An experimental study of both transient and steady state bi-dimensional photorefractive bright spatial solitons in Bi12TiO20 is presented. The results are compared to previous theoretical work, showing qualitative agreement conceming both the transient and the steady soliton widths.

The time behavior of bright spatial solitons in biased photorefractive media is investigated within the framework of a bidimensional band transport model. Biasing the photorefractive media requires an externally applied electric field or the presence of a photovoltaic effect. These two basically different phenomena are shown to be equivalent and additive. The mechanism of space-charge field buildup is analytically expressed, leading to a time-dependent wave propagation equation in generic photorefractive media. The temporal behavior of the soliton solutions to this equation is investigated. It shows the evolution of the soliton beam from the time the external electric field is applied to the final steady-state soliton. On the way, the so-called quasisteady soliton is retrieved, along with its properties. Furthermore, the photovoltaic soliton is described by the wave propagation equation: its behavior is the same as that of the steady-state soliton, the transient states included. Finally, low-power photorefractive bright spatial solitons are generated in a Bi12TiO20 crystal with a He-Ne laser and their temporal behavior is investigated, thus providing an experimental validation of our theoretical considerations.

CETTE THESE PROPOSE UNE MISE EN EVIDENCE THEORIQUE ET EXPERIMENTALE DE L'AUTO-FOCALISATION D'UN FAISCEAU LASER DANS PLUSIEURS ECHANTILLONS DE CRISTAL PHOTOREFRACTIF (BI#1#2TIO#2#0, SR#XBA#1#-#XNB#2O#6 ET BATIO#3). ELLE PROPOSE EN OUTRE D'ETUDIER COMMENT ET DANS QUELLES CONDITIONS L'AUTO-FOCALISATION PEUT CONDUIRE A LA PROPAGATION DE FAISCEAU NON-DIVERGENTS, C'EST-A-DIRE DE SOLITONS SPATIAUX. NOUS AVONS PU, GRACE A DES HYPOTHESES ADAPTEES A LA PROPAGATION DE SOLITONS SPATIAUX, DERIVER DU MODELE DE TRANSPORT PAR BANDE DE KUKHTAREV UNE EQUATION DE PROPAGATION DEPENDANT EXPLICITEMENT DU TEMPS. ELLE PERMET DE DECRIRE LA PROPAGATION DE TOUT PROFIL DE FAISCEAU RESPECTANT LES HYPOTHESES. NOUS AVONS ANALYSE CETTE EQUATION EN MONTRANT QU'ELLE PERMETTAIT LA PROPAGATION DE SOLITONS SPATIAUX, DONT NOUS AVONS ETUDIE LE COMPORTEMENT. NOUS AVONS EGALEMENT PU METTRE EN EVIDENCE DIVERS PHENOMENE QUE CETTE EQUATION DECRIT, TELS QUE LE BRANCHEMENT OPTIQUE. GRACE A LA CONCEPTION D'UN BANC EXPERIMENTAL ORIGINAL, NOUS AVONS OBSERVE LE PHENOMENE D'AUTO-FOCALISATION DANS LES TROIS CRISTAUX MENTIONNE CI-DESSUS, AINSI QUE DES PHENOMENE DE BRANCHEMENT OPTIQUE DANS BATIO#3. L'ANLYSE THEORIQUE CONDUITE CI-DESSUS NOUS PERMET D'AFFIRMER QUE NOUS AVONS OBSERVE LA PROPAGATION DE SOLITONS SPATIAUX PHOTOREFRACTIFS : L’EXPÉRIENCE A PERMIS DE VALIDER NOTRE MODELE THEORIQUE

A (1+1)-dimensional model is studied numerically to evidence the build up mechanisms of photorefractive solitons, from the characteristic carrier recombination time, through the quasi-steady-state soliton, to the screening soliton. Three different build up regimes are evidenced and their domain of existence are computed. The transient quasi-steady-state soliton is shown to be characterized by two constants: its normalized width and its normalized build up time response multiplied by its peak intensity over dark irradiance. This latter assertion allows us to predict the photorefractive soliton response time for various optical powers. It is thus compared to existing experimental results.

Photorefractive spatial solitons have recently been the object of intense fundamental research. This paper proposes two applications of this interesting low power phenomenon. Photorefractive self-focusing can indeed be used to enhance nonlinear processes such as wavelength conversion. Furthermore, its sensitivity to low power optical beams makes it a good candidate towards all optical switching and computing, provided the time response issues have been solved. This paper proposes a switching technology with optical powers on the scale of a few mW/cm2 and provides strong hints towards solving the photorefractive time response issue.

We consider the propagation of light beams in photorefractive media in the framework of a (1+1)-dimensional model. The Kukhtarev band transport model is introduced both in a time-dependent differential equation describing the evolution of the space charge field and in a nonlinear wave propagation equation. This latter is then numerically solved with a beam propagation method routine. The evolution in time and space of an initially diffracting laser beam is simulated as a function of initial profiles and waists. The beam is shown to go through a transient overfocused state prior to relaxing to a steady state soliton. Additional features such as the stability condition of the system or effects such as optical branching and soliton interactions are studied.

This paper presents a theoretical and experimental investigation of the self-focusing of a laser pulse in a photorefractive medium on a nanosecond time-scale. For times shorter than the dielectric recombination time, we analyze the space-charge field build-up with respect to time and space and provide strong hints on the short time self- focusing of a narrow powerful beam. In order to validate our theoretical and numerical analysis, systematic investigations were performed on a Bi12TiO20 crystal, to show the effect of the external applied electric field, the intensity of the laser pulse and the beam waist.