G. Rivoire

Third-order nonlinear optical properties of new bisdithiafulvenyl-substituted tetrathiafulvalene

We measured the third-order susceptibilities χijkl〈3〉 of new bisdithiafulvenyl-substituted tetrathiafulvalene, using the degenerate four-wave mixing method. From these measurements we deduced the values of the second-order hyperpolarizabilities γ and their molecular and electronic contributions. We compared the third-order optical properties of these new extended tetrathiafulvalene derivatives with the properties of acetylenic and ethylenic analogs of tetrathiafulvalene that we studied previously. All organic molecules studied here revealed large third-order nonlinear optical hyperpolarizability.

Degenerate four-wave mixing in absorbing isotropic media

Abstract Degenerate four-wave mixing in absorbing liquid media leading to a quantitative description of the phase conjugation efficiency R is analysed. The influence of linear and two-photon absorption on R is determined. The validity limits of a model based on classical Maxwell approach are investigated. Given the parameters characterising a studied sample (linear and non-linear absorption coefficients, and the thickness of the sample), the proposed method allows to choose an interval of incident intensities in which the model remains valid; beyond these intensities it is necessary to correct the model.

Stochastic resonance and noise-enhanced transmission of spatial signals in optics: the case of scattering

The nonlinear effect of noise-enhanced signal transmission by means of stochastic resonance in optics is studied. We investigate this effect for the novel case of spatial signals or images. With a theoretical model involving a threshold nonlinearity we describe a mechanism whereby the transmission of an image can be improved by the addition of noise. We argue that such a nonlinear mechanism can operate in different types of light scattering. With a stimulated Raman scattering experiment we verify the existence of a stochastic resonance effect in the transmission of a laser image; we show that maximal efficacy is obtained with the assistance of a speckle of sufficient intensity. The results extend the scope of stochastic resonance and can serve as a basis for further development of the effect in optics.

Reconstruction of the laser wave-front by stimulated scatterings in the pico-second range

Abstract The reconstruction of the exciting laser wave-front by the backward Stimulated Raman Scattering (S.R.S.) and Rayleigh Wing Scattering (S.W.R.S.) is observed in benzene and cyclohexane in the case of pico-second pulses. The characteristic properties of this reconstruction are different of those obtained in the nano-second range.

Backward stimulated Raman scattering in water

The backward Stimulated Raman scattering (BSRS) is studied in liquid water. We show that BSRS can be produced by small volumes of active materials. Our experimental results and calculations with different geometries and intensities of excitation allow us to choose the best experimental set-up for BSRS efficiencies. The competition with librational scattering is also observed.

Influence of transverse effects on measurement of third-order nonlinear susceptibility by self-induced polarization state changes

Measurements of the third-order nonlinear electric susceptibility component χxyyx3 by means of self-induced changes in the state of polarization of an intense elliptically polarized laser beam are reported. The simultaneous observation of the transverse intensity profile and of the losses allows us to show the influence of field transverse gradients on the polarization state. We show that the plane-wave model cannot explain our experimental results, even at low input intensities. The influence of spatial dispersion on nonlinear susceptibility is demonstrated.

Third-order nonlinear optical susceptibility of acetylenic analogues of tetrathiafulvalene

Abstract An analysis of picosecond time-resolved degenerate four-wave mixing at 532 nm in soluble acetylenic analogues of tetrathiafulvalene is presented. Different nonlinear mechanisms are isolated and discussed. The molecular hyperpolarizabilities can reach 2 × 10 −44 SI units (10 5 times greater than that of CS 2 ).

Nitrobenzene as a material for the fast-respond degenerate four-wave mixing

Abstract We have revealed a possibility of using nitrobenzene (NB) as a promising for time-resolving degenerate four-wave mixing (DFWM). The DFWM efficiency versus time delay between the probe and pump beams was revealed. The four time-retarded maxima of the DFWM are caused by specific electron–vibration interactions in the NB. An essential contribution of acoustical vibrations is demonstrated. Molecular dynamics geometry optimisation of the NB molecule is done taking into account intermolecular interactions. Superposition of all possible molecular conformations with appropriate weighting factors and higher-order intermolecular multiple interactions is considered. The observed DFWM behavior is explained within the framework of ab inito quantum chemical calculations with inclusion of excited configuration interaction (CI) states. We considered both harmonic as well anharmonic electron–vibration interactions. The average lifetime of MO levels contributing to the third-order nonlinear and time kinetics of the observed DFWM dependences is estimated. Advantages and drawbacks of the prosposed method are discussed.

Stimulated Raman Scattering Thresholds for Ultra-short Excitation

S.R.S. thresholds in different liquids have been measured with a 25 ps pulse duration. An important rise, compared with the measured thresholds in the nanosecond range, has been noted, and it cannot be completely explained by the transient self-focusing or Raman effects as they are usually calculated. Several assumptions are reviewed in order to explain the observations.

Changes in the space structures of light beams induced by nonlinear optical phenomena: Application to phase contrast and image processing

Abstract The possibilities offered by optical nonlinearities (Kerr effect, nonlinear absorption, stimulated scatterings) for real time all optical imaging techniques such as phase contrast, contrast improvement, contrast reversal are demonstrated. The simplicity of the set ups used, and the good reproducibility of the results lead to potential applications.