硒化鎵晶體摻鉺特性及其應用在產生中紅外光源之研究

Abstract

我們利用非等化學計量比的方法與改變石英甘鍋底部的形狀，成功成長了結晶性佳且大尺寸的硒化鎵單晶。此外，本文對於鉺元素摻雜於硒化鎵晶體的光學及電學性質也作了深入的探討，這是由於鉺元素所輻射出的1.54□m的波長，可作為光纖通訊中重要的光源。首先，利用改變光激發光功率及改變溫度的螢光光譜量測與室溫下紅外穿透光譜量測來檢測摻鉺硒化鎵的光學特性。從光學特性的分析中，鉺摻雜於硒化鎵晶體中除了會在高於價帶64meV處形成雜質能階外，也觀測到鉺元素本身所輻射的特徵光譜。在電性量測的實驗中，利用了變溫的霍爾效應實驗以及深層能階暫態光譜實驗來檢測摻鉺硒化鎵的電學特性。從分析中可得知，除了可以觀察到64meV淺的雜質能階外，還可以觀測到更深的158meV深層能階，我們也推測此深層能階扮演著硒化鎵晶體與鉺元素之間能量轉換的關鍵角色。除此之外，對於鉺元素在硒化鎵晶體中的取代機制以及電子在摻鉺後的硒化鎵晶體傳輸所受到的散色機制，也有更進一步的分析討論。 對於利用硒化鎵晶體應用在非線性光學之差頻效應來產生紅外光源的實驗，也是本文的另一個研究課題。我們分別利用兩種不同的雷射架構，成功的產生了奈秒及皮秒級脈衝式連續可調且具同調性的紅外光源，其所產生的光波範圍分別為4.35-14.25um及2.4-30um。不同的脈衝時脈寬度的光源，會有其不同應用的領域。對於兩系統中所產生紅外光源的轉換效率，本文中也有深入的討論與分析。除此之外，利用二次諧波轉換效率的量測，可以定量獲得摻鉺硒化鎵晶體的二階非線性光學係數大於純的硒化鎵晶體的結果。推測二階非線性光學係數產生變化的原因，是由於鉺元素在硒化鎵晶體單層中以及層與層間發生取代行為，進而改變了原本的對稱性結構所致。

In this dissertation, the centimeter size GaSe single crystals were grown by means of the non-stoichiometric mixture and quartz tube with a capillary. Good crystal quality was examined by X-ray rocking cure and EPMA measurements. The major studies are taken concern of the characteristics of Er-doped GaSe, because of an intra-4f transition at a wavelength of 1.54um, which is important as a light source in optical communication technology. The optical properties of Er-doped GaSe crystals have been investigated by using temperature and power dependent photoluminescence, and Fourier-transform infrared spectrometer measurements. The new emission bands resulting from doping erbium and their transition mechanism were discussed. Additionally, the electrical characteristics and Er-related luminescence of Er-doped GaSe samples were also studied by means of Hall, DLTS, and IR photoluminescence measurements. The two impurity levels that act as radiative and nonradiative centers, respectively, were evaluated by DLTS and the temperature dependence of hole concentration measurements. Meanwhile, the possible origin of these impurity levels could be associated with the substitution of two Ga2+ atoms by one Er3+ atom or are related to a Ga vacancy via the interstratifying of an Er3+ ion at the interlayer site. The temperature-dependence of hole mobility was also analyzed by scattering from homopolar optical phonons and ionized impurities. Besides, an optical parametric oscillator (OPO) based on periodically poled lithium niobate pumped by a Nd:YAG laser was demonstrated. Combined signal and idler pulse output from OPO with a maximum energy of 2.7 mJ, which corresponds to a slope efficiency of 25% have been achieved. The tuning range is 1.71 to 1.98 um for the signal wave and 2.81 to 2.30 um for the idler wave. The signal and the idler waves are tuned and difference-frequency mixed in a GaSe crystal to produce tunable mid-IR from 4.35 to 14.25 um. The DFG efficiency was less than the calculated value, mainly due to high reflection loss, poor surface quality, and OPO beam divergence. Additionally, we also demonstrated the infrared light source that provides picosecond pulses on microjoule energy level, widely tunable in the 2.4-30 um wavelength range with pulse durations ~5 ps. The energies of several microjoule are obtained with a maximum of ~5 uJ at wavelength of 3.5 um, corresponding to photon conversion efficiency of 8 %. The pump source is Nd:YAG laser at a wavelength of 1.064 um and signal source is a parametric device based on a 10 Hz Nd:YAG amplifier system. This picosecond light source was applied to evaluate the nonlinear coefficient (deff) of crystals GaSe doped with erbium based on second harmonic generation. The variation in the deff values between undoped and erbium doped GaSe could be probably ascribed to the effect of erbium doping, which resulted in substitution or interstitial of Er atom in GaSe unit cell.