Circular Dichroism Observed by Photoemission from Ultrathin Bi$_{2}$Te$_{3}$ Films

Circular dichroism (CD) observed by photoemission from the surface states of topological insulators has drawn much interest. It was initially attributed to the spin polarization or chiral orbital momentum of the initial states, but later proven to also involve the final states. The detailed mechanism remains controversial. To address this question, we have performed measurements of ultrathin films of the prototypical topological insulator Bi$_{2}$Te$_{3}$ over a wide range of film thickness and photon energy. The results show that the CD depends not only on the photon energy, but also on the film thickness in a nontrivial manner. A theoretical model has been developed that involves dipole transition, surface photoemission, and spin-orbit coupling. The computed results are in good agreement with the general trends of the data including sign reversals as a function of photon energy and film thickness. The complex behavior of the measured CD function is partially caused by modifications of both the initial and final states in the thin film geometry.