Optical Stark effect of a local defect on the TiO2(110) surface

Probing the optical Stark effect at the single-molecule or atomic scale is crucial for understanding many photoinduced chemical and physical processes on surfaces. Here, we report a study about the optical Stark effect of single atomic defects on a $\mathrm{Ti}{\mathrm{O}}₂(110)$ surface with photoassisted scanning tunneling spectroscopy. When a laser is coupled into the tunneling junction, the midgap state of the $\mathrm{OH}\text{\ensuremath{-}}{\mathrm{O}}₂$ defects changes remarkably in differential conductance spectra. As the laser power gradually increases, the energy of the midgap state shifts away from the Fermi level with an increase in intensity and a broadening of the peak width. The observation can be explained as an optical Stark effect with the Autler-Townes formula. This large optical Stark effect is due to a tip enhancement and a strong dipole moment in the transient charged state during electron tunneling. Our study provides different aspects in exploring electron-photon interactions at the microscopic scale.