High Efficiency THz-wave Generation from Nonlinear Frequency Mixing

In the past few years, widely tunable terahertz (THz) generation from parametric frequency mixing in nonlinear optical crystals has drawn much attention. The scope of the interest on and application of THz wave includes molecular science, solid-state physics, biomedical applications, diagnostics, and ultra-high speed optical communications, among many others. In this thesis, we generated coherent THz waves from lithium niobate by two novel schemes: the non-collinearly phase-matched THz-wave parametric generation in waveguide-type lithium niobate (LiNbO3) and the collinearly phase-matched THz-wave difference-frequency generation in periodically poled lithium niobate (PPLN). In the non-collinearly phase-matched scheme, we observed a parametric-generation efficiency of 1.61% from 1064 nm to 1071 nm and 162 um in a 0.5 mm thick, 45 mm long z-cut congruent lithium-niobate waveguide with a pump energy of 2.2 mJ and a pump pulse width of 5.8 ns. We also measured an ultra-low-threshold, narrow-line THz-wave parametric oscillator with an intra-cavity grazing-incidence grating and a 1 mm thick, 45 mm long lithium-niobate planar waveguide. When pumped by an actively Q-switched Nd:YAG laser, the threshold energy and intensity of the parametric oscillator were about 2.2 mJ and 70 MW/cm2, respectively. The linewidths of the output THz wave were 12 GHz and 134 GHz with and without the intra-cavity grating, respectively. The energy conversion efficiency, the pump threshold, and the THz linewidth are the highest, lowest, and narrowest among all the reported values for similar devices. In the collinearly phase-matched scheme, we first generated a dual-wavelength 1.5 um laser with a 17 uJ energy (peak power 45 kW) from an optical parametric amplification system as the pump source. We then generated a THz-wave in the wavelength range of 190~210 um and 457~507 um from a forward and backward difference frequency generator (DFG), respectively, by using a 3.2 cm long multi-grating PPLN crystal. The grating period of the PPLN crystal varied from 63 to 70 um in 1 um increments. The extraordinary refractive index of LiNbO3 in the THz-wave range was precisely deduced from the quasi-phase-matching condition of the difference frequency generations. We estimated that about 0.37 pJ and 0.056 pJ energies of the forward and backward THz waves, respectively, were generated in the PPLN crystal. My work is the first demonstration of forward and backward THz-wave generations from collinearly phase-matched difference frequency mixing in PPLN.

關鍵字

兆赫波；非線性光混頻；鈮酸鋰

並列摘要

過去幾年中，以非線性晶體作為增益介質，利用非線性光學光參數方式來產生兆赫波幅射，這種方式吸引很多人的注意。綜觀兆赫波方面的研究與應用，包含：分子科學、固態物理、生物醫學、非破壞性診斷、與超快光通訊等。在這本論文中，利用非線性晶體鈮酸鋰(LiNbO3)，使用兩種基本方式實現了高同調性並且波長可調的兆赫波幅射在此晶體中，第一種方式為：利用非同向性相位匹配兆赫波參數產生與震盪方式在鈮酸鋰(LiNbO3)晶體中；另一種方式為：使用同向性準相位匹配兆赫波光差頻產生方式在週期性極化鈮酸鋰(PPLN)晶體中。在非同向性相位匹配架構中，我們觀察到當鈮酸鋰只有0.5mm厚度時(其長度為45mm、z-cut 鈮酸鋰兆赫波波導)，在泵浦光(其能量為2.2mJ、脈衝寬度為5.8ns)波長為1064nm轉換到1071nm跟162um的光參數產生轉換效率高達1.61%，這是第一次我們由實驗證明有如此高的轉換效率是因為兆赫波在此晶體中受到光波導效應而提高。另一方面，在一個intra-cavity grazing incident grating幫助下，我們同時也觀察到超低閾值、窄頻兆赫波參數震盪器來產生兆赫波，其增益晶體為一片厚度1mm、長度45mm兆赫波光波導的鈮酸鋰晶體中。本架構是由主動式Q開關雷射當泵浦光供給能量來產生光參數轉換，在此震盪器中我們發現閾值(threshold)只需2.2mJ的能量(或是強度70MW/cm2)。更近一步我們發現，在有或者是沒有intra-cavity grazing incident grating幫助下，震盪器中兆赫波的線寬分別為12GHz與134GHz，這表示我們使用grating可以大大降低兆赫波的頻寬，頻寬的降低對光譜學上可以得到很好的應用。藉由此實驗架構跟同類型的實驗架構相比較，我們所產生的兆赫波參數震盪器的閾值(threshold)是最小的，而兆赫波的頻寬是最窄的。在同向性架構中，首先藉由光參數放大器系統中產生雙波長為1.5um附近的泵浦光源，此雙波長雷射光擁有總能量為17uJ (總峰值功率為45KW)，然後利用此雙波長雷射產生光參數雷射差頻正向與反向兆赫波，其波長分別是190~210um與457~507um，此時使用的晶體為3.2cm長的週期性極化酸鋰(PPLN)晶體。此晶體的極化週期在每1m變化下從63um到70um。並且鈮酸鋰晶體在兆赫波段的異常折射率也藉由在準相位匹配條件下，由雷射差頻方式精準的量測出來。並且我們預估分別有0.37pJ與0.056pJ在正向與反向兆赫波能量。在這實驗中，我們第一次在週期性極化酸鋰晶體中利用雷射光差頻產生器實現了三波同向的正向與反向兆赫波。

並列關鍵字

THz-wave generation ； nonlinear frequency mixing ； LiNbO3

參考文獻

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High Efficiency THz-wave Generation from Nonlinear Frequency Mixing

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