61 research outputs found
400 kHz repetition rate THz-TDS with 24 mW of average power driven by a compact industrial Yb-laser
We demonstrate a high average power terahertz time-domain (THZ-TDS)
spectrometer based on optical rectification in the tilted-pulse front geometry
in lithium niobate at room temperature, driven by a commercial, industrial
femtosecond-laser operating with flexible repetition rate between 40 kHz - 400
kHz. The driving laser provides a pulse energy of 41 uJ for all repetition
rates, at a pulse duration of 310 fs, allowing us to explore repetition rate
dependent effects in our TDS. At the maximum repetition rate of 400 kHz, up to
16.5 W of average power are available to drive our THz source, resulting in a
maximum of 24 mW of THz average power with a conversion efficiency of ~ 0.15 %
and electric field strength of several tens of kV/cm. At the other available
lower repetition rates, we show that the pulse strength and bandwidth of our
TDS is unchanged, showing that the THz generation is not affected by thermal
effects in this average power region of several tens of watts. The resulting
combination of high electric field strength with flexible and high repetition
rate is very attractive for spectroscopy, in particular since the system is
driven by an industrial, compact laser without the need for external
compressors or other specialized pulse manipulation
Dissociative photoionization of NO across a shape resonance in the XUV range using circularly polarized synchrotron radiation.
We report benchmark results for dissociative photoionization (DPI) spectroscopy and dynamics of the NO molecule in the region of the Ï* shape resonance in the ionization leading to the NO+(c3Î ) ionic state. The experimental study combines well characterized extreme ultraviolet (XUV) circularly polarized synchrotron radiation, delivered at the DESIRS beamline (SOLEIL), with ion-electron coincidence 3D momentum spectroscopy. The measured (N+, e) kinetic energy correlation diagrams reported at four discrete photon energies in the extended 23-33 eV energy range allow for resolving the different active DPI reactions and underline the importance of spectrally resolved studies using synchrotron radiation in the context of time-resolved studies where photoionization is induced by broadband XUV attosecond pulses. In the dominant DPI reaction which leads to the NO+(c3Î ) ionic state, photoionization dynamics across the Ï* shape resonance are probed by molecular frame photoelectron angular distributions where the parallel and perpendicular transitions are highlighted, as well as the circular dichroism CDAD(Ξe) in the molecular frame. The latter also constitute benchmark references for molecular polarimetry. The measured dynamical parameters are well described by multichannel Schwinger configuration interaction calculations. Similar results are obtained for the DPI spectroscopy of highly excited NO+ electronic states populated in the explored XUV photon energy range
Nonlinear frequency mixing in quantum cascade lasers: Towards broadband wavelength shifting and THz up-conversion
Terahertz (THz) sideband generation on a near-infrared (NIR) carrier has been recently demonstrated using quantum cascade lasers (QCL), with potential applications in wavelength shifting and THz up-conversion. However, the NIR wavelength range and nonlinear efficiency were severely limited by absorption. Here we overcome this drawback through a novel reflection geometry, whilst preserving a large interaction area. As well as insights into the nonlinear mechanism, this allows a much large range of NIR pump energies, relaxing the criteria of using particular excitation wavelengths
Nematic fluctuations mediated superconductivity revealed by anisotropic strain in Ba(FeCo)As
Anisotropic strain is an external field capable of selectively addressing the
role of nematic fluctuations in promoting superconductivity. We demonstrate
this using polarization-resolved elasto-Raman scattering to probe the evolution
of nematic fluctuations under strain in the normal and superconducting states
of the paradigmatic iron-based superconductor Ba(FeCo)As.
In the non-superconducting parent compound BaFeAs we observe a
strain-induced suppression of the nematic susceptibility which follows the
expected behavior of an Ising order parameter under a symmetry breaking field.
For the superconducting compound, the suppression of the nematic susceptibility
correlates with the decrease of the superconducting critical temperature .
