Light-Induced Ferroelectric Domain Inversion And Optical Frequency Conversion Based On Quasi-Phase-Matching

As Laser showed up, people's knowledge towards optics was dramatically changed. Nonlinear optics became one of the interest subjects when laser interacts with some media with one or more new frequency generated, known as frequency conversion processes, such as sum frequency, difference frequency effects. The efficiency of frequency conversion depends on the phase matching conditions, which can usually be fulfilled by the Birefringence method, while the materials dependence, temperature, input wavelength, and polarization limit its further applications.The concept of Quasi-Phase-Matching (QPM) with periodic modulation of the sign of second-order nonlinear susceptibilityχ(2) was originally suggested by Armstrong and the coworker in 1962. Since then, QPM became an issue of material engineering known as periodically poled crystals, which was successfully fabricated in one-dimensional (1D) lithium niobate crystal by electric poling method by M.Yamada in 1993. While, recent year's studies show that the laser-assisted illumination can decrease the poling electric field of lithium niobate crystal which can make the poling process much easier and also can get much thicker samples, therefore the possibility of light-assisted domain engineering opens up a promising method for domain structuring.1D periodic structure is limited in simultaneous multi-frequency conversion. The conception of nonlinear photonic crystal was proposed by V.Berger in 1998 by employing a two-dimensional (2D) periodic modulation ofχ(2) in plane, known as 2D QPM, which can simultaneously phase match multi-second-order frequency conversion process. Nonlinear photonic crystal is a promising structure in nonlinear frequency conversion, while based on the modulation style ofχ(2), quasi-periodic and even disordered photonic structures can also be achieved, presenting a broad application, such as broadband second-harmonic and cascaded third harmonic generation.As my PhD thesis, the following contents are included in this paper: In the first chapter, firstly, the second-order nonlinearity is shortly discussed followed by a section about the second- and third-harmonic generation process. Then, it gives a short introduction to the theory and engineering method of QPM. In the end, the general idea about the research subjects in my PhD thesis is presented.As the second chapter, the focused-light-induced ferroelectric domain inversion in lithium niobate crystal is deeply investigated. Firstly, the general poling properties and poling process of ferroelectric domains in LN and the corresponding detection method are included. Then, with an emphasis, the origin of light-induced decrease in electric field for domain inversion in LiNbO3 crystals with different dopants are invesigated experiementally and theoritically with focused 514.5nm laser beams. It is found that the light-induced maximum values of electric field for domain reversal are almost the same. Besides, inverted domains always first nucleate on the– c surface and behave very strong stability within the illuminated region. Based on these experimental results and analysis, a qualitative space-electric-filed mode is proposed as a reasonable explanation on the light-induced domain reversal process. In the end, a few methods people using for domain fabrication with the assistance of light-illumination was summarized as a promising way in domain engineering.In the third chapter, the second-harmonic (SH) and third-harmonic (TH) gene- ration process in strontium barium niobate (SBN) crystal with disordered ferroelectric domains is studied. Firstly, the disordered domain structures can provide a continuous reciprocal space to phase match the harmonic generation process, such as SH and TH, resulting in broadband harmonic generation in space and frequency spectrum. Then, based on the two-coupled-beams experiment, by analyzing the intensity distribution of SH in space, the implementation for characterization of short-pulses is demonstrated, such as enabling the estimation of pulse width, pulse sequence and so on; In the end of this chapter, it is stressed that we demonstrated firstly in the experiment that the disorder allows the realizing of broadband TH generation, followed by the analysis of the quasi-phase-matching conditions and polarization properties. The harmonic generation process is studied numerically showing that the harmonic intensity increases with the propagation distance.In the fourth chapter, the nonlinear Cerenkov and Raman-Nath diffraction is studied in quasi- and periodic nonlinear photonic structures. Firstly, we introduce the conception of these two kinds of nonlinear diffraction and the corresponding phase-matching condition followed by an emphase on the analysis of intensity distribution and polarization properties of Cerenkov diffraction. Then, based on the two-beams-coupled experiment, multi-Cerenkov second harmonic rings are firstly obtained in experiment including the observation of the virtual one, followed by the analysis of the corresponding nonlinear process and the intensity distribution. Finally, it is worth stressed that it is the first time the multi-TH rings can also be generated in the experiment. Based on the detailed analysis of the corresponding nonlinear processes and intensity distribution in space, we demonstrated that the TH generation is a second-order cascaded process based on the Cerenkov and Raman-nath emission of SH generation.As the last chapter, we give a summarization of the previous work and the prospect for the further research.