  The nonlinear fluorescence emission has been widely applied for the high
spatial resolution optical imaging. Here, we studied the fluorescence anomalous
saturating effect of the nitrogen vacancy defect in diamond. The fluorescence
reduction was observed with high power laser excitation. It increased the
nonlinearity of the fluorescence emission, and changed the spatial frequency
distribution of the fluorescence image. We used a differential excitation
protocol to extract the high spatial frequency information. By modulating the
excitation laser's power, the spatial resolution of imaging was improved
approximate 1.6 times in comparison with the confocal microscopy. Due to the
simplicity of the experimental setup and data processing, we expect this method
can be used for improving the spatial resolution of sensing and biological
labeling with the defects in solids.
