A theme in condensed matter physics is to characterize and study various elementary excitations. These excitations can be regarded as emergent particles of a (usually complicated) system. This view is particularly valued in the study of topological states of matter in the past twenty years. In this talk, I will introduce our works on classifying emergent particles in crystals and their interesting physics, such as the anomalous shifts in interface scattering and pronounced nonlinear response effects.

In this talk, I will first share some stories about my research experience and career, and how I become an applied physicist on semiconductor technologies. Then I will talk about the working principle of quantum cascade lasers and the reasons why they have become the first choice of electrically pumped semiconductor lasers for Mid-infrared and Terahertz photonics. To conclude the talk, I will introduce a recently developed electrically pumped topological laser based on the quantum cascade laser platform which is immune to defects and sharp corners.