超快雙色光激發-探測技術用於多鐵材料鋱錳氧化物相變之研究

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研究生：	徐國謙 Hsu, Kuo-Chien
論文名稱：	超快雙色光激發-探測技術用於多鐵材料鋱錳氧化物相變之研究 Study of Multiferroic Phase Transtion of TbMn2O5 with Two Colors Ultrafast Pump-Probe Measurement
指導教授：	齊正中 Chi, Cheng-Chung
口試委員:	牟中瑜洪勝富吳光雄羅志偉齊正中
學位類別：	博士 Doctor
系所名稱：	理學院 - 物理學系 Department of Physics
論文出版年：	2011
畢業學年度：	99
語文別：	英文
論文頁數：	61
中文關鍵詞：	時間解析、激發探測技術、超快雷射、複鐵、鋱錳氧化物
相關次數：	點閱：103 下載：0
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我們用雙色(400nm激發，800nm探測)的時間解析激發探測技術來研究複鐵材料單晶鋱錳氧化物(TbMn2O5)的相變。在只有探測光的情況下，我們量到沿著晶軸b軸和c軸的反射率分別為21.3%和22.5%。
在時間解析的反射訊號中，上升時間(rise time)大約與脈衝寬度相同。接著上升訊號後面，只有沿著b方向的探測光的量測訊號，在降到一個平台的過程中有幾個皮秒(ps)的快速弛裕時間。沿著b軸探測的快速弛裕的速率(relaxation rate)，和沿著b軸激發c軸探測的訊號平台，隨著溫度的趨勢與沿著b軸的自發電極化(spontaneous electric polarization)十分吻合。此外，我們發現訊號的平台高度隨著溫度的趨勢與反鐵磁的相轉變的啟動一致。激發沿著c軸探測沿著b軸和激發沿著b軸探測沿著b軸，隨著溫度的變化並不明顯，但是對應了在低溫的非相稱性的(incommensurate)反鐵磁相變。

We use two colors (400nm pump and 800nm probe) ultrafast pump-probe technique to study multiferroic phase transition of TbMn2O5 single crystal. The reflectivity of the probe beam alone we measured for the laser polarization parallel to the crystalline b and c axes respectively are about Rb=21.3 % and Rc=22.5 %.
The rapid rise of the transient reflectivity is comparable to the pulse duration. Following the initial signal rise, only the “probe b” signals showed a fast signal drop to a plateau within several picoseconds. The fast relaxation rates of (pump c, probe b), (pump b, probe b) and the plateau level (pump b, probe c) correlate well with the temperature dependence of the spontaneous of electric polarization along b-axis of TbMn2O5. Furthermore, the temperature dependence of the plateau of (pump c, probe c) correlates with the onset of the anti-ferromagnetic transition of our sample. The temperature dependence for (pump c, probe b) and (pump b, probe b) are relatively weak but they seem to correlate with the onset of the lower incommensurate anti-ferromagnetic transition.

Chapter 1: Overview of Multiferroics    1
1    Brief history of multiferroics    1
2    Magnetoelectric coupling    3
3    Symmetry properties    5
4    Mutually exclusiveness of ferroelectric and ferromagnetic    6
5    Ferroelectrics    7
6    Spiral spin    10

Chapter 2: Overview of TbMn2O5    16
1 Introduction to TbMn2O5    16
2 The structure and magnetic order of TbMn2O5    16
3 Dielectric and magnetic properties of TbMn2O5    19
4 Incommensurate and commensurate magnetic phase of TbMn2O5    22
5 Optical spectrum and charge transfer of TbMn2O5    25

Chapter 3: Experimental Setup    28
1 Light source    28
2 The spectrum of the light source    30
3 The pulse-width measurement    31
4 Optical cryostat    33
5 Time-resulted spectroscopy    34

Chapter 4: Ultrafast properties of TbMn2O5    37
1 The DC reflectance measurements    38
2 Time-resolved photoreflectance (PR) experiments    39

Chapter 5: Conclusion    56

References    59

                                

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