  We report the first observation of multi-photon photoluminescence excitation
(PLE) below the resonant energies of nitrogen vacancy (NV) centers in diamond.
The quadratic and cubic dependence of the integrated fluorescence intensity as
a function of excitation power indicate a two-photon excitation pathway for the
NV- charge state and a three-photon process involved for the neutral NV0 charge
state respectively. Comparing the total multi-photon energy with its
single-photon equivalent, the PLE spectra follows the absorption spectrum of
single photon excitation. We also observed that the efficiency of
photoluminescence for different charge states, as well as the decay time
constant, was dependent on the excitation wavelength and power.
