多鐵材料具有被電場、磁場以及應力調控的特徵，且材料具有記憶效應，意味著我們可以透過外加電場、磁場或是應力來寫入資訊，在非揮發性記憶體等元件中有其相當之應用價值。然而除了透過傳統利用電場、磁場或是應力的方式進行操控，在本研究中，我們更進一步引入了光控的方式來達到操控多鐵材料的鐵電性與磁性等功能特性之目的。而透過光照來操控鐵電材料特性的優勢在於，全過程屬於非接觸模式，不需要接觸樣品表面，在製程上有一定程度的方便性；此外，照光區域大小具有可調性，可由雷射光聚焦程度來達到照光區域大小的改變。在現存的多種多鐵材料中，混相鐵酸鉍因處於型態相邊界上(Morphotropic phase boundary, MPB)而具有較高的介電性、鐵電極化與晶相可調性，故我們選擇混相鐵酸鉍作為光誘發相變的研究材料。本研究中我們深入了解光對於混相鐵酸鉍相變的影響與機制，並分析照光區域結構的改變並提出其動態成長模型並且輔以相場理論計算來驗證，最後我們展示了該如何有規律地控制鐵電域與反鐵磁域的排列，以及如何透過照光的方式來調控一個區域的導電度與壓電系數。

The physical properties of crystals are predominately determined by the group symmetry of the crystal lattice. As a result, a change of crystal lattice would directly result in the corresponding modification of physical properties in crystals. The direct modulation of crystal lattices via external stimuli provides a pathway to control the functionalities of a wide spectrum of materials. In this work, the light stimulus is adopted to tailor the local crystal structure in multiferroic thin films, leading to optical control of the inherent ferroelectricity at room temperature.
Multiferroic BiFeO3 has both ferroelectric and antiferromagnetic properties at room temperature, which serves as a promising candidate for next-generation nanoelectronics. When the BiFeO3 is grown on LaAlO3, it becomes a mixed-phase system which is composed of tetragonal-like (T-like) and rhombohedral-like (R-like) phases. It is worth mentioning that the phase transition energy in such system is low so that we can control the phase by applying an electric field or stress according to the previous researches. In this study, we propose a new method to control these two phases and the resulting morphotropic phase boundaries in BiFeO3 via the illumination of 532 nm laser. AFM (atomic force microscopy) is adapted to modulate the as-grown mixed phase into pure T-like phase as the initial state. Our result shows that the center of laser spot prefers to adapt the T-like phase whereas the edge of laser spot favors the R-like phase. As the power becomes stronger, the domain wall of T-like phase shows a 90-degree rotation and while R-like phase grows perpendicular to the T-like phase. We analyzed the difference of light induced structures after the illumination process and proposed the growth model, further verified by phase field simulation. Moreover, we can control the T-like phase by moving the laser spot along different direction. Through the elegant control of stimulation process, not only we offer an efficient way to control the lattice structure of complex materials, but also a pavement towards photonic modulation of multifunctionalities.

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