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Three investigations of difference frequency generation (DFG) of far-infrared radiation by optical mixing are described: a theory of DFG by monochromatic, focused Gaussian pump laser beams, a theory of DFG by a picosecond pump laser pulse, and an experiment using ruby-pumped dye lasers. First, the theory of far-infrared generation by optical mixing of monochromatic, focused Gaussian beams in a uniaxial crystal is developed, taking into account the effects of diffraction, absorption, double refraction, and multiple reflections and total reflection at the boundary surfaces. (Reflection and transmission coefficients of a uniaxial crystal slab are derived by a new matrix technique.) Results of numerical calculations are presented. Focusing the pump beams appreciably enhances the far-infrared output despite the strong far-infrared diffraction. In a 1-cm long crystal, the optimum focal spot size is approximately equal to or smaller than the far-infrared wavelength for output frequencies less than 100 cm−1. Double refraction of the pump beams is relatively unimportant. Both far-infrared absorption and boundary reflections have major effects on the far-infrared output and its angular distribution. The former is often the factor which limits the output power. We show that a simple model treating the nonlinear polarization as a constant lie-radius Gaussian distribution of radiating dipoles adequately describes the effect of pump-beam focusing. We also compare the results of our calculations with those for second-harmonic generation. Second, a theoretical calculation of far-infrared power spectra generated by picosecond pulses in a nonlinear crystal is developed. The results are illustrated with two practical examples: LiNbO3 slabs oriented for rectification of the optical e-ray and for beating of the optical o-ray with the optical e-ray. The former is phase matched at 0 cm−1; the latter, at both the forward-(FCPM) and backward-collinear phase-matching frequencies. The one-dimensional, time-dependent electric field is discussed and then used to explain the origin of the oscillation periods in the power spectra. Finally, a series of experiments using a pair of ruby-pumped dye lasers and a novel dual-frequency dye laser system is described. With these two laser systems, continuously tunable far-infrared radiation in the frequency range 20 to 190 cm−1 was generated. Forward-, backward-, and non-collinear phasematching in LiNbO3 at frequencies between 20 and 160 cm−1 were investigated; 90{sup o} noncritical FCPM was also observed in ZnO at 190 cm−1, CdS at 180 cm−1, and ZnS at 91 cm−1. The highest peak power ((almost equal to)200 mW) and the broadest tuning range using a single crystal sample (40 to 160 cm−1) were obtained with noncollinear phase matching in LiNbO3.
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This dissertation deals with the generation of parametric light in the range 1 to 12 μm. Parametric infrared generation turns out to be a challenge at the interface between the fields of nonlinear optics and materials science embodied by the two approaches used to achieve efficient frequency conversion. Birefringent Phase-Matching (BPM) in anisotropic materials has been the traditional solution used in most frequency converter devices. But since the 90's, the quick success of microstructured materials has paved the way for Quasi-Phase-Matching (QPM) even in isotropic materials, leading to a renewed interest in Optical Parametric Oscillators (OPO). The high degree of engineering offered by this technology is now widely recognized as a key competitive advantage. We obtained original results concerning parametric infrared generation using BPM as well as QPM.We have built the first OPO pumped by a 1.064 μm Nd:YAG laser and based on a 5-mm-thick crystal of 5%MgO:PPLN cut as a partial cylinder. This OPO combines a wide and continuous tunability over the range 1.4 μm - 4.4 μm with a good conversion efficiency, up to 30%. Despite the need to resort to pump intensities almost an order of magnitude higher than in a slab OPO, we have shown that the energetical performance of a partial cylinder OPO is now equivalent to that of a slab OPO besides a wider tunability that can be continuously addressed. When the same Nd:YAG laser pumps two such independent OPOs in parallel, we dispose of a highly versatile QPM dual wavelength source with two widely and independently tunable beams. We have built this unique source allowing versatile Difference Frequency Generation (DFG) towards the mid- and far- infrared. We carried out the first BPM DFG experiments with this source in a CdSe slab oriented for angular noncritical phase-matching at two different pump wavelengths, respectively 2.72 μm and 2.79 μm. The second set of DFG experiments were performed in a CdSe crystal cut and polished as a 5-mm-diameter full cylinder. Using a pump wavelength of 2.79 μm, we were able to tune the DFG wavelength from 8.3 μm up to 10.3 μm by rotating the crystal over an angular range of 18°. Contrary to all the BPM DFG experiments reported so far in the single crystal CdSe, tuning was achieved while keeping normal incidence of both the incident and generated beams in the crystal. The implementation of spectral narrowing techniques is already anticipated and will contribute to more accurate measurements of the phase-matching directions of a crystal as well as to a higher DFG conversion efficiency.These experiments with our dual wavelength source are preliminary and encouraging validations of our capability of performing DFG in small crystals and at any pump wavelength between 1.4 μm and 3.5 μm. Even though we investigated the promises held by CdSiP2 when it is only pumped with a Nd:YAG laser at 1.064 μm, there is tremendous prospect in terms of tunable infrared generation between 3.5 μm and 8 μm when this crystal is pumped around 2.4 μm. Such early demonstrations will be highly valuable for future applications requiring compact and tunable sources spanning the infrared spectrum. From a more fundamental point of view, performing DFG experiments at different pump wavelengths in the mid-infrared can lead to a highly accurate determination of the values of the refractive indices of a nonlinear crystal. In this dissertation, we have cast the first stone of a method that leads to the determination of the values of the refractive indices of a nonlinear crystal in the mid- to far- infrared. This new method is based on the unique measurements of the DFG phase-matching angles in spheres or cylinders, and should contribute to further advances in the field of phase-matching metrology.