Electrodynamics of topological and quantum materials

量子力学发明后不久，像爱因斯坦这样的物理学家开始应用它来理解固体的性质。通常，我们把量子力学与原子或亚原子粒子之类的小东西联系在一起。但是固体的量子理论是20世纪最伟大的智力成就之一，它使晶体管、半导体激光器和高密度磁存储器的发明成为可能，从而改变了社会。然而，尽管我们知道这些知识，研究这些东西的物理学家仍然会遇到各种各样的现象，这些现象对教科书上的理论提出了根本性的挑战。在一些材料中，电子可以通过相互排斥来阻止导电，这导致它们被卡住，就像量子停车场中的汽车一样；在其他情况下，电子自发地自组织排列，导致整个材料的对称性改变；还有一些是电绝缘体，表面包裹着一层薄薄的金属片，由于占用空间的原因，它们被称为“拓扑绝缘体”。所有这些都代表了我们当前理解的前沿，为了强调这种统一性，加拿大研究人员创造了“量子材料”一词来描述它们。量子材料的研究是当代固体研究的一个重要主题。就像之前对固态的研究一样，它既能促进我们对物质的理解，又能扩大我们设计新型电子材料的能力，为未来的技术奠定基础，同时加深我们对自然的欣赏。关于固体，我们能问的最基本的问题之一就是它对光的反应，所以光学在这项研究中起着重要的作用。我们可以看到电绝缘体（如玻璃）和金属（如铝）之间的区别，只需要观察它们。通过将其扩展到可见光以外的波长，我们在实验光谱学方面的研究使我们能够比肉眼看到更多关于固体的东西。我的研究计划的目标是利用现代激光研究量子材料的研究。我们可以使用可见波长的脉冲激光来产生远红外光，也就是太赫兹波，我们可以用它来很容易地区分超导体和金属。利用来自激光的强偏振光，我们可以以极高的灵敏度识别固体的许多基本对称性，这进一步使我们能够探测到如果电子自发地自组织，这些对称性是如何变化的。通过强烈的光爆发，我们还可以改变固体，观察它们的演变，将它们激发出平衡状态，然后描述它们是如何回到原始状态的，或者将它们推到一个完全不同的状态。通过这些技术，我们的目标是更深入地了解量子材料，以便有一天我们可以像研究硅一样使用它们。

Soon after quantum mechanics was invented, physicists like Einstein began to apply it to understand the properties of solids. Normally we associate quantum mechanics with small things, like atoms or subatomic particles. But the quantum theory of solids is one of the great intellectual achievements of the twentieth century, and has transformed society by enabling the invention of the transistor, the semiconductor laser, and high-density magnetic storage. Yet for all our know-how, physicists who study these things still encounter a variety of phenomena that present fundamental challenges to the theories found in textbooks. In some materials, electrons can be prevented from conducting electricity by their mutual repulsion, which causes them to get stuck, like cars in a quantum parking lot; in others, the electrons spontaneously self-organize in arrangements that cause the symmetry of the whole material to change; still others are electrical insulators that are encased by a thin metallic sheet at the surface, which have been dubbed “topological insulators” for reasons that take more than this space allows. All of them represent frontiers of our current understanding, and to emphasize this unity, Canadian researchers coined the term quantum materials to describe them. Research on quantum materials is now a dominant theme in contemporary research on solids. Like earlier research on the solid state, it promises to both advance our understanding of matter and expand our capacity to engineer new electronic materials, laying the foundation for future technology as it deepens our appreciation of nature. One of the most basic questions we can ask about a solid is how it responds to light, so optics plays an important role in this research. We can see the difference between an electrical insulator, such as glass, and a metal, such as aluminum, just by looking at them. By extending this to wavelengths beyond the visible, our research in experimental optical spectroscopy allows us to see more about the solid than we could with our eyes. The goal of my research program is to exploit modern lasers in research on quantum materials research. We can use pulsed lasers at visible wavelengths to generate far-infrared light, known as terahertz waves, which we can use to readily tell the difference between a superconductor and a metal. With intense polarized light from a laser we can identify many of the fundamental symmetries of solids with exquisite sensitivity, which further allows us to detect how these symmetries change if the electrons spontaneously self-organize. And with intense bursts of light we can also change solids and watch them evolve, exciting them out of equilibrium and then characterizing how they return to their original state, or pushing them into an altogether different one. Through these techniques, we aim to understand quantum materials more deeply, so that someday we might put them to use as we have with silicon.