Calculation of divergent photon absorption in ultrathin films of a topological insulator

We perform linear and nonlinear photon absorption calculations in ultrathin films of a topological insulator on a substrate. Due to the unique band structure of the coupled topological surface states, novel features are observed for suitable photon frequencies, including a divergent edge singularity in the one-photon absorption process and a significant enhancement in the two-photon absorption process. The resonant frequencies can be controlled by tuning the energy difference and coupling the top and bottom surface states. Such unique linear and nonlinear optical properties make ultrathin films of topological insulators promising material building blocks for tunable high-efficiency nanophotonic devices.