利用高品質超短脈衝及超寬頻高靈敏度偵測器研究微結構之超快變化

Abstract

由小林孝嘉教授構思研發的非共線光參數放大器(NOPA)光學系統，已在交大先進雷射 研究中心(ALRC)完成設置，目前這個NOPA 系統可產生脈衝寬度在10 飛秒以下，波長 範圍在500-700 nm(可見光-近紅外光，VIS-NIR )之間的超短脈衝雷射，搭配多頻道的 鎖相放大器及光偵測系統，可同步量測128 頻道的瞬時訊號。 在本計劃中，我們擬將把NOPA 產生的脈衝雷射波長延伸至紫外光範圍(UV, 350-450 nm)，我們擬採用三種方法，即利用可見光波段的NOPA 脈衝的諧波產生，或利用鈦藍 寶石雷射產生的四波混成，或利用以氬氣填充中空的光纖，來得到UV 脈衝。所產生的 寬頻UV 脈衝，再經過菱鏡對壓縮至10 飛秒以下。我們同時擬建立UV 光的多頻道偵 測系統，並將擴增光偵測系統至256 頻道，以便增加頻譜的解析能力。 結合上述的高品質NUV-VIS-NIR 超短雷射脈衝(即實驗穩定、頻譜穩定及平坦)與256 頻道的高頻譜解析力的偵測系統，我們將建立一套在NUV-VIS-NIR(1.65 eV to 3.4 eV) 的波段中，具有10 飛秒的時間解析力和1 nm 的光譜解析力的時間解析光譜量測系 統。激發探測實驗系統可以是UV 激發/UV 探測，或是UV 激發/可見光探測，或是可 見光激發/UV 探測等不同組合。 上述強大功能的雷射和偵測系統，將被應用於化學與生物科學中的”能階躍遷光譜”以 及”即時分子振動光譜”的量測。當光譜延伸至UV 波段時，我們將可以觀測不同分子(包 含簡單分子或如聚合物之複雜分子)的動力學行為及能態結構。許多激發態分子結構的 重要資訊將可獲取，這是瞭解分子在每一個激發態反應機制的關鍵資訊，並可進一步 利用Woodward-Hoffman rule 來做驗證。 輸出脈衝頻寬增加後的NOPA 系統，可以利用相同的三氯甲烷激發氧化方式，來對蛋白 質的反應過程作解析，進一步得到蛋白質摺疊機制的重要訊息。此系統也可以用來探 測以往只能用理論來預測的微結構改變，像是在光－生物反應的過程裡，視紫紅質的 光致順反異構化以及氧基血紅素的光解等等，透過解析來自物質的鍵結在基態以及激 發態的振動訊號來得到結構的改變量，解析度約為20 mÅ。 此外，更可以利用此超高時間解析和超寬頻的光譜技術，來研究凝態物質中的強關聯 電子系統(如超導體、超巨磁阻以及多鐵材料等)的準粒子超快動力行為。我們可以同 時量測在不同波長和溫度的條件下，不同型態的準粒子的弛緩行為，這些結果將提供 有關這些奇特材料的物理機制的關鍵訊息，如相共存、極化子和電荷有序之動力行為、 同調聲子行為、電子-聲子耦合強度、電子能帶結構與能隙對稱等。它提供我們探索這 些材料一個嶄新的技術。

A novel non-collinear optical parametric amplifier (NOPA) system, which was proposed by Kobayashi originally, has been reconstructed in the Advanced Laser Research Center (ALRC) in National Chiao-Tung University (NCTU). This NOPA system can generate sub-10 fs ultrashort laser pulse whose spectrum is broadened in visible-near infrared (VIS-NIR) region of 500-750 nm. Together with the pulse light source, a multi-channel lock-in amplifier to observe signals at 128 channels simultaneously was also developed. In this project we are going to extend the NOPA output to UV spectral range (350-450 nm). Three methods, include the harmonics generation of the visible NOPA, the four-wave mixing of the Ti-sapphire laser, and a hollow core fiber filled with Argon gas, are proposed to generate the UV pulses. The generated broadband UV pulse is going to be compressed by a prism pair. We are also going to construct a broadband with higher spectral resolution 256 channels detector system, which can obtain data simultaneously at every probe channel. By the combination of the high quality (in experimental stability, spectral stability, and smoothness of the spectrum) ultrashort laser pulse in the NUV-VIS-NIR spectral range and the broadband 256 channels detector system, it is possible to perform time-resolved spectroscopy with 10 fs resolution and 1 nm spectral resolution in the NUV- VIS-NIR spectral region from 1.65 eV (750nm) to 3.40 eV (350nm). Difference absorption spectrum was found to be changing in 1 fs. The experiments can be one of the schemes of UV-pump/ UV-probe, UV-pump/visible-probe, and visible-pump/UV-probe. The powerful laser systems and detection systems developed above will be used for “transition spectroscopy” and "real-time vibration spectroscopy" measurements in chemistry and bioscience. By extending the spectral region of output light up to UV region, we can observe the dynamics and potential structures of various molecules including very basic molecules and more complicated systems such as polymers and biopolymers. There is much important information relevant to the molecular structure in the excited states in molecules including singlet state and triplet state can be obtained. These information will provide the key to understand the reactivity of molecules in each excited state, which may de used to understand the reaction mechanism and pathway utilizing theoretical formalism such as Woodward-Hoffman rule. Broadening of the NOPA spectrum enables to study potential landscape of protein by utilizing the same excitation mechanism as used in the oxidation of chloroform. This information is essential to understand the protein folding mechanism. It can also be applied for direct observation of micro structural changes, which have been only predicted theoretically, like initial structuralchange in photobiological processes such as photo-isomerization in rhodopsin and photo-dissociation of oxygen from oxy-hemoglobin. Concerning vibration of basic molecules in electronic ground state and excited state, the experimental result of the real-time spectroscopy determines the structural change of molecules with resolution of about 20mÅ with the help of empirical equation of relation between bond length and frequency. Besides, the optical spectroscopy with ultra high time-resolution and ultra broadband will also be used to study the ultrafast dynamics of quasiparticles in solid state/condensed matter physics, especially the strongly correlated electron system (SCES), which include superconductors, colossal magnetoresistance materials, and multiferroics. The wavelength and temperature dependences of the relaxation behavior of quasiparticles in various levels will be measured simultaneously. These results may reveal the valuable information about the physical mechanisms governing the intriguing properties of these materials, for examples, the coexistence of various phases, polaron and charge ordering dynamics, electron-phonon coupling strength, electronic structure and gap symmetry… etc. The ultrafast dynamics of the SCES materials studied by this powerful equipment may create an entirely new way to explore the intriguing properties of these materials.