Electrodynamics of topological and quantum materials

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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.