以時間解析光譜研究La2-xSrxCuO4與SnSe之超快動力學

本論文透過控制飛秒雷射光偏振方向的激發-探測實驗量測時間解析光譜。透過偏振方向解析以及時間解析的超快雷射光譜，藉由改變雷射光偏振方向與晶體軸向之夾角，可觀察晶體內部的載子在動態上之各向異性（Anisotropy），並比較於不同溫度下各向異性的行為差異。　　本論文中成功於溫度30 K時觀察到La1.95Sr0.05CuO4薄膜之瞬時反射率變化(ΔR/R)對應雷射光偏振方向有二重對稱的現象，此現象隨溫度上升而減弱。理論上晶體結構的對稱與電子結構的對稱應同時存在，因此在La2-xSrxCuO4薄膜之tetragonal結構下觀察到各向異性是十分特殊，我們將持續檢驗不同厚度以及不同摻雜量的La2-xSrxCuO4薄膜，期望能更加理解高溫超導銅氧化物之行為與機制。　　本論文中也研究了新穎的熱電材料SnSe單晶在不同軸向與不同溫度下的超快動力學。實驗結果發現當探測光偏振方向與b軸或與c軸平行時，瞬時反射率變化(ΔR/R)呈現極大的差異。當探測光偏振方向平行c軸時，ΔR/R中有 45 GHz之震盪項。而當探測光偏振方向轉與b軸平行時，則無 45GHz項之訊號，且弛緩時間約100 ps的項較強。此外兩軸皆有 1THz之震盪項，在不同溫度下的實驗結果能發現1 THz之震盪項隨著溫度增加而減弱，推測此為聲子能量弛緩之表現。我們將對不同能量弛緩時間之訊號進一步分析，期待得以更深入理解SnSe之熱電性質之來源。

關鍵字

超快動力學；鑭鍶銅氧；硒化錫

並列摘要

In this thesis, we use the pump-probe method to measure time-resolved spectroscopy of the La2-xSrxCuO4 thin films and the SnSe single crystal. To understand the anisotropy properties of the sample, we measured the temperature-dependent and polarization-dependent transient reflectivity changes (ΔR/R) by dual-color pump-probe setup, which provide angle-resolved ultrafast dynamics of the La2-xSrxCuO4 thin films and the SnSe single crystal. The transient reflectivity change (ΔR/R) of La1.95Sr0.05CuO4 thin films showed two-fold symmetric at 30 K, The two-fold symmetric was weakened as the temperature increased. In general, if the crystal structure is symmetrical, the electronic structure will also be symmetrical. Therefore, it is very surprising to observe the anisotropy in the La2-xSrxCuO4 thin films with the tetragonal structure. We will measure the La2-xSrxCuO4 thin films with different doping and thickness. We wish this can help provide more information about the mechanism of the high temperature superconducting cuprate . We also measured the temperature-dependent and oriental-dependent pump-probe spectroscopy of the SnSe single crystal. The time-resolved spectra exhibit significant differences between the ΔR/R measured with probe beam polarization along b-axis and c-axis of the single crystal. A 45GHz oscillation term was observed as the polarization iii of the probe beam is along the c-axis. When the polarization of the probe beam is along the b-axis, the 100 ps relaxation term is more intense. In addition, a 1THz oscillation term observed along both b-axis and c-axis, this term decreased as the temperature increased. It’s presumed to be the energy relaxation from phonon. We will further analyze these energy relaxation terms to provide more information of the mechanism of SnSe single crystals in thermoelectric properties.