Observation of a Warped Helical Spin Texture in Bi2Se3 from Circular Dichroism Angle-Resolved Photoemission Spectroscopy

A differential coupling of topological surface states to left- versus right-circularly polarized light is the basis of many optospintronics applications of topological insulators. Here we report direct evidence of circular dichroism from the surface states of Bi[subscript 2]Se[subscript 3] using laser-based time-of-flight angle-resolved photoemission spectroscopy. By employing a novel sample rotational analysis, we resolve unusual modulations in the circular dichroism photoemission pattern as a function of both energy and momentum, which perfectly mimic the predicted but hitherto unobserved three-dimensional warped spin texture of the surface states. By developing a microscopic theory of photoemission from topological surface states, we show that this correlation is a natural consequence of spin-orbit coupling. These results suggest that our technique may be a powerful probe of the spin texture of spin-orbit coupled materials in general.United States. Dept. of Energy (Grant number DE-FG02-08ER46521)Defense University Research Instrumentation Program (U.S.) (Grant number W911NF-09-1-0170)National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Grant number DMR - 0819762