硒化鎵晶體中兆赫波光參數放大之研究

Abstract

以兆赫波時域光譜技術測量純的及□鉺的硒化鎵晶體在兆赫波段的光學性質，我們發現二者的折射率在0.2-2.5THz波段約為3.2，純硒化鎵晶體的吸收係數平均為5(cm-1)， □鉺的硒化鎵晶體的吸收係數平均為55(cm-1)，二者在0.589 THz處都有一聲子吸收□。利用Lorentz-Drude model計算其導電率並以理論擬合可得知此二晶體的弛緩率及平均碰撞時間，我們進一步推算其載子遷移率分別為81(cm^2/Vs) 及39(cm^2/Vs)。

我們也架設了一套由高功率飛秒雷射聚焦游離空氣產生電漿，以空氣的三階非線性係數滿足四波混頻的兆赫波產生源。改變雷射基頻(800 nm)及二倍頻(400 nm)間的相位差、偏振方向夾角及強度，量測其產生的兆赫波的特性。並利用此光源來作硒化鎵晶體中兆赫波光參數放大的研究，初步結果顯示在1THz此光參數放大器有150% 的增益。

Optical constants of pure and 0.2% Er:GaSe in 0.2 – 1.2 THz region are determined by THz-TDS. The refractive indexes for both crystals are 3.2 and absorption coefficients are 5(cm-1) and 55 (cm-1), respectively. A phonon vibration is observed at 0.589 THz for both crystals. By use of Lorentz – Drude model, the conductivity can be further calculated from experimental measurement. The parameters such as relaxation rate and momentum relaxation time are also derived. The mobility u=81(cm^2/Vs)for pure GaSe and u=39(cm^2/Vs) for Er:GaSe are also proposed. Femtosecond Laser induced plasma based on the third order nonlinearity is successfully utilized to construct the THz-TDS. The properties of the THz radiation from this configuration is characterized by altering the phase difference, the angle of polarization and intensity between fundamental beam (800nm) and second harmonic beam (400nm). Terahertz enhancement/amplification is preliminarily performed in our studies. The gain could be as high as 150% under the phase matching condition around 1THz.