Study Of Second-Harmonic Generation Properties In BaTio₃ Nano-arrays

The field of Nonlinear Optics(NLO)has greatly developed in the past half-century with advancement of laser.There are different types of nonlinear optical(NLO)effects such as harmonic generation,nonlinear optical absorption and refraction,ultra-fast process,and optical soliton.Among these effects,second harmonic generation(SHG)is the most investigated nonlinear optical process since its discovery by Franken in the 1960 s.SHG is a second order nonlinear coherent process in which two lower energy photons are up-converted to exactly twice the incident frequency.Based on the generation mechanism of SHG,its signal has the characteristics of stability,tunability,coherence,ultra-fast response and polarization sensitivity and to further advance its in the fields of life sciences and optoelectronics,the study of nanoscale material properties is of particular importance.In this thesis,we have studied the preparation of barium titanate(BTO)nanoarrays and the characteristics of their second harmonics.The main contents are as follows:(1)Periodically aligned BTO nano-triangular arrays were prepared by nanosphere lithography(NSL)and pulse laser deposition(PLD)techniques,with the nano-film prepared by the latter method only.The surface morphology of the BTO nano-arrays were measured by atomic force microscopy(AFM)and scanning electron microscopy(SEM).The results show that the BTO nano-triangle arrays were uniformly distributed and the surface of BTO films was uniform and flat.(2)Optical transmission spectra of BTO nano-arrays and films were performed by UV-visible spectrophotometer.The results show that BTO nano-triangle arrays and films have high transmittance in the UV-visible region and good transmission in the long-wavelength portion.The optical band gap of different size nano-arrays is slightly different.Moreover,the optical band gap value of BTO nano-arrays and film varies with the annealing temperature.(3)The SHG properties of BTO nano-arrays and film were studied by a self-built transmission microscopic confocal setup.The polarization-dependent characteristics of BTO nano-arrays and film were tested and the relationship between the SHG intensity and the polarization state of the pump laser established.The results show that the fitting and the experimental data agrees well.The SHG signal of BTO nano-arrays after annealing treatment is enhanced.By calculating the area normalization coefficient,we compared the SHG signal intensity of the BTO nano-arrays and film under the same excitation conditions,and found that the BTO signal of the BTO nano-arrays was significantly enhanced compared to the BTO film.