Strong Self-Enhancement of Optical Nonlinearity in a Topological Insulator with Generation of Tamm State

As new states of matter currently attracting broad attention in the optics field, topological insulators (TIs) present topologically-protected conducting surface states surrounding insulating bulk and prominent third-order optical nonlinearities in an ultrawide frequency range. Optical frequency conversion in TI nanofilms plays a crucial role in TI nonlinear optical functionalities, whose enhancement is in demand for high-performance devices. Herein, an optical Tamm state is observed between the Sb₂Te₃ TI nanofilm and 1D photonic crystal (PC) with a self-enhancement behavior of optical nonlinearity in the TI nanofilm. The experiments demonstrate that there exists a distinct reflection dip in the forbidden band of the PC, which is attributed to the appearance of Tamm state in the TI/PC structure. The wavelength of the TI-based Tamm state possesses a nearly linear redshift with increasing TI film thickness. The measurements are in excellent agreement with simulation and theoretical calculations. Moreover, a strong third-harmonic generation reinforcement of 30-fold is experimentally observed in a 90 nm Sb₂Te₃ film, which stems from the field enhancement induced by the TI self-excited Tamm state at the fundamental frequency. This work opens a new door for generation of optical Tamm states and applications of TIs in high-efficiency nonlinear optical functionalities, especially frequency conversion.