O. Faucher

Measurement of high order Kerr refractive index of major air components

We measure the instantaneous electronic nonlinear refractive index of N(2), O(2) and Ar at room temperature for a 90 fs and 800 nm laser pulse. Measurements are calibrated by post-pulse molecular alignment through a polarization technique. At low intensity, quadratic coefficients n(2) are determined. At higher intensities, a strong negative contribution with a higher nonlinearity appears, which leads to an overall negative nonlinear Kerr refractive index in air above 26 TW/cm(2).

An experimental investigation of the nonlinear refractive index (n2) of carbon disulfide and toluene by spectral shearing interferometry and z-scan techniques

The recently proposed spectral shear interferometry and the well-known z-scan techniques were employed for the determination of the nonlinear refractive index n2 of CS2, toluene and fused silica. The determined n2 values by both techniques were found to be in very good agreement. In addition, the role of the repetition rate of the laser is also investigated revealing its importance for the correct determination of both the size and the sign of the nonlinearity.

Measurement of high order Kerr refractive index of major air components: erratum

A clarification is missing concerning the high order Kerr non-linearities deduced from our experimental data published in [Opt. Express 17, 13429-13434 (2009)]. Here, we rectify this omission by making explicit the distinction between cross-Kerr and Kerr effects, and by extrapolating the value of the nonlinear refractive index for the last effect. Since the occurrence of sign inversion in the Kerr effect is not affected, the overall report in [Opt. Express 17, 13429-13434] remains valid.

Transition from Plasma-Driven to Kerr-Driven Laser Filamentation

While filaments are generally interpreted as a dynamic balance between Kerr focusing and plasma defocusing, the role of the higher-order Kerr effect (HOKE) is actively debated as a potentially dominant defocusing contribution to filament stabilization. In a pump-probe experiment supported by numerical simulations, we demonstrate the transition between two distinct filamentation regimes at 800 nm. For long pulses (1.2 ps), the plasma substantially contributes to filamentation, while this contribution vanishes for short pulses (70 fs). These results confirm the occurrence, in adequate conditions, of filamentation driven by the HOKE rather than by plasma.

Analysis of femtosecond Raman-induced polarization spectroscopy (RIPS) in N2 and CO2 by fitting and scaling laws

Raman-induced polarization spectroscopy (RIPS) experiments were conducted at room temperature and pressures below 2 atm in pure N2 and CO2. Both homodyne and optically heterodyne detection were used. Calculations of the signal as a function of the pump–probe delay were performed taking into account the pulse duration of the laser, the rotational dependence of the dephasing rates and the small instantaneous electronic Kerr effect. The observed decay and shape of the rotational quantum beats are in good agreement with the calculations. Several sets of linewidth coefficients corresponding to the diagonal part of the collisional–rotational relaxation matrix were used. This matrix was choosen among available models derived from wavenumber domain studies (fitting and scaling laws). The energy-corrected sudden model derived from extensive wavenumber domain studies reproduces the experimental data well. A comparison between different models is discussed. Copyright

Field-free two-direction alignment alternation of linear molecules by elliptic laser pulses.

We show that a linear molecule subjected to a short specific elliptically polarized laser field yields post-pulse revivals exhibiting alignment alternatively located along the orthogonal axis and the major axis of the ellipse. The effect is experimentally demonstrated by measuring the optical Kerr effect along two different axes. The conditions ensuring an optimal field-free alternation of high alignments along both directions are derived.

High-Field Quantum Calculation Reveals Time-Dependent Negative Kerr Contribution

The exact quantum time-dependent optical response of hydrogen under strong-field near-infrared excitation is investigated and compared to the perturbative model widely used for describing the effective atomic polarization induced by intense laser fields. By solving the full 3D time-dependent Schrödinger equation, we exhibit a supplementary, quasi-instantaneous defocusing contribution missing in the weak-field model of polarization. We show that this effect is far from being negligible, in particular when closures of ionization channels occur and stems from the interaction of electrons with their parent ions. It provides an interpretation of the higher-order Kerr effect recently observed in various gases.

Pulse trains produced by phase-modulation of ultrashort optical pulses: tailoring and characterization

In this paper, creation of pulse doublets and pulse trains by spectral phase modulation of ultrashort optical pulses is investigated. Pulse doublets with specific features are generated through step-like and triangular spectral phase modulation, whereas sequences of pulses with controllable delay and amplitude are produced via sinusoidal phase modulations. A temporal analysis of this type of tailored pulses is exposed and a complete characterization with the SPIDER technique (Spectral Phase Interferometry for Direct Electric-field Reconstruction) is presented.

On negative higher-order Kerr effect and filamentation

As a contribution to the ongoing controversy about the role of higher-order Kerr effect (HOKE) in laser filamentation, we first provide thorough details about the protocol that has been employed to infer the HOKE indices from the experiment. Next, we discuss potential sources of artifact in the experimental measurements of these terms and show that neither the value of the observed birefringence, nor its inversion, nor the intensity at which it is observed, appear to be flawed. Furthermore, we argue that, independently on our values, the principle of including HOKE is straightforward. Due to the different temporal and spectral dynamics, the respective efficiency of defocusing by the plasma and by the HOKE is expected to depend substantially on both incident wavelength and pulse duration. The discussion should therefore focus on defining the conditions where each filamentation regime dominates.

Femtosecond polarization spectroscopy in molecular gas mixtures: Macroscopic interference and concentration measurements

Raman-induced polarization spectroscopy (RIPS) experiments combined with homodyne detection have been conducted with a femtosecond laser at room temperature and low pressure (p<2 atm) in CO2–N2 mixtures as well as in air (O2–N2 mixtures). Each molecule of the mixture produces its own time-dependent signal, measured as a series of recurring transients. Macroscopic interference is observed when transients of both molecules overlap in the time domain. This interference leads to a large modification of the signal, which is well reproduced by calculations. The total signal recorded in CO2–N2 or O2–N2 mixtures of known concentration is analyzed in order to measure the polarizability anisotropy ratio of the two components at 800 nm. The ratio measured in an O2–N2 (air) mixture is compared with values of previous works. The knowledge of the ratio for CO2/N2 allows us to determine the concentration of CO2–N2 unknown gas mixtures. The method is presented as a relevant technique for concentration measurements in the...