Several Effects Of Second Harmonics In One And Two-Dimensional Ferroelctric Superlattices

This thesis focuses on the nonlinear phenomena occuring during the interaction between near infrared laser fundamental waves and ferroelectric materials. It keeps second harmonic (SH) generation as its thread, and set congruent LiTaO3(CLT) crystal samples as the main subject under examination. Throughout the thesis, several hot topics of the field in recent years are discussed, which include-nonlinear Cherenkov radiation (NCR) in bulk materials, elastic scattering quasi-phase matching (ESQPM) conical radiation, and local quasi-phase matching (LQPM) process, et al. The contents of the whole thesis are as the following:The first chapter is set as a background report of the thesis, making an outline to the latest research works in the fields which closely related to this thesis, and of my interest, of course, with personal bias. Its next chapters are designed to fixate on the details of three individual topics mentioned herein separately.The second chapter takes a brief tour of the material properties of the ferroelectric crystal CLT, including its nonlinear optical tensor properties. The rest of this chapter was put to illustrate the procedure of the room temperature poling technique of the CLT, as to complete this thesis as a self-contained source. Within this chapter, in the hope of making this thesis a self-contained source, I also make a brief introduction to the theory of quasi-phase matching.The third chapter gently leads the way through the Cherenkov radiation, a classical electromagnetic shocking wave to its counterpart in the nonlinear optical regime. It explains the formation of NCR and its mutation, radiation of the similar root, yet with a longitudinal phase tuning. With a space-time confined plane wave model, it succeeds in formulating the output angle spectrum of NCR, and finds out the relationship of NCR and the conventional two-dimensional quasi-phase matching. Careful experiments verified the validity of these output conditions.The fourth chapter deals with what are elastic scattering SHG conical beams and their present researching status. Considering its resemblance with NCR in its spatial distribution profile, I tried to argue the differences between these two effects. Most of the efforts were dedicated to high-order performance of elastic scattering conical beams, both in theory and in experiments. Based on the known output angle formula, I presented the algebric equaton of the SH projection curve set. And in keeping with the popular idea of harmonics intensities being related to the effective nonlinear coefficiencts, I deduced the effective nonlinear coefficiencts in this case. Experiments showed that phase-matched conical radiation with high-orders reciprocal vectors present a collective phenomenon other than low-orders, usually, only their spatial envelopes show up due to overlapping effect. I invented a vector shell method to decide the possible quasi-phase matching proccess in the experiment. And based on this method, I concluded and then verified it with experiments that "All quasi-phase matching processes with collinear incident fundamental waves are elastic scattering quasi-phase matching processes." I also proposed a rotational dynamic experiment of the high-order ESQPM, which could reveal more Fourier coefficients of the superlattice structure, and a degeneration phenomenon of the conical beams during the dynamic process was discussed.The fifth chapter describes the local quasi-phase matching theory. By paraphrasing the whole theory and observing the theory-determined domain structures with discrete Fourier transformation, a second discovery of the theory was achieved. In the reciprocal space of the local quasi-phase matching structure, the conventional phase matching reciprocal lattice vector has a transverse expansion, and the expansion is locating on the nonlinear Ewald sphere, which leads to the focusing effect of the LQPM.Then I proposed to extend this theory to manipulate wave front and a simple example was provided with both ferroelectric structure and numeric results of the outputs. At the end of this chapter, I discussed the potential weakness and possible improvements of the theory.The sixth chapter summarizes the thesis, and points out some potential research directions in the near future and my on-going research works.