标题: | 硒化镓晶体中远红外波段的光学性质与应用 A study of optical properties and application of GaSe crystal in the mid- and far-infrared |
作者: | 陈晋玮 Ching-Wei Chen 潘犀灵 Ci-Ling Pan 光电工程学系 |
关键字: | 硒化镓;中远红外光源;兆赫辐射;光学性质;差频;光整流;光参数放大;GaSe;NIR , FIR;THz;Optical properties;DFG;Optical rectification;OPA |
公开日期: | 2007 |
摘要: | 在本论文中,主要重点在探讨由实验室自制的高品质硒化镓单晶于中远红外光波段范围的光学性质与应用。利用硒化镓晶体其高非线性的特质与低吸收系数的特性来结合非线性光学的过程来产生同调的光源。首先,利用傅利叶变换红外光谱仪(FTIR)与兆赫时域光谱仪(THz-TDS)来研究硒化镓晶体于此宽波段的光学性质。并针对实验数据的拟合提出硒化镓晶体于此波段的常态与非常态介电函数修正方程式。对于晶体在高吸收区域,也以实验检视出横向与纵向声子振动模态分别为6.39 and 7.62 THz。此外,于兆赫波段的一个位于0.586 THz的低频声子模态,可进一步确认硒化镓晶体为ε型式的晶相。我们也针对Sellmeier方程式提出了修正的参数,并能够有效地描述晶体的色散特性。在本研究中所提出硒化镓晶体的介电函数修正方程式能利用于兆赫波段实际光学组件的应用设计。 近一步,我们利用此材料良好的性质并结合非线性光学中差频的技术来产生同调的红外光源输出,其可调范围从2.4到28 μm。而237.0 cm-1 和 213.5 cm-1两个红外波段的吸收声子模态也和光色散性质相互关联。而Sellmeier方程式常态与非常态折射率的两个吸收峰的波长分别为42.2 □m和46.8 □m。其中,常态色散的吸收峰符合于红外声子模态的E’对称,并反应出硒化镓晶体内镓原子与硒原子于键结平面上的交互振荡。另一方面,Sellmeier方程式非常态色散的吸收峰符合于红外声子模态的A2”对称,并反应出硒化镓晶体内镓原子与硒原子于光轴上垂直方向的交互振荡。另外,我们并完成了利用硒化镓晶体产生同调红外光源时,光学吸收特性对输出特性所造成的影响。输出范围从红外光至兆赫辐射波段的同调光源,其输出功率的变化和此非线性差频过程的增益相关,并且和硒化镓晶体本身的吸收系数也有关联。而吸收系数对差频过程所造成的影响,进一步可利用在硒化镓晶体内微量掺杂铒原子来做部分地补偿。 接着,我们在实验与理论上提出利用多级的光整流技术于硒化镓晶体中产生同调的兆赫辐射光源,利用精确地调控两级中激发光源的时间延迟,可将来自硒化镓晶体中产生的第二级兆赫辐射,同调叠加于第一级兆赫辐射光场。此两级之间的高同调特性证实了光整流的同调过程,并可应用于兆赫辐射的光谱调控技术。此多级的光整流技术不但可以克服晶体长度与群速度色散的限制,此技术亦有发展高功率兆赫辐射光源输出的潜力。并在此研究中进一步讨论双光子吸收所产生的自由载子对兆赫辐射输出的影响,并定量计算出兆赫辐射于硒化镓晶体中的非线性吸收截面系数σTHz,其估计范围为(1.3-5.9)×10-17 cm2。 我们也架设了一套由高功率飞秒雷射聚焦游離空气产生电浆,以空气的三阶非线性系數满足四波混频的兆赫辐射产生源。改变雷射基频及二倍频间的相位差、偏振方向夹角以及量测进入BBO晶体之前激发光源与产生兆赫辐射强度之间的功率相依关系,来量测以此方法所产生兆赫辐射的特性。此外,硒化镓晶体为产生高功率兆赫辐射的良好非线性介质,并利用来做兆赫辐射光參數放大的研究。本研究中,实验上证实了兆赫辐射的放大现象,初步结果显示中心频率于1 THz的兆赫辐射经过此光參數放大器后有2.7倍的功率增益。此技术提供了一个方法来提升兆赫辐射的电场强度以利用于未来兆赫辐射非线性光谱学的应用。 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. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#GT009024815 http://hdl.handle.net/11536/37958 |
显示于类别: | Thesis |
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