Chiral exciton in the topological insulator Bi$_2$Se$_3$*

Materials with novel band structures can host ``chiral excitons'', where the exciton emission preserves the helicity of the excitation photon, as recently demonstrated in transition metal dichalcogenide monolayers~\footnote{H. Zeng, J. Dai, W. Yao, D. Xiao and X. Cui \textit{Nature Nanotech.} \textbf{7} 490} \footnote{K.F. Mak, K. He, J. Shan and T.F. Heinz \textit{Nature Nanotech.} \textbf{7} 494}. Here, we report the observation of a highly polarized photoluminescence peak, which is due to chiral exciton emission in the topological insulator Bi$_2$Se$_3$. Surprisingly, the energy of the emission is centered at 2.26\,eV, much higher than the 0.3\,eV bulk band gap of Bi$_2$Se$_3$. The excitation profile shows maximum polarization around 2.60\,eV excitation, suggesting the chiral exciton is due to interband transition between the topological surface states and a bulk band. We demonstrate that the polarization of the exciton emission is insensitive to temperature and Bi$_2$Se$_3$ film thickness, providing a convenient and robust platform for optoelectronic applications.