy-level model, in which the material parameters are limited to few input values (scattering rates and radiative cross sections). To provide an experimental case study, we have epitaxially grown samples consisting of two asymmetric coupled quantum wells forming a three-level system, which we optically pump with a free electron laser. The benchmark quantity selected for model testing purposes is the saturation intensity at the 1!3 intersubband transition. The numerical quantum model prediction is in reasonable agreement with the experiments and therefore outperforms the discrete-energy-level analytical model, of which the prediction of the saturation intensity is off by a factor 3. © 2019 by the authors.
- ItemFew-cycle 65-µJ pulses at 11.4 µm for ultrafast nonlinear longwave-infrared spectroscopy(Washington, DC : Optical Society of America, OSA, 2022) Fuertjes, Pia; Bock, Martin; Grafenstein, Lorenz von; Ueberschaer, Dennis; Griebner, Uwe; Elsaesser, ThomasLow-energy excitations can provide insight into the basic ultrafast nonequilibrium dynamics of condensed matter. High-energy femtosecond pulses in the long-wavelength infrared are required to induce such processes, and can be generated in an optical parametric chirped pulse amplification (OPCPA) system comprising three GaSe stages. A femtosecond Cr:ZnS laser serves as the front-end, providing the seed for the 2.0-µm pump and the 2.4-µm signal pulses without nonlinear conversion processes. The OPCPA system is pumped at 2.05 µm by a picosecond Ho:YLF regenerative amplifier at a 1-kHz repetition rate. The recompressed idler pulses at 11.4 µm have a duration of 185 fs and an unprecedented energy of 65 µJ, corresponding to a pump-to-idler conversion efficiency of 1.2%. Nonlinear transmission experiments in the range of the L2 infrared band of liquid water demonstrate the potential of the pulses for nonlinear vibrational spectroscopy of liquids and solids.
- ItemUltrafast vibrational dynamics of the DNA backbone at different hydration levels mapped by two-dimensional infrared spectroscopy(Melville, NY : AIP Publishing LLC, 2015) Guchhait, Biswajit; Liu, Yingliang; Siebert, Torsten; Elsaesser, ThomasDNA oligomers are studied at 0% and 92% relative humidity, corresponding to N < 2 and N > 20 water molecules per base pair. Two-dimensional (2D) infrared spectroscopy of DNA backbone modes between 920 and 1120 cm(-1) maps fluctuating interactions at the DNA surface. At both hydration levels, a frequency fluctuation correlation function with a 300 fs decay and a slow decay beyond 10 ps is derived from the 2D lineshapes. The fast component reflects motions of DNA helix, counterions, and water shell. Its higher amplitude at high hydration level reveals a significant contribution of water to the fluctuating forces. The slow component reflects disorder-induced inhomogeneous broadening.
- ItemMolecular couplings and energy exchange between DNA and water mapped by femtosecond infrared spectroscopy of backbone vibrations(Melville, NY : AIP Publishing LLC, 2017) Liu, Yingliang; Guchhait, Biswajit; Siebert, Torsten; Fingerhut, Benjamin P.; Elsaesser, ThomasMolecular couplings between DNA and water together with the accompanying processes of energy exchange are mapped via the ultrafast response of DNA backbone vibrations after OH stretch excitation of the water shell. Native salmon testes DNA is studied in femtosecond pump-probe experiments under conditions of full hydration and at a reduced hydration level with two water layers around the double helix. Independent of their local hydration patterns, all backbone vibrations in the frequency range from 940 to 1120 cm-1 display a quasi-instantaneous reshaping of the spectral envelopes of their fundamental absorption bands upon excitation of the water shell. The subsequent reshaping kinetics encompass a one-picosecond component, reflecting the formation of a hot ground state of the water shell, and a slower contribution on a time scale of tens of picoseconds. Such results are benchmarked by measurements with resonant excitation of the backbone modes, resulting in distinctly different absorption changes. We assign the fast changes of DNA absorption after OH stretch excitation to structural changes in the water shell which couple to DNA through the local electric fields. The second slower process is attributed to a flow of excess energy from the water shell into DNA, establishing a common heated ground state in the molecular ensemble. This interpretation is supported by theoretical calculations of the electric fields exerted by the water shell at different temperatures.
- ItemCrosspolarization with imperfect infrared polarizers(Amsterdam [u.a.] Elsevier, 2022) Furchner, Andreas; Hinrichs, KarstenThe analysis of vibrational bands is a core application of infrared (IR) spectroscopy. Polarization-dependent measurements enable the study of anisotropic materials. However, imperfect IR polarizers exhibit polarizer leakage, which causes pronounced bandshape and baseline distortions for samples with weak optical anisotropy. Based on the 4 × 4 Mueller-matrix formalism, we propose a polarimetric measurement scheme for handling imperfect polarizers and source prepolarization that delivers correct co- and crosspolarized transmission and reflection IR spectra. The scheme is applied to a weakly anisotropic polypropylene sheet, resolving crosspolarized signatures as small as 5⋅10−5. We determine the polymer's direction-dependent complex refractive index in the vibrational fingerprint range.
- ItemUltrafast phosphate hydration dynamics in bulk H2O(Melville, NY : American Institute of Physics, 2015) Costard, Rene; Tyborski, Tobias; Fingerhut, Benjamin P.; Elsaesser, ThomasPhosphate vibrations serve as local probes of hydrogen bonding and structural fluctuations of hydration shells around ions. Interactions of H2PO4− ions and their aqueous environment are studied combining femtosecond 2D infrared spectroscopy, ab-initio calculations, and hybrid quantum-classical molecular dynamics (MD) simulations. Two-dimensional infrared spectra of the symmetric (𝜈𝑆(PO−2)) and asymmetric (𝜈𝐴𝑆(PO−2)) PO−2 stretching vibrations display nearly homogeneous lineshapes and pronounced anharmonic couplings between the two modes and with the δ(P-(OH)2) bending modes. The frequency-time correlation function derived from the 2D spectra consists of a predominant 50 fs decay and a weak constant component accounting for a residual inhomogeneous broadening. MD simulations show that the fluctuating electric field of the aqueous environment induces strong fluctuations of the 𝜈𝑆(PO−2) and 𝜈𝐴𝑆(PO−2) transition frequencies with larger frequency excursions for 𝜈𝐴𝑆(PO−2). The calculated frequency-time correlation function is in good agreement with the experiment. The 𝜈(PO−2) frequencies are mainly determined by polarization contributions induced by electrostatic phosphate-water interactions. H2PO4−/H2O cluster calculations reveal substantial frequency shifts and mode mixing with increasing hydration. Predicted phosphate-water hydrogen bond (HB) lifetimes have values on the order of 10 ps, substantially longer than water-water HB lifetimes. The ultrafast phosphate-water interactions observed here are in marked contrast to hydration dynamics of phospholipids where a quasi-static inhomogeneous broadening of phosphate vibrations suggests minor structural fluctuations of interfacial water.