Ce travail porte sur l’étude des mécanismes de conversion de fréquences dans les cristaux photoniques non linéaires bidimensionnels. En particulier, nous avons étudié la génération paramétrique optique (GPO) dans les cristaux de LiTaO3 (PPLT- 2D) à réseau carré. Nous nous sommes d’abord intéressés à l’étude théorique et numérique de structures PPLT- 2D. Une étude numérique, nous a permis de comprendre la contribution des vecteurs de réseau réciproque (kmn) pour différents types de réseaux, en particulier les réseaux carrés et rectangulaires. Par simulations numériques, nous avons également étudié l’influence de l’angle incidence du faisceau de pompe sur l’efficacité de conversion paramétrique dans ses structures. Les mesures expérimentales nous ont permis de mettre en évidence, pour la première fois, l’existence de plusieurs singularités où le signal est partagé pour générer deux idlers et vice versa. En effet, en analysant les propriétés spectrales et angulaires des faisceaux à la sortie du cristal, nous avons, expérimentalement et numériquement, identifié les contributions impliquées dans ces processus communs de GPO. De plus, nous avons étudié le management du gain paramétrique optique dans un réseau carré de PPLT-2D. Les résultats montrent que l’exploitation de la direction non colinéaire permet d’augmenter le gain suivant deux angles d’incidences de la pompe : 0.8° et 1.6°. Cela implique, une meilleure efficacité de conversion en minimisant le walk-off entre la pompe et l’idler. Dans ces conditions, le gain pourrait être proche de celui de la direction colinéaire. Un bon accord a été constaté entre les résultats expérimentaux et ceux de la simulation. / This work deals with the study of the mechanisms of frequency conversion in two-dimensional nonlinear photonic crystals. In particular, we have studied optical parametric (OPG) generation in square-lattice LiTaO3 crystals (PPLT-2D). We first focused on the theoretical and numerical study of PPLT-2D structures. A numerical study allowed us to understand the contribution of the reciprocal lattice vectors (kmn) for different types of gratting, in particular square and rectangular lattices. By numerical simulations, we also studied the influence of the incidence angle of the pump beam on the efficiency of the parametric conversion in these structures. Experimental measurements have allowed us to demonstrate, for the first time, the existence of several singularities where the signal is shared to generate two idlers and vice versa. Indeed, by analyzing the spectral and angular properties of the beams at the output of the crystal, we have experimentally and numerically identified the contributions involved in these common OPG processes. In addition, we studied the management of the optical parametric gain in a square lattice of PPLT-2D. The results show that the use of the non-collinear direction makes it possible to increase the gain according to two angles of incidence of the pump: 0.8 ° and 1.6 °. This implies, better conversion efficiency by minimizing the walk-off between the pump and the idler. Under these conditions, the gain could be close to that of the collinear direction. A good agreement was found between the experimental results and those of the simulation.

Optical parametric frequency conversion based on quasi phase matching (QPM) in nonlinear optical crystals is a powerful technique for generating coherent radiation in wavelength ranges spanning from the mid-infrared (mid-IR) to the blue, displaying low thermal load and high efficiency.This thesis shows how QPM in one- (1D) or two-dimensional (2D) lattices can be employed to engineer novel devices for parametric downconversion in the IR, af-fording freedom in designing both spectral and angular properties of the parametric output. Experimental demonstrations of parametric devices are supported by theoreti-cal modelling of the nonlinear conversion processes.In particular, broadband parametric downconversion has been investigated in 1D QPM lattices, through degenerate downconversion close to the point of zero group-velocity dispersion. Ultra-broadband optical parametric generation (OPG) of 185 THz bandwidth (at 10 dB), spanning more than one octave from 1.1 to 3.7 μm, has been achieved in periodically poled 1 mol% MgO-doped near-stoichiometric LiTaO3 (MgSLT) of 25 μm QPM period, pumped at 860 nm. Such broadband gain is of high interest for ultrashort optical pulse amplification, with applications in high harmonic generation, ultrafast spectroscopy and laser ablation. Furthermore, the det-rimental impact of parasitic upconversion, creating dips in the OPG spectrum, has been investigated. By altering the pump pulse duration, energy can be backconverted to create peaks at the involved OPG wavelengths, offering a possible tool to enhance broadband parametric gain spectra.The engineering of the angular properties of a parametric output benefits greatly from 2D QPM, which is investigated in this thesis by the specific example of hexagonally poled MgSLT. It is demonstrated how two OPG processes, supported by a single 2D QPM device, can exhibit angularly and spectrally degenerate signals (idlers). This degeneracy results in a coherent coupling between the two OPG pro-cesses and a spectrally degenerate twin-beam output in the mid-IR (near IR). 2D QPM devices exhibiting such coherently coupled downconversion processes can find applications as compact sources of entangled photon-pairs. This thesis further illus-trates the design freedom of 2D QPM through the demonstration of a device support-ing multiple parametric processes, thus generating multiple beams from the mid-IR to the blue spectral regions. / <p>QC 20131204</p>