Our results indicate a significant contribution of nematic fluctuations to
electron pairing and validate theoretical scenarios of enhanced near a
nematic quantum critical point.Comment: 5 pages, 3 figures + S
Circular dichroism in photoionization of H2
ABSTRACT: Circular dichroism is a consequence of chirality. However, nonchiral molecules can also exhibit it when the measurement itself introduces chirality, e.g., when measuring molecular-frame photoelectron angular distributions. The few such experiments performed on homonuclear diatomic molecules show that, as expected, circular dichroism vanishes when the molecular-frame photoelectron angular distributions are
integrated over the polar electron emission angle. Here we show that this is not the case in resonant dissociative ionization of H2 for photons of 30â35 eV, which is the consequence of the delayed ionization from molecular doubly excited states into ionic states of different inversion symmetry
Circular dichrosim in photoionization of H2 and D2
ABSTRACT: In this work, circular dichroism in H2 (D2) photoionization is studied in detail. We have selected several
photon energies for a case study: 19 eV for which only direct ionization to the 1s_g ionization channel is present, 27 eV where autoionization of Q1 doubly excited states takes place, and 32.5 eV for which autoionization from Q1 and doubly excited states and direct ionization to 1s_g and 2p_u channels strongly interfere. The latter case shows clear evidence of different behavior of the photoionization against radiation helicity
A classical Over Barrier Model to compute charge exchange between ions and one-optical-electron atoms
In this paper we study theoretically the process of electron capture between
one-optical-electron atoms (e.g. hydrogenlike or alkali atoms) and ions at
low-to-medium impact velocities (v/v_e <= 1) working on a modification of an
already developed classical Over Barrier Model (OBM) [V. Ostrovsky, J. Phys. B:
At. Mol. Opt. Phys. {\bf 28} 3901 (1995)], which allows to give a
semianalytical formula for the cross sections. The model is discussed and then
applied to a number of test cases including experimental data as well as data
coming from other sophisticated numerical simulations. It is found that the
accuracy of the model, with the suggested corrections and applied to quite
different situations, is rather high.Comment: 12 pages REVTEX, 5 EPSF figures, submitted to Phys Rev
Optical sideband generation up to room temperature with mid-infrared quantum cascade lasers
Room temperature sideband generation on an optical carrier is demonstrated using midinfrared quantum cascade lasers. This is achieved via an enhancement of the nonlinear susceptibility via resonant interband and intersubband excitations, compensating the large phase-mismatch
Circular dichroism in molecular-frame photoelectron angular distributions in the dissociative photoionization of H2 and D2 molecules
ABSTRACT: The presence of net circular dichroism in the photoionization of nonchiral homonuclear molecules has been put in evidence recently through the measurement of molecular-frame photoelectron angular distributions in
dissociative photoionization of H2 [Dowek et al., Phys. Rev. Lett. 104, 233003 (2010)]. In this work we present a detailed study of circular dichroism in the photoelectron angular distributions of H2 and D2 molecules, oriented perpendicularly to the propagation vector of the circularly polarized light, at different photon energies (20, 27, and 32.5 eV). Circular dichroism in the angular distributions at 20 and to a large extent 27 eV exhibits the usual pattern in which inversion symmetry is preserved. In contrast, at 32.5 eV, the inversion symmetry breaks down, which eventually leads to total circular dichroism after integration over the polar emission angle. Time-dependent ab initio calculations support and explain the observed results for H2 in terms of quantum interferences between direct photoionization and delayed autoionization from the Q1 and Q2 doubly excited states into ionic states (1sÏg and 2pÏu) of different inversion symmetry. Nevertheless, for D2 at 32.5 eV, there is a particular case where theory and experiment disagree in the magnitude of the symmetry breaking: when D+ ions are produced with an energy of around 5 eV. This reflects the subleties associated to such simple molecules when exposed to this fine scrutiny
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