Nonlinear Optical Effect of LiNbO3 under External dc Electric Field

• Main Authors: Liu, Shyan-Der, 劉劉賢德
• Other Authors: A.Y.Wu
• Format: Others
• Language: zh-TW
• Online Access: http://ndltd.ncl.edu.tw/handle/73815134212754411067

碩士 === 國立海洋大學 === 電機工程學系 === 85 === LiNbO3為非中心對稱離子晶體,具有優異的非線性光學效應,在強雷射光場 的作用下,產生各種非線性光學效應.倘若在晶體中再加一直流電場,則高 階非線性光學 X^(3)會出現, 這對整體的非線性光學效應有增強或減弱的 效果.在這篇論文中 ,我們利用一直流電場,分別加在LiNbO3晶體的Z方向 及Y方向,並以波長為1064nm 的Nd:YAG雷射為光源,測量其外加直流電場和 倍頻光強度的關係.外加電場的介入,改變了LiNbO3晶體內Nb-O鍵結上的電 荷位移量,使得整體的非線性極化量增加,倍頻光強因而增強. 由實驗結果 顯示,在Z 方向加電場時,所得的效應比在Y方向加電場時來得明顯,前者為 二次曲線關係, 後者則為一次直線關係.另外,經由不同電場下所測得的 Maker fringes, 可得當電壓為1540V/mm時,計算相對應的二階非線性光學 係數X^(2) (或d33) 增加了15.8% . 這證實了外加電場可以提高非線性光 學係數,是提高轉換效率的一種有效方法. Lithium niobate(LiNbO3) is a noncentral-symmetric ionic crystal, whichpossesses excellent nonlinear optical properties and may be used forgenerating many nonlinear optical effects when irradiated by high power laser. If we apply an external dc electric field upon the materialunder the irradiation of laser, X^(3),the higher order nonlinear opticalcoefficient will appear, which may enhance or reduce the total nonlinearoptical effects. In this thesis, we apply an dc electric field to LiNbO3 crystal alongthe Z and Y directions, and choose a Nd:YAG laser with wavelength1064nm as the fundamental light source and measure and obtain the relationship between the dc electric field and the intensity of the second harmonic generation.In LiNbO3 , the displacement of bond charge will be changed by theapplication of an external dc electric field , which enhances thetotal nonlinear polarization, and consequently,increases the intensityof SHG. According to our experimental results, the effect of theapplication of an electric field along the Z- direction is strongerthan the effect along the Y-direction . There is a quadradicrelationship between the electric field and the intensity of theSHG in the former case , and a linear relationship in the later case. Besides, according to the measured Makers fringes under differentelectric field, when the electric field is 1540V/mm, the correspondingsecond order nonlinear coefficient,X^(2), increases 15.8%. This confirms the idea that the total effective nonlinear optical coefficientcan be increased by applying an electric field, an effective method toincrease the conversion efficiency.