畴工程铁电BiFeO$_3$薄膜用于高效无偏置太赫兹发射
Domain-engineered ferroelectric BiFeO$_3$ thin films for efficient bias-free THz emission
AI总结：
本研究通过工程化铁电畴构型，证明多畴BiFeO$_3$薄膜可增强太赫兹发射，并揭示超快屏蔽为主要机制，为无偏置太赫兹源提供设计参数。
AI中文摘要：
超短光脉冲可以驱动固体中的瞬态光电流，这是现代太赫兹（THz）发射器的基础。铁电材料近来在此背景下作为发射源出现，它们既不需要光电导天线的偏置电压，也不需要自旋电子发射器的磁场，然而产生发射的超快光电流机制一直存在争议。在此，我们在具有工程化铁电畴结构（从单畴到周期性条纹图案）的外延BiFeO$_3$薄膜中解决了这一问题。条纹畴薄膜发射的太赫兹场强度是单畴构型的四倍，达到金属自旋电子参考发射器输出的大约一半，同时在我们最高激发通量下仍未饱和。通过解析发射太赫兹场的振幅和相位随泵浦光偏振和晶体取向的变化，我们根据其对称性分离了共存的光电流。这确定了光生载流子对铁电极化的超快屏蔽是面内极化薄膜中的主导发射机制，在多畴薄膜中通过畴壁处的强内建场得到增强，而面外极化则有利于体光伏电流。我们的结果确立了铁电畴构型作为设计参数，同时决定了光驱动太赫兹发射的强度和微观起源。
英文摘要：
Ultrashort light pulses can drive transient photocurrents in solids, the basis of modern terahertz (THz) emitters. Ferroelectrics have recently emerged in this context as sources that require neither the bias voltage of photoconductive antennas nor the magnetic field of spintronic emitters, yet the ultrafast photocurrent generating the emission has remained debated. Here, we resolve this question in epitaxial BiFeO$_3$ thin films with engineered ferroelectric domain configurations, from single-domain to periodic stripe patterns. The stripe-domain film emits a THz field four times stronger than the single-domain configurations, reaching about half the output of a metallic spintronic reference emitter while remaining unsaturated at our highest excitation fluences. Resolving the emitted THz field in amplitude and phase as a function of the pump light polarization and the crystal orientation, we separate the coexisting photocurrents by their symmetry. This identifies the ultrafast screening of the ferroelectric polarization by photocarriers as the dominant emission mechanism in the in-plane-polarized films, enhanced in multidomain films by the strong built-in fields at the domain walls, while an out-of-plane polarization favors the bulk photovoltaic current. Our results establish the ferroelectric domain configuration as a design parameter setting both the strength and the microscopic origin of light-driven THz emission.
