Fabrice Charra

Light-induced second-harmonic generation in azo-dye polymers.

The permanent all-optical poling of an azo-aromatic acrylic copolymer is experimentally demonstrated by seeding preparation in a backward phase-conjugation geometry. The microscopic mechanism involves an orientational hole burning followed by orientational redistribution caused by trans-to-cis isomerization of the azo-dye chromophores. The characteristic kinetics of monitored by second-harmonic generation.

Quasi-permanent all-optical encoding of noncentrosymmetry in azo-dye polymers

We present an original poling technique that uses a purely optical excitation process. The experiment consists of a seeding-type process. Writing and probing periods are alternated. Writing periods correspond to simultaneous irradiation of the sample by the coherent superposition of the 1064-nm fundamental and the 532-nm second-harmonic light of a picosecond-pulsed Nd:YAG laser. The sample is a spin-coated film of a poly(methyl methacrylate) copolymer onto which the azo-dye molecule Disperse Red 1 is grafted. We demonstrate efficient and quasi-permanent poling of the molecules with a spatial period that satisfies the phase-matching condition for second-harmonic generation. The influence of seeding parameters such as the relative phase and the relative intensities between the writing beams is studied both theoretically and experimentally. Tensorial properties and the spatial profile of the photoinduced χ(2) are analyzed from a microscopic point of view. Dark and photostimulated relaxation processes are investigated from a chemical-physics point of view. The physical origin of the photoinduced molecular orientation process is discussed. A minimal model involving the relevant experimental parameters is developed. Numerical simulations are in agreement with the experiment.

Picosecond photoinduced dichroism in solutions of thiophene oligomers

Abstract The picosecond photoinduced absorption of a series of α-coupled thiophene oligomers in solution have been measured using the Kerr ellipsometry configuration. We present spectral and dynamic studies of the sharp transient absorption peak observed in the transparency region of these oligomers. Its origin and the reasons for its narrow line shape are discussed. The conversion towards the triplet state is observed in terthiophene.

Transient optically induced non-centrosymmetry in a solution of octupolar molecules

Abstract The transient polar orientation of a solution of the octupolar molecule ethyl violet is achieved by optical means in an experimental geometry of phase conjugation. The corresponding resonant six-wave mixing process is described in terms of anisotropic nonlinear absorption. Orientation results from the consistent octupolar symmetry of both the molecular species and the orienting field tensor. The 26 ps decay time of the induced non-centrosymmetry is limited by electronic excitation relaxation. A 510×10−30 esu second-harmonic hyperpolarizability at 1064 nm fundamental wavelength with 13 D transition dipole moment between excited states is inferred. Octupolar nature of the hyperpolarizability tensor symmetry is verified by polarization analysis of the induced second-harmonic susceptibility components.

Six-wave mixing probe of light-induced second-harmonic generation: example of dye solutions

Using a phase-conjugation configuration based on six-wave mixing interactions, we have investigated in detail the mechanism of photoinduced second-harmonic generation (SHG) in initially centrosymmetric materials. After some theoretical study of the process we discuss how different complementary tests (with the same experimental setup), such as investigations of the polarization properties or the lifetime dependence of the induced χ(2) grating on the solvent, can reveal the microscopic characteristics of the process. We present experimental studies performed in various solutions of an azo-dye molecule (Disperse Red 1) with the 532-nm harmonic light along with the 1.064-μm light from a picosecond-pulsed Nd:YAG laser. The results show good agreement with the theoretical predictions. It is shown moreover that the light-induced χ(2) grating is due to oriental hole burning of the molecules, whereas reorientation of the molecules in the solvent is one major cause of relaxation.

Light-induced second-harmonic generation in an octupolar dye.

Quasi-permanent light-induced second-order susceptibility χ(2) of octupolar molecules is demonstrated. The octupolar molecule Ethyl Violet was oriented in a solgel matrix by a purely optical poling technique. The method consists of a seeding process based on a nonclassical holographic process with coherent two-frequency mixing. The growth of the induced χ(2) is monitored in real time by frequency doubling of the fundamental beam in the sample. The polarization dependence of the signal is used to demonstrate the octupolar symmetry of the induced χ(2).

Phase and frequency resolution of picosecond optical Kerr nonlinearities

We report on a method aimed at measuring both the real and imaginary parts of the third-order Kerr-type nonlinear-optical susceptibility of isotropic materials. It is based on Kerr ellipsometry in which analysis of the polarization state of transmitted light between a nearly crossed polarizer and an analyzer allows separation of pump-induced dichroism and birefringence. The method provides a natural measurement of nonlinear phase retardations in angle units, without any calibration procedure. By using a white-light continuum as a probe, this property is used to get a simultaneous measurement of the nonlinear dispersion in the whole visible spectrum. It is illustrated in tetramethylsilane, where stimulated Stokes and anti-Stokes Raman contributions to the nonlinearity are observed.

Efficient all-optical poling of an azo-dye copolymer using a low power laser

Abstract Efficient all-optical poling of an azo-dye copolymer (consisting of a PMMA backbone to which DR1 molecules were grafted as pendant side groups) using a low power microsecond laser is demonstrated. Conversion efficiencies are as large as those usually achieved using more standard methods such as Electric Field poling. Periodic poling of the sample is evidenced. Influence of the preparation conditions on the second-harmonic generated signal is presented.

All-optical induction of noncentrosymmetry in a transparent nonlinear polymer rod

What is believed to be the first evidence of an all-optical orientation of a highly transparent polymer is presented. The samples are bulk rods of poly(methyl methacrylate) grafted with paranitroaniline (PNA) obtained by use of copolymerization of methyl methacrylate (MMA) monomers mixed with grafted MMA-PNA monomers. Bulk samples were obtained after molding and polishing of the copolymer with standard techniques. Experiments show that the photoinduced second-order(1) susceptibility chi((2)) may occur through molecular reorientation following selective polar excitation of the nonlinear chromophores by simultaneous two- and three-photon absorption on the same electronic level.

Nondegenerate multiwave mixing in polydiacetylene: phase conjugation with frequency conversion

We present an analysis of optical phase conjugation with frequency doubling in terms of nondegenerate six-wave mixing fifth-order nonlinear susceptibility. We show how the Bragg phase-matching condition is achieved at small angles. The process under picosecond-pulse conditions in a polydiacetylene solution pumped at two-photon resonance and probed at one-photon resonance is experimentally demonstrated. Results are discussed in terms of a one-dimensional quantum three-level system. The proximity of one- and two-photon resonance in conjugated polymers makes these polymers efficient nonlinear-optical materials for this original application. The experiment permits artifact-free measurement of the two-photon state recovery time of the red form of polydiacetylene.