具有週期性光柵結構之可調式液晶兆赫波元件之研究

Abstract

近二十年來，兆赫波科技與應用的發展非常迅速，但應用於兆赫波段的可調式元件仍然缺乏。目前大部分的可調式兆赫波元件有著可調範圍過小及操作溫度過低的缺點。我們實驗室發表過利用液晶製作用在兆赫波段並可在室溫操作的可調式元件。根據這些經驗，我們製作出具有光柵結構之可調式液晶元件並探討這些元件於此波段下的性質與研究。 本實驗中，我們架設一套鈦藍寶石為增益介質的飛秒脈衝雷射以用於激發及偵測兆赫波。此飛秒脈衝雷射的特性為擁有450 mW的光強度，90MHz的重複率，及64飛秒的脈衝寬度。利用此雷射我們架設一套利用光導天線激發與接收的兆赫波時域分析儀，此套儀器可解析兆赫波範圍為0.1-4.0THz。另外，為了探討我們製作元件於兆赫波束方向轉換器的研究，我們利用光纖將兆赫波時域分析儀改良為可移動偵測端，此改良後的裝置可以解析兆赫波範圍為0.1-1.0THz。相較於使用液態氦冷卻之熱輻射偵測儀測量不同位置之兆赫波訊號，改良後的兆赫波時域分析儀具有可解析寬頻訊號與價格便宜等優點。 本實驗用於設計兆赫波元件的材料為石英玻璃、向列型液晶E7、MDA-00-3461，利用兆赫波時域分析儀我們發現這些材料在兆赫波段其吸收小且沒有特殊的共振吸收譜線。且此兩款向列型液晶具有高的雙折射性，非常適用於兆赫波段製作可調式元件。經過量測，石英玻璃的折射係數為: n=1.95, k< 0.01。E7的折射係數為: no = 1.58，ne = 1.71，□ko=0.03，□ke=0.01。以及MDA-00-3461的折射係數為: no = 1.54，ne = 1.72，ko=0.03，ke=0.01。 本實驗利用上述之材料製作具有光柵結構之可調式液晶元件，包含了磁控與電控之液晶相位光柵及兆赫波束方向轉換器。磁控式與電控式之液晶相位光柵可做為可調式之兆赫波分束器。兆赫波束方向轉換器是利用相位延遲器陣列分別調整不同位置的兆赫波相位，使兆赫波具有一梯度的相位差，達到兆赫波轉向的效果。

In the past two decades, terahertz photonics and applications have progressed remarkably. However, tunable components in terahertz range are relatively underdeveloped. The present tunable terahertz devices, however, have limited range of tunability and have to be operated at cryogenic temperatures far below room temperature. To conquer the drawbacks, various tunable terahertz devices operated at room temperature based on nematic liquid crystal have been demonstrated in our group. In this work, based on the experience of fabricating the nematic liquid-crystal-based tunable devices, we demonstrated several tunable devices with grating structure to manipulate THz waves. We constructed a Kerr-lens mode-locking Ti:sappire laser with maximum power 450 mW, 90 MHz repetition rate, and 64 fs in pulse width. A conventional photoconductive antenna based THz-TDS was constructed that can characterize the broadband terahertz signal in the range, 0.1 - 4.0 THz. To investigate the terahertz beam steering, an improved THz-TDS with movable detecting part was constructed with an optical fiber. The improved THz-TDS can characterize the terahertz signal below 1.0 THz. It can detect the terahertz pulse directly and can be more convenient and lower cost than that using the liquid-helium-cooled Si bolometer. We choose the fused silica and nematic liquid crystal E7 and MDA-00-3461 to fabricate our devices. The materials for tunable devices design should have the properties of large birefringence and small absorption. The complex refractive indices of these materials were characterized by the conventional THz-TDS at room temperature 24.5□C. In the THz range, 0.2-2.0 THz, the complex refractive indices of fused silica are n=1.95, k < 0.01. The ordinary and extraordinary indices of E7 are no = 1.58, ne = 1.71, □ko=0.03, and □ke=0.01, and which of MDA-00-3461 are no = 1.54, ne = 1.72, ko=0.03, and □ke=0.01, respectively. In this frequency range, these materials do not show any sharp resonant absorption and clear dispersion. The studies of the optical constants of NLC E7 and MDA-00-3461 in terahertz range show the attractive potential of the applications due to the comparable large birefringence and relative small extinction coefficient. In this work, we demonstrated several liquid-crystal-based tunable devices with grating structure. These devices including magnetically and electrically controlled phase gratings, and the electrically phase shifter array. We designed the phase gratings that can be utilized as tunable beam splitters, and the phase shift array can be utilized as a beam steerer.