Bulletin of the American Physical Society
2013 Joint Meeting of the APS Division of Atomic, Molecular & Optical Physics and the CAP Division of Atomic, Molecular & Optical Physics, Canada
Volume 58, Number 6
Monday–Friday, June 3–7, 2013; Quebec City, Canada
Session H6: Intense Field Filamentation and Acceleration |
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Chair: See Leang Chin, Université Laval Room: 302 |
Wednesday, June 5, 2013 10:30AM - 10:42AM |
H6.00001: Fast Shape Evolution of Laser Filaments in the Wake of Femtosecond Driving Pulse Dmitri Romanov, Robert Levis A theoretical model is developed for subnanosecond evolution of highly nonequilibrium, inhomogeneous free-electron gas in a laser filament/microfilament wake channel. The evolution is driven by two interrelated mechanisms: (i) impact ionization of residual neutral atoms inside the channel and on its surface, and (ii) thermal conduction in the electron gas. The simulation results for the cases of weak and moderate initial ionization show crucial importance of incorporating the spread effects, especially as regards the electron temperature. The calculated evolution patterns determine the transient optical and electronic properties of filament wake channels. Accordingly, we propose tracing the wake channel evolution via linear and nonlinear light-scattering experiments. The evolving shape of the electron density distribution can be extracted from longitudinal and/or transverse Fraunhofer diffraction patterns. Complementarily, the evolving temperature distribution may be deduced either from angular-resolved four-wave-mixing experiments or from the spatial-spectral patterns of giant Rabi sidebands. Medium-specific estimates are made for atmospheric-pressure argon gas. In molecular-gas cases, the model can be straightforwardly augmented to incorporate the processes of dissociative recombination and vibrational excitations. [Preview Abstract] |
Wednesday, June 5, 2013 10:42AM - 10:54AM |
H6.00002: Modification of ellipticity of light propagating in air Ladan Arissian, Shermineh Rostami, Jean Claude Diels We measure the polarization of IR pulses propagating in air. The linearly and elliptically polarized light of 60, 110 and 300 fs interact with air molecules for 4 meters. The peak intensity of light is determined by intensity clamping in laser filaments. The ellipticity of light changes when the light is close to circular polarization, a change that is measured for various pulse widths and energies. We observe the change of polarization even in the shortest pulse width of 60 fs, when alignment is mostly neglected. We do {\em not} observe a polarization change for linearly polarized light and polarization of lowest energy (9.6 mJ, 300 fs, the only case without filamentation) pulses. We show that elliptical polarization is not preserved in filament formation, consistent with the prediction by Close1966 that molecular orientation induces cross-phase modulation resulting in stronger self-focusing of the weaker polarization. Our high power filament polarization analyzer includes a grazing incidence ($84^{\rm o}$) plate followed by a Brewster plate (both 1 mm thick fused silica). The polarization is scanned by rotating a half wave plate by an angle $\theta$ prior to the fixed quarter plate. We measure energy and pulsewidth dependent polarization change around $\theta=22.5^{\rm o}$. [Preview Abstract] |
Wednesday, June 5, 2013 10:54AM - 11:06AM |
H6.00003: Observation of molecular alignment by impulsively excited rotational wavepacket in $O_2$ and $N_2$ using scattered probe pulse Sima Hosseini, Ali Azarm, See Leang Chin We report a new optical technique to observe molecular alignment and periodic recurrences after impulsively exciting rotational wave packet, experimentally. The nonadiabatic laser-induced molecular alignment in a gaseous medium has been previously established. In the new current method, we collected the Rayleigh scattering of a weak probe pulse (400 nm/18 $\mu$J/$\sim$ 50 fs) in the direction at right angle to the propagation direction of the filament in either oxygen or nitrogen induced by a pump pulse (800 nm/0.9 mJ/ 42 fs). The aligned molecules in the filament would induce time dependent spectral modulation in the probe pulse. We measured the wavelength modulation of the elastically scattered probe pulse and obtained information of the alignment and revivals. Using this method we found the rotational revival period of $N_2$ and $O_2$. Moreover, we could measure the rotational constants of these gases in excellent agreement with those reported in the literature. This method has a potential application in filament remote sensing in the atmosphere to detect and identify molecular pollutants in the back scattered direction. Specially, this technique is a key to distinguish pollutants which their excited fragments inside the filament have the same fluorescence spectra. [Preview Abstract] |
Wednesday, June 5, 2013 11:06AM - 11:18AM |
H6.00004: Direct measurements of the nonlinear refractive index at high intensity in gases Jared Wahlstrand, Yu-Hsiang Cheng, Sina Zahedpour, Howard Milchberg The intensity-dependent nonlinear refractive index of air and other gases is important in understanding the propagation of intense optical pulses in the atmosphere, as well as in applications such as supercontinuum generation in gas cells and hollow core fibers. In addition to the third-order response of the bound electrons, molecular alignment and the response of electrons freed by ionization may contribute substantially to the time-dependent nonlinear refractive index. Recently, there has been a resurgence of interest in the fundamental nonlinearity because of the possibility of a large higher-order Kerr effect. We present direct, absolute measurements of the optical nonlinearity in the noble gases and components of air using single-shot spectral interferometry and a thin gas target. We discuss artifacts caused by interference effects between the pump and probe beams when the probe beam is the same wavelength as the pump beam. [Preview Abstract] |
