鹵化物非線性光學晶體合成與光學特性探討

Abstract

本論文係採用化學合成法來成長光學非線性AGeX3 (A = Rb, Cs; X = Cl, Br)晶體，並藉由電子微探儀(EPMA)量測晶體成分比例、微差掃描熱卡計(DSC)量測晶體相變溫度、粉末X光繞射儀(PXRD)量測晶體結構、拉曼散射光譜儀(Raman)量測晶體振動型態、傅立葉紅外線光譜儀(FTIR)量測晶體可穿透範圍、紫外光可見光光譜儀(UV-visible)量測晶體能隙、光激發螢光光譜儀(PL)量測晶體發光特性、粉末二倍頻(PSHG)量測晶體非線性係數、橢圓儀(ellipsometry)量測晶體折射係數。由成分分析結果顯示，陰陽離子置換比例如同預期；且熱分析結果顯示，晶體的居禮轉換溫度及熔點皆會隨著溴原子含量的增加而提高，顯示所合成的AGeX3晶體皆以固溶體型態存在。從X光繞射結果得知，CsGe(BrxCl1−x)3晶體結構會隨著溴原子含量的增加而歪斜扭曲；但(RbzCs1−z)GeBr3晶體結構則是隨著銣原子含量的增加而朝正方對稱結構變化。由拉曼散射光譜儀分析顯示，晶體的拉曼訊號會隨著溴原子含量的增加而有紅位移的現象產生。而光激發螢光光譜分析顯示，晶體的光激發光訊號會隨著晶體使用環境溫度的降低而有紅位移的現象產生。紫外光可見光光譜儀分析顯示，陰離子取代的比例能調變晶體能隙值的大小。而傅立葉紅外線光譜儀分析顯示，晶體的穿透範圍會隨著溴原子含量的增加而擴大。由粉末二倍頻檢測結果得知，晶體的二階非線性係數會隨著溴原子含量的增加而變大，但會隨著銣原子含量的增加而變小。

In this thesis, we grow the NLO crystals by chemical synthesis. Most studies have examined the structural, linear optical and nonlinear optical properties of the AGeX3 (A = Rb, Cs; X = Cl, Br) crystals by varying the alloy composition to satisfy the demands of specific applications. In the analysis of EPMA, bromine replaced chlorine in CsGeCl3, and vice versa in CsGeBr3 while cesium was substituted by rubidium in CsGeBr3. According to DSC analysis, the Curie temperature and the melting temperature of the crystals rose with Br content. The XRD analysis indicated that the structural distortion of CsGe(BrxCl1−x)3 (R3m) increases with Br content, while the structure of (RbzCs1−z)GeBr3 slowly becomes centro-symmetric as Rb content increases. The results of Raman spectroscopy agree with the expectation based on effective-mass that the oscillation frequency increases as the Br content falls because Br is heavier than Cl. The atomic vibration modes of AGeX3 (A = Rb, Cs; X = Cl, Br) system were also defined herein. Regarding transparency characteristics, the longest infrared transparency wavelength is typically limited by the phonon absorption of the crystal and the absorption edge is limited by the energy band-gap of the crystal. UV-visible spectra show that the absorption edge declines from 3.43 to 2.38 eV with the composition of bromine (x = 0 to 1), but remains constant for z = 0 to 3/4. The infrared phonon absorption edge of CsGe(BrxCl1−x)3 with x = 0 to 1 is approximately from 30 to 47 μm. Hence, the transmission range of the crystals increases with Br. Furthermore, the force constant increases as the Br content declines, such that the oscillation frequency increases as Br content decreases. The photoluminence spectra revealed that the emission bands of CsGe(BrxCl1-x)3 and (RbzCs1-z)GeBr3 were red-shifted as the temperature fell, because cooling reduced the lattice constant. Detection of the generated second-harmonic of the powder demonstrates that all of the crystals, CsGe(BrxCl1-x)3 (x = 0, 1/6, 1/4, 2/6, 3/6, 4/6, 3/4, 5/6, 1) and (RbzCs1-z)GeBr3 (z = 1/4, 2/4, 3/4), were phase-matchable. The structural distortion and the off-center Ge ion in the unit cell govern the SHG responses. The XRD results that the lattice constant increased with Br content while the cell angle decreased as Br increased. Therefore, the structural distortion of CsGe(BrxCl1−x)3 increases with Br content and the position of the B-site cation, Ge, becomes closer to the cell corner. However, (RbzCs1−z)GeBr3 yields opposing results as the Rb content is increased. Thus, the nonlinearity properties increased with Br content, but fell as Rb content increased. This result is identical to that for PSHG, for which that second-order NLO susceptibility increased with Br content, but declined as Rb content increased.