以雙色飛秒時間解析光譜研究鉍鐵氧薄膜之超快動力學

Abstract

本篇論文我們已成功地以雙色飛秒時間解析光譜來研究鐵酸鉍(BiFeO3) 薄膜的超快動力學。這個系統可以搭配單調變鎖相放大技術來增加時間解析光譜的訊雜比，或是更進一步地，搭配雙調變鎖相放大技術來抑制激發光的散色光雜訊。在搭配雙調變鎖相放大技術下，此系統可用來量測具時間解析的磁光柯爾效應(Time-Resolved Magnetic-optical Kerr Effect)，結合共焦顯微鏡則可進一步量測具次微米空間解析的時間解析光譜(Time-Resolved Confocal Microscopy)，另外，可藉由調變激發光的光程(頻率小於100 赫茲) 來量測微分形態的時間解析光譜(Differential-Form Femtosecond Spectroscopy)。

本論文中，我們研究的樣品是(001)-和(110)-軸向的鐵酸鉍薄膜與三種不同鐵電方向組成的鐵酸鉍薄膜。利用時間解析光譜，我們可以解析出電子、晶格與自旋子系統之間複雜的交互作用，這是其他量測系統所無法做到的。時間解析光譜是利用一超短脈衝光在一瞬間(<100 飛秒) 將光子能量傳給電子子系統，接著，觀察此能量在不同子系統之間隨時間如何傳遞。因為不同子系統之間能量的傳遞都有其特徵時間，所以可以利用此特徵時間來解析複雜材料的物理特性。又因為能量傳遞的過程會反應在瞬時反射率變化(ΔR/R) 上，藉由分析瞬時反射率隨時間的關係，就能得到不同子系統間交互作用的特徵時間。

我們所使用的激發光能量是3.1 電子伏特(eV)，能將位於價帶(valance band) 的電子激發到導帶(conduction band) 上。而探測光能量是1.55 電子伏特，用來探測鐵離子的low-lying 能階。因為此能階不滿足電偶極激發(dipole-forbidden transition)，而且鐵離子又位於氧八面體的中心，所以結構對稱性的破壞與聲子、自旋的行為會反應在探測光的反射率上。

本論文中，首先我們比較不同厚度鐵酸鉍薄膜的時間解析光譜，估算出應變脈衝波(strain pulse) 沿著[110] 方向傳播的聲速。同時，也可以根據應變脈衝波模型(strain pulse model)，代入當時探測光的折射率，算出應變脈衝波的聲速。接著，從極化光相依時間解析光譜的實驗，我們可以得到應變脈衝波的產生是各相異性的(anisotropic)。此結果與極化光相依的二階非線性效應實驗做比較，可得知在各相異性的部分是由光整流(optcial rectification) 效應所貢獻的。

最後，我們量測(001)-軸向鐵酸鉍薄膜在不同溫度下的時間解析光譜。(001)-軸向鐵酸鉍薄膜的時間解析光譜主要由氧八面體(oxygen octahedral) 的形變(氧化面體關係到鐵酸鉍的鐵磁向量) 和自旋熱化效應所主導(spin thermalization)。我們發現，當鐵酸鉍薄膜處在自旋玻璃相(spin-glass phase) 時，自旋的熱化是透過兩相鄰磁疇的偶合振盪來進行; 而在反鐵磁相(AFM phase) 時，則是透過E-聲子來進行。當溫度高於340 K 時，自旋的熱化過程被從缺陷能階熱激發到導帶的電子所影響，並同時影響了樣品表面的自發性鐵電極化(spontaneous ferroelectric polarization)。氧八面體的回復速率recovering rate) 在反鐵磁相時會隨溫度的遞減而線性的增快；然而，當鐵酸鉍進入自旋玻璃相時，會因為反鐵磁有序程度的減低而變慢。根據這些變溫量測結果，我們已詳細分析鐵酸鉍在溫度TN 以下，各個相變的物理機制。

In this dissertation, we developed a dual-color femtosecond spectroscopy to study the ultrafast dynamics of BiFeO3 (BFO) thin films. The system can combine with single-modulation technique to enhance the signal-to-noise ratio or double modulation technique to further suppress the noise from the pump scattering light. The double-modulation technique can be also applied to time-resolved magnetic-optical Kerr effect, time-resolved confocal microscopy with degenerated pump and probe beams, and differential-form femtosecond spectroscopy. By applying this system to (001)-, and (110)-oriented BFO thin films and the BFO thin film with various ferroelectric domain walls, the complex dynamics between electrons, lattice, and spins can be resolved. The complex dynamics are hard to be resolved by other measuring techniques in such multiferroic materials due to the order parameters are coupled to each other. Because the difference interaction between electrons, lattice, and spins has its characteristic time, femtosecond spectroscopy with time resolution can exactly probe the transferring time of the energy via perturbing the electrons by a pump beam. The transient reflective change ΔR/R reflects the energy transferring processes in materials. By extracting out the relaxation times of ΔR/R, we can study the ultrafast dynamics of order parameters in materials.

The photon energy of the pump beam used in this study is 3.1 eV for exciting electrons from the valance band to the conduction band of BFO. The photon energy of the probe beam used in this study is 1.55 eV for detecting the low-lying energy levels of Fe3+ which are dipoleforbidden transitions. However, due to Fe3+ locates at the center of the oxygen octahedral and the probe beam is sensitive to the symmetry breaking caused from the structure or the spins, 1.55 eV photon can be used to probe the dynamics of the phonons and the spins in BFO.

By thickness-dependent ΔR/R of (110)-oriented BFO thin films, we can evaluate the strain pulse velocity propagating along [110] direction in BFO. According the strain pulse model, we can also calculate the strain pulse velocity of BFO with the refractive index of BFO for the probe beam. The polarization-dependent measurements shows the generation mechanism of the strain pulse is anisotropic. Comparing with the polarization-dependent second harmonic generation, we found the anisotropic part of the strain pulse generation is caused by the optical rectification effect.

We also measured the temperature-dependent ΔR/R of (001)-oriented BFO thin films. The ΔR/R of (001)-oriented BFO is dominated by the dynamics of the oxygen octahedral which is associated with the ferromagnetic moment, and the spin thermalization. The spins thermalize is through the polarization domain vibration (between two neighboring domains) in spin-glass phase and E-phonon in AFM phase. Above 340 K, moreover, the defect states dominate the spin thermalization and also affect the surface electric polarizations. The recovering rate of the oxygen octaheral linearly increase as decreasing temperature in AFM phase, but drops in spin-glass phase due to the degradation of AFM ordering. According to above results, we had revealed the physical origin of the anomalies below TN.