Observation of chiral surface excitons in a topological insulator Bi2Se3 — Optical orientation of exciton’s angular momentum

Calculated band structure projected onto the top quintuple layer for Jz=1/2. The blue arrow indicates an optical transition with circularly polarized light used in the experiment. (right) The photoluminescence spectra from right and left-handed chiral excitons with Jz=+1 and -1, respectively. The majority of the photoluminescence intensity preserves the helicity of the incident photon due to the chiral nature of the excitons, as shown in the schematic drawing.
Calculated band structure projected onto the top quintuple layer for Jz=1/2. The blue arrow indicates an optical transition with circularly polarized light used in the experiment. (right) The photoluminescence spectra from right and left-handed chiral excitons with Jz=+1 and -1, respectively. The majority of the photoluminescence intensity preserves the helicity of the incident photon due to the chiral nature of the excitons, as shown in the schematic drawing.

We observe composite particles -- chiral excitons -- residing on the surface of a topological insulator (TI), Bi2Se3. Unlike other known excitons composed of massive quasiparticles, chiral excitons are the bound states of surface massless electrons and surface massive holes, both subject to strong spin–orbit coupling which locks their spins and momenta into chiral textures. Due to this unusual feature, chiral excitons emit circularly polarized secondary light (photoluminescence) that conserves the polarization of incident light. This means that the out-of-plane angular momentum of a chiral exciton is preserved against scattering events during thermalization, thus enabling optical orientation of carriers even at room temperature. The discovery of chiral excitons adds to the potential of TIs as a platform for photonics and optoelectronics devices.