Arne Keller

TWO-FREQUENCY IR LASER ORIENTATION OF POLAR MOLECULES. NUMERICAL SIMULATIONS FOR HCN

Abstract Using ab initio nuclear-coordinate-dependent dipole moments and polarizabilities, we study the orientation dynamics of HCN, by numerically solving the time-dependent Schrodinger equation, in the presence of a superposition of intense, linearly-polarized infrared laser pulses of frequency ω and 2ω. We show that polarizability acts in concert with permanent dipole moments to orient polar molecules, as opposed to alignment which occurs alone with a single laser frequency or one moment only (permanent or induced). Optimal orientation occurs for the field configuration E (t)= E 0 (t) cos ωt+0.5 cos 2ωt , where 2ω is resonant with a 0 → 1 vibrational transition and E 0 (t) is a picosecond pulse.

State selective reactions prepared through the excitation of orbital states in van der Waals complexes of Ca–HX*

We have observed the chemiluminescent reaction Ca*+HX→CaX*+H, where the reactants are prepared in a van der Waals complex formed in a supersonic expansion. This preparation, combined with tunable laser excitation, allows access to well‐defined electronic states of the reaction complex corresponding to different orientations of the calcium excited orbital. In the case of the Ca–HBr complex, a remarkable effect of this preparation is observed on the branching ratio to the final excited states A 2Π and B 2Σ of CaBr: Depending upon the selected state of the complex, the A/B ratio varies by a factor of 2. This is interpreted by the conservation of the orbital orientation during the reaction involving the departure of the hydrogen.

Observation and spectroscopy of metallic free radicals produced by reactive collisions during a supersonic expansion

Using the techniques of laser vaporization, supersonic jet, and laser‐induced fluorescence spectroscopy, we have studied a number of small calcium containing free radicals, formed by the reaction of calcium atoms with the appropriate precursor molecule. We present spectra for CaCCH (from HCCH), CaNH2 (from NH3 ), and CaCN (from CH3CN ) in the region of the first electronically excited states. The cooling achieved in the jet has revealed structure hidden in previous high temperature studies. In the case of CaCN, we report the first rotationally resolved spectra of this molecule.

Direct bell states generation on a III-V semiconductor chip at room temperature

Summary form only given. In these last years, a great deal of effort has been devoted to the miniaturization of quantum information technology on semiconductor chips. In the context of photon pair sources, the bi-exciton cascade of a quantum dot and the four-wave mixing in Silicon waveguides have been used to demonstrate the generation of entangled states. Spontaneous parametric down-conversion in III-V semiconductor waveguides combines the advantages of room temperature and telecom wavelength operation, while keeping open the possibility of electrically pumping of the device. Here we present a source consisting of a multilayer AlGaAs waveguide grown on a GaAs substrate and then chemically etched to achieve lateral confinement in a ridge. The structure design is such that a pump beam (around 775 nm), impinging on the surface of the waveguide with an incidence angle θ, generates two counterpropagating orthogonally polarized beams (around 1550 nm). The waveguide core is surrounded by distributed Bragg reflectors to enhance the pump field. We demonstrate the direct emission of polarization entangled photons by pumping the device with two symmetric angles of incidence corresponding to frequency degeneracy and performing a quantum tomography measurement. Most common entanglement witnesses are satisfied and a raw fidelity of 0.8 to the Bell state ( Hν +eiφ vx ) is obtained. A theoretical model, taking into account the experimental parameters, provides ways to understand and control the amount of entanglement.These results open the route to the demonstration of other interesting features of our device such as the generation of hyper-entangled states via the control of the frequency correlation degree through the spatial and spectral pump beam profile, leading to a new generation of completely integrated devices for quantum information.

Transition state observation of excited harpoon reactions, within Ca‐HX van der Waals complexes

The observation of reactions through the excitation of van der Waals precursors allows for the spectroscopic investigation of the reaction surface of the excited state adducts. This method has been applied to the reaction of calcium 1D and 1P with the various hydrogen halide molecules, the reaction being endothermic with ground state calcium by less than 1 eV. The complexes were produced by supersonic expansion following laser ablation of the calcium atom. The chemiluminescent channel producing the CaX (A 2Π, B 2Σ) molecules is found to be an important channel and was investigated through action spectra. These spectra reveal the local electronic excitation of the calcium atom within the complex with the local orbital symmetry A′ or A″ of the Cs molecular complex. The band structure has been analyzed in terms of the intermolecular modes, the bending mode being prominent in the A″ state correlating with Ca 1P1. This state can be characterized by a single generic two dimensional reaction surface (bending+H‐X...

Reaching optimally oriented molecular states by laser kicks

We present a strategy for postpulse molecular orientation aiming both at efficiency and maximal duration within a rotational period. We first identify the optimally oriented states which fulfill both requirements. We show that a sequence of half-cycle pulses of moderate intensity can be devised for reaching these target states.

Laser-assisted conductance of molecular wires

An explicit expression is worked out for the conductance of a system consisting of a molecular wire, described within a homogeneous tight-binding model, bridging two electrodes illuminated by a weak laser, in the limit of linear radiative response, zero temperature and low bias voltage. Frequency tuning leads to valuable spectroscopic information on the system. More interesting is the prediction of an enhancement of the total current by several orders of magnitude, opening the possibility of nanoscale switch design.

Quantum phase gate and controlled entanglement with polar molecules

We propose an alternative scenario for the generation of entanglement between rotational quantum states of two polar molecules. This entanglement arises from dipole-dipole interaction, and is controlled by a sequence of laser pulses simultaneously exciting both molecules. We study the efficiency of the process, and discuss possible experimental implementations with cold molecules trapped in optical lattices or in solid matrices. Finally, various entanglement detection procedures are presented, and their suitability for these two physical situations is analyzed.

Ultrafast molecular imaging by laser-induced electron diffraction

We address the feasibility of imaging geometric and orbital structures of a polyatomic molecule on an attosecond time scale using the laser-induced electron diffraction (LIED) technique. We present numerical results for the highest molecular orbitals of the CO{sub 2} molecule excited by a near-infrared few-cycle laser pulse. The molecular geometry (bond lengths) is determined within 3% of accuracy from a diffraction pattern which also reflects the nodal properties of the initial molecular orbital. Robustness of the structure determination is discussed with respect to vibrational and rotational motions with a complete interpretation of the laser-induced mechanisms.

Time-dependent unitary perturbation theory for intense laser-driven molecular orientation

Molecular orientation dynamics by ultrashort pulses was investigated by applying a time-dependent unitary perturbation theory (TDUPT) combined with averaging techniques. A perturbative propagator was constructed which allowed to elucidate the postpulse orientation dynamics. It was observed that an effective orientation could be achieved for standard short laser pulses of zero time average. It was suggested that chosing a appropriate free parameter available in the TDUPT could help in improving the accuracy of the approximate propagator.