The Study Of Polarization-involved Nonlinear Optical Effects In Optical Superlattice

On the basis of the multiple quasi-phase-matching (MQPM) theory, we systematically studied the coupling processes of polarization and harmonics. The main content is listed below:1. We studied a special mode of polarization coupling MQPM third harmonic generation (THG), and gained the analytical form of the highest THG conversion efficiency. The findings say that the highest THG conversion efficiency of this system is larger than conventional non-polarization coupling THG and has a novel quasi-phase-matching (QPM) transition effect. This effect first of all, indicates the similarity between the THG behavior and the energy band structure when the incident polarization crosses the critical angle. In addition, the transition effect leads to the discontinuous changes of the third harmonic intensity with the change of the incident polarization through critical angle. Using these findings, we could design the polarization-tuned nonlinear optical switch.2. Combined with the conventional non-polarization coupling THG and polarization coupling THG, we proposed a series of theories about the MQPM THG. Taking THG by frequency manipulation and sum frequency processes as the research objects, we found that there are in total twenty-four types of coupling modes. According to the different ways of utilizing the fundamental polarization, the twenty-four types of coupling modes could be divided into three groups. with each group containing eight different modes characterized by a set of coupling wave equations and common coupling properties. The conventional non-polarization coupling THG mode and the special mode of polarization coupling THG could be classified into their corresponding group, representing the common properties within their own group.3. Using the aforementioned findings, we predicted the coupling characteristics and the dynamic properties of a group of novel THG coupling modes. Although this group of THG could not reach100%conversion efficiency, it has the most advantageous harmonic conversion properties. Meanwhile, a novel structure-free effect exists in this group, which means the highest THG conversion efficiency is a constant ratio under the same polarization angle and has no relation to the domain engineered structures that compensate the phase mismatches of the harmonics. Such a property is immensely beneficial for achieving higher harmonic conversion efficiency. We also found that it is similar to the first order phase transition in some aspect. When the incident polarization crosses the critical angle, a sudden change appears in the harmonics evolution mode, but the highest THG conversion efficiency is changing constantly. The relationship between polarization and conversion efficiency confirmed previous experimental work.4. Utilizing the polarization coupling principle, we designed the program of polarization-tuned three harmonics switching in lithium niobate. With the cascading of two periodic structures, the frequency multiplication and the sum frequency processes are spatially separated. Utilizing the coupling effects of the two processes, it is possible to observe a high-efficiency fundamental wave, second harmonic or third harmonic generation at different incident angles.5. Combined with the fundamental polarization and local QPM method, we proposed a nonlinear polarization-tuned beam splitter. The two focusing points designed by local QPM are from different polarization coupling processes, so the energy of the two focusing points is only related to the corresponding polarization component. The energy distribution of the two points is therefore decided only by the incident polarization. The local QPM method is more useful for the integration of the polarization function.6. We proposed the two-dimensional nonlinear axion by local QPM method. With the axisymmetric domain engineered structure, the generated second harmonics interfere with each other and generate an approximate non-diffracted beam. We additionally proposed the idea of two-dimensional nonlinear lens and extending the nonlinear interaction length with total internal reflection.