Ultrafast Zero-Bias Surface Photocurrent in Germanium Selenide: Promise for Terahertz Devices and Photovoltaics
Ultrafast Zero-Bias Surface Photocurrent in Germanium Selenide: Promise for Terahertz Devices and Photovoltaics
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DOI：
10.1021/acsami.8b17225
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发表时间：
2019-02-06
影响因子：
9.5
通讯作者：
Titova, Lyubov V.
中科院分区：
文献类型：
--
作者：
Kushnir, Kateryna;Qin, Ying;Titova, Lyubov V.
理论预测，一个大的自发电极化和随之而来的反转对称性破缺GeSe单分子膜的结果在一个强大的移位电流响应于它们的激发在可见光范围内。位移电流是在带隙以上光激发时在晶格常数的量级上的电子密度的相干位移。二阶非线性效应，它是禁止在大块GeSe晶体的反转对称性。在这里，我们使用太赫兹（THz）发射光谱来证明，波长跨越可见光谱的两个边缘，400和800 nm的超快光激发，在大块GeSe晶体的表面层中发射移位电流，其中反转对称性被破坏。从所观察到的发射的太赫兹脉冲的极性确定的表面移位电流的方向仅取决于样品的取向，而不是激发的线性偏振方向。大量的GeSe中的低频红外活性声子的强吸收限制了所发射的THz脉冲的带宽和幅度。我们预测，减少锗硒厚度为单层或几层将导致在一个高效的宽带太赫兹发射。通过大块GeSe晶体中的表面位移电流的太赫兹发射的实验演示将这种2D材料作为下一代位移电流光电转换器、非线性光子器件和太赫兹源的候选材料提出。
Theory predicts that a large spontaneous electric polarization and concomitant inversion symmetry breaking in GeSe monolayers result in a strong shift current in response to their excitation in the visible range. Shift current is a coherent displacement of electron density on the order of a lattice constant upon above-bandgap photoexcitation. A second-order nonlinear effect, it is forbidden by the inversion symmetry in the bulk GeSe crystals. Here, we use terahertz (THz) emission spectroscopy to demonstrate that ultrafast photoexcitation with wavelengths straddling both edges of the visible spectrum, 400 and 800 nm, launches a shift current in the surface layer of a bulk GeSe crystal, where the inversion symmetry is broken. The direction of the surface shift current determined from the observed polarity of the emitted THz pulses depends only on the orientation of the sample and not on the linear polarization direction of the excitation. Strong absorption by the low-frequency infrared-active phonons in the bulk of GeSe limits the bandwidth and the amplitude of the emitted THz pulses. We predict that reducing GeSe thickness to a monolayer or a few layers will result in a highly efficient broadband THz emission. Experimental demonstration of THz emission by the surface shift current in bulk GeSe crystals puts this 2D material forward as a candidate for next-generation shift current photovoltaics, nonlinear photonic devices, and THz sources.