拓扑绝缘体表面态的非线性光学研究

In recent years, topological insulators have attacked a huge number of physicists due to their unique electronic structures that result in peculiar physical properties. Meanwhile, they also show exciting potential application in fields such as spintronics and quantum computation, etc. Theorists have predicted that new physics and phenomena may emerge from heterogeneous interfaces formed between topological insulators and other materials. Such interfaces are also crucial in the application of topological insulators. However, commonly used experimental tools, including ARPES and STM, often have difficulties in reaching those buried interfaces. We plan to probe surfaces and interfaces of topological insulators using the Sum-frequency spectroscopy (SFS) technique. Being a second-order nonlinear optical probe, SFS is highly surface specific and sensitive; moreover, it probes wherever photons can reach and is particularly suitable for studying buried interfaces. With SF spectra, we can extract properties (such as polarization and lifetime, etc) of topological surface states. By modulating the system via chemical doping or field excitation, etc, we can study the response of topological surface states in situ under working conditions, which helps to unveil the underlying mechanism. Our study can also benefit the future development of new classes of electronics and spintronics based on topological insulators.

近年来，拓扑绝缘体以其独特的电子结构及相关的奇异特性引起了物理学界的强烈兴趣，并因其在自旋电子学、拓扑量子计算等领域的潜在巨大应用而受到广泛关注。理论预言了拓扑绝缘体与其他材料间形成的异质界面蕴含着丰富的新奇物理现象，这些界面的属性也对实现拓扑绝缘体的应用至关重要。然而,常用的角分辨电子能谱与扫描隧道电子显微镜等技术很难对这类隐藏界面进行探测。本项目计划使用表面光学混频方法研究拓扑绝缘体的表面与界面态。作为二阶非线性光学技术的一种，该方法不但拥有非常高的表面、界面灵敏度，且可探测光子所能到达的任何区域，是研究隐藏界面的强有力手段。通过混频光谱，我们可获取拓扑绝缘体界面、表面电子态的极化、寿命等信息；结合掺杂、外场激励等电子态调控手段，可实现对拓扑绝缘体表面态在工作状态下实时、原位的测量，观察其对调控的响应，探讨其物理机制。我们的研究将也有助于未来拓扑绝缘体器件的研发，有着显著的现实意义。

本项目深入研究了拓扑绝缘体的光学二次谐波响应，发展了针对多层薄膜非线性光学响应的特征矩阵方法，从中获取了源于拓扑绝缘体表面态的信号，并在此基础上探测到体掺杂与表面掺杂导致的不同二次谐波响应，有望帮助我们深入理解不同掺杂对拓扑绝缘体表面态的影响。在此过程中，我们针对红外波段非线性谱学技术开发了可有效提升差频设备输出功率的延迟补偿装置、以及针对强吸收界面的微纳光栅表面等离激元增强方法。此外，我们也利用非线性光学方法对具有重要应用价值的光催化氧化物表面、以及二维材料单晶边界开展了研究，展示了相关技术广阔的应用空间。发表SCI论文7篇，申请国家发明专利1项，受邀在国内外学术会议进行多次报告，培养毕业博士2名、硕士1名。