The physics of terahertz negative photoconductivity in low-dimensional materials

Enhancement of conductivity is the common photoresponse when incident photons temporarily generate free carries after photoexcitation in a solid-crystalline-material. In sharp contrast, some emerging low dimensional materials such as graphene, transition-metal dichalcogenides, topological insulators, MXenes, and carbon nanotubes possess reduced terahertz-range conductivity after photoexcitation, a phenomenon that has attracted significant interest in the research community in recent years. Negative terahertz photoconductivity reveals a plethora of fascinating ultrafast processes involving photoexcited states and unveil their unique intrinsic characteristics. This review highlights these unconventional responses of charge carriers and discusses the underlying physics for contemporary layered and one-dimensional materials. These understandings reveal extraordinary photophysical properties of materials which are essential for designing high-frequency advanced optoelectronic devices.