Optical Gating of Photoluminescence from Color Centers in Hexagonal Boron Nitride.

We report on multicolor excitation experiments with color centers in hexagonal boron nitride at cryogenic temperatures. We demonstrate controllable optical switching between bright and dark states of color centers emitting around 2 eV. Resonant, or quasi-resonant, excitation of photoluminescence also pumps the color center, via a two-photon process, into a dark state, where it becomes trapped. Repumping back into the bright state has a step-like spectrum with a defect-dependent threshold between 2.25 and 2.6 eV. This behavior is consistent with photoionization and charging between optically bright and dark states of the defect. Furthermore, a second zero phonon line, detuned by +0.4 eV, is observed in absorption with orthogonal polarization to the emission, evidencing an additional energy level in the color center.