Z-掃瞄量測硒化鎵晶體之非線性效應

Abstract

利用飛秒超短脈衝鈦藍寶石雷射（波長820nm，脈衝寬度70飛秒）從事變化強度Z-掃瞄量測技術研究利用垂直布式長晶法所長的ε相硒化鎵半導體晶體，我們發現此樣品除了量測出非線性雙光子吸收係數β＝13.28±0.2cm/GW，並且測出非線性折射率之變化現象現象，所得出的三階非線性折射係數γ=-3.779x10-13cm2/W，以及五階非線性折射係數σr=1.25x10-18cm3。並且改變雷射入射光與晶體的光軸之相對角度θ，我們可以證實在θ>5°（θ≒5°為相位匹配角）負的非線性折射率是因為串連的二階效應所造成的。此外等效的三階非線性係數在θ< 5°是可以確定的也是因為串連的二階效應所引起的引起的。

By using a femtosecond Ti:sapphire laser (wavelength = 820nm) as z-scan light source, we have studied intensity dependent nonlinear refractive index of the ε-GaSe single crystal grown by Vertical Bridgman growth method . We not only measure the two photon absorption coefficient to be 13.28±0.2 cm/GW, but also the third order nonlinear refraction index coefficient γ= ﹣3.779x10-13cm2/W and the fifth order nonlinear refraction indexσ=1.25x10-18cm3. By rotating the incident angle of the crystal axis with respect to the optical axis of the laser beam, we verify the negative refraction index atθ﹥5° by cascadeχ(2):χ(2) process .Furthermore the effective χ(3) becomes positive for θ< 5°