硒化鎵晶體中遠紅外波段的光學性質與應用

Abstract

在本論文中，主要重點在探討由實驗室自製的高品質硒化鎵單晶於中遠紅外光波段範圍的光學性質與應用。利用硒化鎵晶體其高非線性的特質與低吸收係數的特性來結合非線性光學的過程來產生同調的光源。首先，利用傅利葉變換紅外光譜儀(FTIR)與兆赫時域光譜儀(THz-TDS)來研究硒化鎵晶體於此寬波段的光學性質。並針對實驗數據的擬合提出硒化鎵晶體於此波段的常態與非常態介電函數修正方程式。對於晶體在高吸收區域，也以實驗檢視出橫向與縱向聲子振動模態分別為6.39 and 7.62 THz。此外，於兆赫波段的一個位於0.586 THz的低頻聲子模態，可進一步確認硒化鎵晶體為ε型式的晶相。我們也針對Sellmeier方程式提出了修正的參數，並能夠有效地描述晶體的色散特性。在本研究中所提出硒化鎵晶體的介電函數修正方程式能利用於兆赫波段實際光學組件的應用設計。 近一步，我們利用此材料良好的性質並結合非線性光學中差頻的技術來產生同調的紅外光源輸出，其可調範圍從2.4到28 μm。而237.0 cm-1 和 213.5 cm-1兩個紅外波段的吸收聲子模態也和光色散性質相互關聯。而Sellmeier方程式常態與非常態折射率的兩個吸收峰的波長分別為42.2 □m和46.8 □m。其中，常態色散的吸收峰符合於紅外聲子模態的E’對稱，並反應出硒化鎵晶體內鎵原子與硒原子於鍵結平面上的交互振盪。另一方面，Sellmeier方程式非常態色散的吸收峰符合於紅外聲子模態的A2”對稱，並反應出硒化鎵晶體內鎵原子與硒原子於光軸上垂直方向的交互振盪。另外，我們並完成了利用硒化鎵晶體產生同調紅外光源時，光學吸收特性對輸出特性所造成的影響。輸出範圍從紅外光至兆赫輻射波段的同調光源，其輸出功率的變化和此非線性差頻過程的增益相關，並且和硒化鎵晶體本身的吸收係數也有關聯。而吸收係數對差頻過程所造成的影響，進一步可利用在硒化鎵晶體內微量摻雜鉺原子來做部分地補償。 接著，我們在實驗與理論上提出利用多級的光整流技術於硒化鎵晶體中產生同調的兆赫輻射光源，利用精確地調控兩級中激發光源的時間延遲，可將來自硒化鎵晶體中產生的第二級兆赫輻射，同調疊加於第一級兆赫輻射光場。此兩級之間的高同調特性證實了光整流的同調過程，並可應用於兆赫輻射的光譜調控技術。此多級的光整流技術不但可以克服晶體長度與群速度色散的限制，此技術亦有發展高功率兆赫輻射光源輸出的潛力。並在此研究中進一步討論雙光子吸收所產生的自由載子對兆赫輻射輸出的影響，並定量計算出兆赫輻射於硒化鎵晶體中的非線性吸收截面係數σTHz，其估計範圍為(1.3－5.9)×10-17 cm2。 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輻射產生源。改變雷射基頻及二倍頻間的相位差、偏振方向夾角以及量測進入BBO晶體之前激發光源與產生兆赫輻射強度之間的功率相依關係，來量測以此方法所產生兆赫輻射的特性。此外，硒化鎵晶體為產生高功率兆赫輻射的良好非線性介質，並利用來做兆赫輻射光參數放大的研究。本研究中，實驗上證實了兆赫輻射的放大現象，初步結果顯示中心頻率於1 THz的兆赫輻射經過此光參數放大器後有2.7倍的功率增益。此技術提供了一個方法來提昇兆赫輻射的電場強度以利用於未來兆赫輻射非線性光譜學的應用。

