Inverted Dirac-electron population for broadband lasing in a thermally activated p-type topological insulator

Maintaining a population inversion in electron distributions is the first step towards lasing. There is a strong interest in realizing the inversion in a Dirac conical band structure, because broad-band lasing may then be realized owing to the zero-gap nature of the Dirac cone. Here we show that the population inversion can be elongated to >7 ps at 8 K and >10 ps at 300 K on the surface of a p-type topological insulator (Sb0.73Bi0.27)2Te3. Time-and angle-resolved photoemission spectroscopy gives us the direct evidence for the elongated duration of the inversion in the topological surface states. We hereby provide a guideline to prolong the population inversion at finite temperatures. Our study strengthens the route toward the Dirac materials to be a lasing medium.