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 σ* shape resonance in the ionization leading to the NO+(c3Π) 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+(c3Π) ionic state, photoionization dynamics across the σ* 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(θe) 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(Fe1−x_{1-x}Cox_x)2_2As2_2

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(Fe$_{1-x}$Co$_x$)$_2$As$_2$. In the non-superconducting parent compound BaFe$_2$As$_2$ 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 $T_c$. Our results indicate a significant contribution of nematic fluctuations to electron pairing and validate theoretical scenarios of enhanced $T_c$ near a nematic quantum critical point.Comment: 5 pages, 3 figures + S

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 (1sσg and 2pσu) 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

Giant optical nonlinearity interferences in Terahertz quantum structures

Second-order optical nonlinearities can be greatly enhanced by orders of magnitude in resonantly excited nanostructures. However, they are frequently not as heightened as predicted, limiting their exploitation in nanostructured THz nonlinear optics. Here, we show that the second-order nonlinear susceptibility can vary by orders of magnitude as a result of giant destructive interference effects. Using THz quantum-cascade-lasers as a model source to investigate interband and intersubband nonlinearities, we show that these giant interferences are a result of an interplay of the second-order nonlinear contributions of multiple light and heavy hole states. As well as of importance to engineer the resonant optical properties of nanostructures, this framework can be employed as a novel, sensitive tool to elucidate the bandstructure properties of complex materials

Giant optical nonlinearity interferences in quantum structures

Second-order optical nonlinearities can be greatly enhanced by orders of magnitude in resonantly excited nanostructures. These resonant nonlinearities continually attract attention, particularly in newly discovered materials. However, they are frequently not as heightened as currently predicted, limiting their exploitation in nanostructured nonlinear optics. Here, we present a clear-cut theoretical and experimental demonstration that the second-order nonlinear susceptibility can vary by orders of magnitude as a result of giant destructive, as well as constructive, interference effects in complex systems. Using terahertz quantum cascade lasers as a model source to investigate interband and intersubband nonlinearities, we show that these giant interferences are a result of an unexpected interplay of the second-order nonlinear contributions of multiple light and heavy hole states. As well as of importance to understand and engineer the resonant optical properties of nanostructures, this advanced framework can be used as a novel, sensitive tool to elucidate the band structure properties of complex materials

Short THz pulse generation from a dispersion compensated modelocked quantum cascade laser

Dispersion compensation is vital for the generation of ultrashort and single cycle pulses from modelocked lasers across the electromagnetic spectrum. However, no such scheme have been successfully applied to terahertz (THz) quantum cascade lasers (QCL) for short and stable pulse generation in the THz range. Here we show a monolithic on-chip compensation scheme for a modelocked QCL, permitting THz pulses to be considerably shortened from 16ps to 4ps. This is based on the realization of a small coupled cavity resonator that acts as an 'off resonance' Gires-Tournois interferometer (GTI), permitting large THz spectral bandwidths to be compensated