In this dissertation, the optical properties and applications of high quality, home-made GaSe single crystals are investigated in the mid- to far-infrared ranges. The major part of this study is focused on the coherent light generation by means of the nonlinear optical processes associated with the GaSe crystal, which possesses the promising characteristics including high nonlinearity and low absorption properties. First, the optical constants of a GaSe crystal are measured by the Fourier-transform infrared spectrometer (FTIR) and terahertz time-domain spectroscopy (THz-TDS) in a wide frequency range. Based on experimental data, a modified complex ordinary and extraordinary dielectric function of GaSe is presented. The transverse and longitudinal optical phonons in the reststrahlen band for the ordinary refraction index are experimentally determined to be 6.39 and 7.62 THz, respectively. Besides, a low-frequency rigid-layer phonon mode at 0.586 THz confirms the pure GaSe crystal to be in the ε-phase. Furthermore, the revised parameters of Sellmeier equation, which is expressed in an empirical formula form and that effectively describes the dispersion of this GaSe crystal, is also reported. The proposed dielectric functions of the ε-GaSe crystal in this study are applicable to practical photonic devices at terahertz frequencies. Moreover, we apply this promising material for the generation of coherent infrared radiation widely tunable from 2.4 to 28 μm through difference-frequency generation (DFG). The infrared-active modes of □-GaSe crystal at 237.0 cm-1 and 213.5 cm-1 were found to be responsible for the observed optical dispersion and infrared absorption edge. The poles of the modified Sellmeier equations occur at 42.2 □m for the e-ray and 46.8 □m for the o-ray, respectively. The pole of the o-ray dispersion corresponds to an infrared active mode of E’-symmetry with vibration involving both Ga and Se atoms on the basal plane of GaSe crystal. The pole of the e-ray dispersion corresponds to an infrared active mode of A2”-symmetry with vibration involving both Ga and Se atoms along the optical axis (c-axis). We perform a study of the effect of optical absorption on generation of coherent infrared radiation from mid-IR to THz region from GaSe crystal. The output power variation with wavelength can be properly explained with the spectral shape of parametric gain and absorption coefficient of GaSe. The adverse effect of infrared absorption on DFG process can partially be compensated by doping GaSe crystal with erbium ions. Subsequently, we propose and experimentally demonstrate the generation of single-cycle terahertz radiation with two-stage optical rectification in GaSe crystals. By adjusting the time delay between the pump pulses employed to excite the two stages, the terahertz radiation from the second GaSe crystal can constructively superpose with the seeding terahertz field from the first stage. The high mutual coherence between the two terahertz radiation fields is ensured with the coherent optical rectification process and can be further used to synthesize a desired spectral profile of output coherent THz radiation. The technique is also useful for generating high amplitude single-cycle terahertz pulses, not limited by the pulse walk-off effect from group velocity mismatch in the nonlinear optical crystal used. In addition, free carriers induced nonlinear absorption of THz radiation is also investigated in this study. The absorption cross-section, σTHz, of GaSe at terahertz frequency in the presence of free carriers are estimated in the range of (1.3－5.9)×10-17 cm2. Specially, femtosecond laser induced plasma in ambient air based on the third order nonlinearity is employed to construct a THz-TDS system in this study. The properties of the THz radiation from this configuration are characterized by altering the phase shift, the angle between polarizations of the fundamental and second harmonic beams. The dependence of the THz signal as a function of the fundamental pulse energy before the BBO crystal is also examined. Furthermore, GaSe crystal is a promising nonlinear optical medium to perform the generation of intense THz radiation. Herein, we report the experimental demonstration of terahertz wave amplification in GaSe crystal. Terahertz power amplification factor of about 2.7 times is preliminarily performed under the phase matching condition around 1 THz. The demonstration provides a potential way to further increase the terahertz electric field for nonlinear spectroscopic applications with a desktop femtosecond laser system.