Shulha D. Nonlinear optical interactions and their spatial anisotropy in crystalline materials and nanocomposites

This dissertation is devoted to the task of increasing the efficiency of nonlinear optical devices by optimizing the characteristics of known materials and by creating new nonlinear optical materials based on nanocomposites. Justification of the relevance of the chosen topic of the dissertation research, the purpose of the research, the main tasks, scientific novelty and practical value of the work are highlighted in the introduction. The first chapter of the dissertation presents a review of literature sources on the physics of nonlinear optical processes – generation of the second harmonic, sum and difference frequencies, parametric generation, as well as the variants of their application in optical devices. The issues of the conditions of phase matching, both scalar and vector, the implementation of which allows to increase the magnitude of the observed nonlinear optical effects are considered. A review of the physics of nanocomposites is presented, a method for manufacturing nanocomposites, their use in optoelectronics and photonics is described. The second chapter describes the method used in the work to solve the problem of optimizing the geometry of nonlinear optical interactions in crystalline materials in the general case of non-collinearity of interacting beams (vector phase matching). The method is based on the construction and analysis of the properties of extreme surfaces, which reflect the maximum achievable efficiency values for all possible directions of the output (generated) beam. A description of the crystals of the orthorhombic and monoclinic systems, for which the study is conducted, is presented. A description of the special software developed for calculations is provided. Data on the technology for manufacturing nanocomposites, which are NH4H2PO4 nanocrystals (ADP) located in the pores of the Al2O3 (AAO) nanomatrix, as well as a description of the experimental setup on which the study was carried out, are presented. The third chapter presents the results of determining the maximum achievable values of the generation efficiency of the second harmonic, sum and difference frequencies in a number of biaxial nonlinear optical crystals of the rhombic system - KTP, KTA, KB5, KNbO3, LBO, CBO, LRB4. The optimal geometries of vector phase matching are determined, i.e. such directions of the wave vectors of the input and output beams that provide the highest achievable values of the efficiency of nonlinear processes. It is shown that for a significant number of cases, vector phase matching ensures higher efficiency than the scalar one. In particular, for the case of second harmonic generation (type I or ssf phase matching) in KB5 (the relative efficiency increase is 145%), KTP (49%), KNbO3 (39%) and KTA (17%). The fourth chapter presents the results of the optimization of the geometry of nonlinear optical interactions for monoclinic nonlinear optical crystals GdCOB, YCOB, BiBO, LCB. It is shown that for these materials the efficiency increase caused by the use of vector phase matching does not exceed tens of percents with respect to the scalar case. The maximum value of the relative efficiency increase is about 53% for the BiBO crystal in the case of difference frequency generation. The highest absolute efficiency values also occur for the BiBO crystal, while the lowest ones are for LCB. The fifth chapter presents the results of studies of a nanocomposite nonlinear optical material AAO:ADP obtained by introducing NH4H2PO4 nanocrystallites into the nanopores of anodized aluminum oxide. Experimental studies using the Maker fringes method revealed a strong third-order nonlinear response, while second-harmonic generation was absent, probably due to phase mismatch and surface roughness. Theoretical modeling using the comparative model and the Reintjes model allowed to determine the value of the third-order nonlinear optical susceptibility for the studied samples. The study also revealed the key role of nanopore size in influencing the efficiency of third-harmonic generation.