| Since nonlinear Cerenkov radiation was introduced into the area of nonlinear optics for the first time in the year of1970, the study of such an effect has attracted more and more attentions of researchers. After years of study, there are mainly two researching systems of nonlinear Cerenkov radiations:optical waveguides and bulk nonlinear photonic crystals, respectively. Our thesis has discussed the novel properties of nonlinear Cerenkov radiations in both the two systems in depth, starting with the fabrications and characterizations of LiNbO3, LiTaO3dielectric superlattice and dielectric waveguide. The main contents of our thesis are as follows:1. The fabrication processes of annealing-proton-exchanged (APE) waveguide and the standardization of its basic technical parameters are discussed, using benzoic acid (pyrophosphoric acid) as the source of proton-exchanging. Also, we set up a more comprehensive waveguide characterization and measurement system, including prism-coupling and end-face-coupling, which are used for charactering the optical properties of our waveguide sample, such as the number of guide mode, the effective depth of waveguide, the distributions of the effective refractive index and so on. Based on the basic researches of the fabrication of the sample, we have investigated nonlinear coupling effect between distinct guided modes of APE multi-mode planar waveguide in detail, and first experimentally realized the mode-coupling Cerenkov sum-frequency generation in nonlinear waveguide, which was induced by the coupling of different modes with the same frequency but different propagation constants. At the same time, it was demonstrated that the elastic scattering light of each guide mode could be involved in such nonlinear interaction as well.2. Based on the theory of micro-structure design and qui-phase-matching (QPM), we realized Cerenkov third harmonic generation in the experiment via two cascaded second-order nonlinear processes for the first time, using the APE nonlinear photonic crystal waveguide. Also, we systematically discussed the characterization of the generated Cerenkov third harmonic using such processes in theory. In a periodic-poled LiTaO3waveguide, the procedure of Cerenkov third harmonic generation consists of two processes:guided-to-guided waveguide second harmonic generation cascaded with guided-to-radiated sum-frequency generation. Compared with the method of directly using the third-order nonlinear coefficient to generate Cerenkov third harmonic, such cascaded phase-matching condition has much higher nonlinear efficiency. Our experiment demonstrated that, such a nonlinear process can be modulated by the available QPM reciprocal vectors involved, including both the forward ones and the backward ones, accordingly leading to various Cerenkov third harmonic angles. Also, the temperature-tuning and power-tuning of such interaction were studied in detail. In addition, using coupled mode equations, we numerically discussed the characterizations of the generated Cerenkov third harmonic via such processes, including the temperature-tuning with different pump power density, the variations of mode-overlapping and radiation angles with temperature, the intensity of Cerenkov third harmonic dependence on interaction length and so on. These theoretical results were in substantial agreement with our experimental measurements.3. The study of Cerenkov up-conversion in two-dimensional nonlinear photonic crystal. Based on the background of nonlinear Cerenkov radiation researches in the system of bulk nonlinear photonic crystals, we develop the research system from one-dimension to two-dimension. In two-dimensional photonic crystals, there are not only radiation sources, but also the modulation to radiation sources by the micro-structures in the direction of the propagation of nonlinear polarization wave. By using of two-dimensional rectangular periodic-poled LiTaO3crystal in our experiment, the modulated Cerenkov up-conversions can be achieved under fs-laser pump condition. It was found that, the modulation of second-order nonlinear coefficient was introduced only in the direction of the propagation of nonlinear polarization wave, and then Cerenkov radiations which depended on the domain walls were modulated. And Cerenkov radiation intensity changed with the pump beam polarizations. At the same time, we found that, in the condition of forward reciprocal vectors involved, the velocity of nonlinear polarization wave could be decreased down to the threshold of generating Cerenkov up-conversions, which resulted in the appearance of cutoff-wavelength in Cerenkov up-conversion processes. Using such a method, we can further achieve tunable Cerenkov radiations via the electro-optic effect or acousto-optic effect.4. The study of Cerenkov down-conversion in a two-dimensional nonlinear photonic crystal. The prerequisite of realizing Cerenkov radiations is that, the phase velocity of nonlinear polarization wave is larger than that of the harmonics. We demonstrated that, in normal dispersion crystals, the phase velocity condition of achieving Cerenkov up-conversion is automatically satisfied, while for Cerenkov down-conversions, there is a phase-velocity threshold. Our earlier researches demonstrated that, the phase velocity of nonlinear polarization wave could be accelerated by the reciprocal vectors provided by domain structures of superlattice. We go on with this idea, and in the experiment, using two-dimensional rectangular periodic-poled LiTaO3crystal, we designed a small domain-inverted period in the direction of propagation of nonlinear polarization wave. The QPM structure provided a large backward reciprocal vector, thus the phase velocity of nonlinear polarization wave could be greatly accelerated to be larger than that of the harmonics. So the phase velocity threshold of generating Cerenkov down-conversions was broken, and we achieved the Cerenkov difference-frequency radiation for the first time in this normal dispersion medium. In addition, we realized optical parametric generation process in waveguide using an APE LiTaO3waveguide, which made a good groundwork for our further investigation of other Cerenkov down-conversion processes. |
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