Multiphoton-Excited Fluorescence of Silicon-Vacancy Color Centers in Diamond

Silicon-vacancy color centers in nanodiamonds are promising as fluorescent labels for biological applications, with a narrow, nonbleaching emission line at 738 nm. Two-photon excitation of this fluorescence offers the possibility of low-background detection at significant tissue depth with high threedimensional spatial resolution. We measure the two-photon fluorescence cross section of a negatively charged silicon vacancy (Si-V−) in ion-implanted bulk diamond to be 0.74ð19Þ × 10−50 cm s=photon at an excitation wavelength of 1040 nm. Compared to the diamond nitrogen-vacancy center, the expected detection threshold of a two-photon excited Si-V center is more than an order of magnitude lower, largely due to its much narrower linewidth. We also present measurements of twoand three-photon excitation spectra, finding an increase in the two-photon cross section with decreasing wavelength, and we discuss the physical interpretation of the spectra in the context of existing models of the Si-V energy-level structure.