Langasite Crystal Photoelectric Performance And Application Of Research

Since the first report of langasite (La₃Ga₅SiO₁₄, LGS) in 1980s, the study on LGS has been constantly paid attention to by researchers. At first, LGS was mainly studied as laser crystal. After the discovery of zero temperature dependence cut of LGS single crystal, the studies on its piezoelectric performance, bulk acoustic wave and surface acoustic wave performances became the focus of researches. LGS crystal exhibits many advantages for the above application fields, such as large electromechanical coupling factors which are as high as twice to three times of those of quartz (α-SiO₂) while the same temperature properties, lower velocities of bulk acoustic and surface acoustic waves compared with those of quartz, and no phase transitions from room-temperature to the melting point (1470℃). All the advantages make LGS a very attractive subject in piezoelectric field. Like quartz, LGS is also a kind of material with multiple usages. It has proper electro-optic properties, such as stable physical and chemical properties, adjustable half-wave voltage, and medium damage threshold. Therefore, the prospect of LGS applied in optical and electro-optic fields attracts the attention of researchers. The electro-optic Q switch fabricated with LGS was used in medium power laser cavity, and the stable output was obtained. In this dissertation, the optical and electro-optic properties of LGS were studied by different optical techniques, and the photoelectric performance of LGS crystal was systematically described. Also the propagating characteristics of BAW and SAW in LGS were theoretically analyzed which might provide some theoretical guidance for study on its acousto-optic performance. The main content of this dissertation is as follows:Ⅰ.The refractive indices of LGS in visible light region were measured without impairment by the reflection ellipsometry method. The refractive dispersion curve of LGS was fitted to Sellmeier's equation.Ⅱ. According to interfering theory of parallel polarized light, the birefringence of LGS was calculated through recording the interfering spectrum. The result was n_e-n₀≈0.01.The birefringence was so little that the phase-matched optical second harmonic generation for LGS is impossible.IIl.The optical activity of LGS was investigated by the same parallel interferometric method as above. The specific rotation dispersion was fitted to Boltzmann's equation. The optical rotation of LGS is relatively small compared with that of quartz.IV.The electro-optic phenomena were investigated, which was not in common with that of KD2PO4 (DKDP) for the optical activity. The linear electro-optic coefficient yn of LGS sample with a small ratio of length to width was conveniently determined by placing one DKDP electro- optic crystal in the unique beam path to compensate the phase difference induced by LGS. Thereafter, the experiment of linear modulation for the voltage signal in sonic and ultrasonic frequency by LGS crystal was carried out.V.Theoretically analysis on the propagating characteristics of BAW in LGS crystal was finished. The slowness curves of BAW in different planes of LGS were plotted. The propagating characteristics of SAW were also described.