Background-free two-photon fluorescence readout via a three-photon charge-state modulation of nitrogen-vacancy centers in diamond.

We demonstrate that a background-free readout of two-photon fluorescence from nitrogen-vacancy (NV) centers in a strongly fluorescing environment can be accomplished by all-optical means via a multiphoton charge-state modulation of NV centers in a mixture of negatively charged and neutral NV centers. A 100 fs, 1060 nm output of an ytterbium fiber laser is ideally suited for this modality of multiphoton microscopy, providing, as our experiments show, an efficient two-photon excitation of both NV and NV charge states, but keeping the nonlinearity of n-photon ionization needed for NV/NV charge-state modulation to a minimum, n=3.