Wednesday, June 5, 2013 11:18AM - 11:30AM |
H6.00005: Higher-Order Nonlinearity of Refractive Index: the Case of Argon Maryam Tarazkar, Dmitri Romanov, Robert Levis Higher-order dynamic Kerr effect (HOKE) is currently at the center of a controversy regarding the mechanisms of laser filamentation. A strong HOKE with a crossover from positive to negative nonlinear refractive index at intensities well below the ionization threshold, would engender plasma-free filamentation and exotic new effects in light propagation. Experimental evidence of HOKE crossover or lack thereof is being hotly debated. Motivated by this debate, we report the frequency-dependent nonlinear refractive index coefficients $n_{2}$ and $n_{4}$ for atmospheric-pressure argon gas, calculated via developed coupled cluster cubic response approach implemented in Dalton program. All calculations are performed at the CCSD level of theory with t-Aug-cc-PV5Z basis set. The benchmark dispersion curve for $n_{2}$ reproduces correctly the available experimental data and agrees well with previously-reported theoretical calculations. The nonlinear refractive index $n_{4}$ is obtained using the relations between different hyperpolarizability coefficients, and the latter are calculated via the auxiliary static electric field approach on the basis of $n_{2}$. We found that the higher-order nonlinear refraction index $n_{4}$ is positive over the wavelengths 300 nm-1500 nm. This result runs counter to the HOKE crossover hypothesis. [Preview Abstract] |
Wednesday, June 5, 2013 11:30AM - 11:42AM |
H6.00006: Accelerating neutral atoms on a Table top Krishnamurthy M, Rajeev Rajendran, Madhu Trivikram T, Rishad KPM, Narayanan V, Krishnakumar E Plasma accelerators driven by super strong laser fields couple unusually large energies to charged particles. Acceleration of neutral atoms from such strongly ionized plasmas have remained elusive. A laser based neutralizer can convert laser accelerated fast ion source to fast neutral atom source. We report a scheme [1] to generate fast Argon atoms (up to 1 MeV) from an optical-field-ionized dense nano-cluster ensemble. Intense, ultrashort pulses ionize each atom in a Ar nanocluster to 8+ and coulomb explode ions to energies up to MeV. We show that in a dense cluster ensemble, the electrons that stream out of the focal volume collisionally excited clusters in the periphery of the focus to high lying Rydberg excited states and form a sheath of electronically excited clusters. Cross sections for reducing ions by charge transfer collisions are orders of magnitude larger with the electronically exited systems. Fast ions that stream through the excited cluster sheath are reduced to neutral atoms with no change in momentum. We show that the scheme can covert ions to neutral atoms with nearly 100\% efficiency, transferring 8 electrons per atom in a few mm span of the supersonic jet.\\[4pt] [1] R. Rajeev et.al., Nature Physics 10.1038/NPHYS2526. [Preview Abstract] |
Wednesday, June 5, 2013 11:42AM - 11:54AM |
H6.00007: On the validity of the paraxial approximation for electron acceleration with radially polarized laser beams Vincent Marceau, Charles Varin, Michel Pich\'e Radially polarized laser beams possess unique properties that make them of great interest for many innovative applications. In particular, they can be used as driver beams for the acceleration of electrons in vacuum. To model electron acceleration with radially polarized laser beams, it is customary to work in the framework of paraxial wave optics. In other words, the paraxial approximation is made to obtain the lowest-order radially polarized fields, which are used to simulate the trajectory of electrons according to the Lorentz force equation. In this contribution, we question the validity of the paraxial approximation in the above mentioned context. To this end, we use an exact solution to Helmholtz equation as well as its paraxial counterpart, which consists of the lowest-order fields. In a parameter regime where the paraxial approximation was previously considered valid in the literature, we point out significant differences between the results obtained with the exact and the paraxial solutions. The origin of these discrepancies is analyzed with the help of a perturbation series expansion. More specifically, we highlight the important role played by the first nonparaxial corrections to the field components. [Preview Abstract] |
Wednesday, June 5, 2013 11:54AM - 12:06PM |
H6.00008: Resonantly enhanced electron-positron pair production in ultra-intense laser-matter interaction Francois Fillion-Gourdeai, Emmanuel Lorin, Andre Bandrauk A new mechanism for pair production from the interaction of a laser with two nuclei is presented. The latter takes advantage of the Stark effect in diatomic molecules and the presence of molecular resonances in the negative and positive energy continua. Both move in the complex energy plane as the interatomic distance and the electric field strength are varied. We demonstrate that there is an enhancement of pair production at the crossing of these resonances. This mechanism is studied in a very simple one-dimensional model where the nuclei are modelled by delta function potential wells and the laser by a constant electric field. The position of resonances is evaluated by using the Weyl-Titchmarch-Kodaira theory, which allows to treat singular boundary value problems and to compute the spectral density. The rate of producing pairs is also computed. It is shown that this process yields a positron production rate which is approximately an order of magnitude higher than in the single nucleus case and a few orders of magnitudes higher than Schwinger's tunnelling result in a static field. [Preview Abstract] |
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