Doubly-charged silicon vacancy center, photochromism, and Si-N complexes in co-doped diamond.

We report the first experimental observation of a doubly-charged defect in diamond, SiV2-, in silicon and nitrogen co-doped samples. We measure spectroscopic signatures we attribute to substitutional silicon in diamond, and identify a silicon-vacancy complex decorated with a nearest-neighbor nitrogen, SiVN, supported by theoretical calculations. Samples containing silicon and nitrogen are shown to be heavily photochromic, with the dominant visible changes due to the loss of SiV0/- and gain in the optically-inactive SiV2-.