Over the past decade, the negatively charged nitrogen vacancy (NV−) center in diamond has been studied extensively because of the stable fluorescence and long coherence time of its spin state. To further extend the study of the interaction between a multi-NV center and the nanoscale sensing with the NV center, researchers detected and controled the NV center spin-state dynamics with high spatial resolution.

SUN Fangwen and his team at Key Lab of Quantum Information and Synergetic Innovation Center of Quantum Information and Quantum Physics of University of Science and Technology of China (USTC) of Chinese Academy of Sciences achieved subdiffraction charge-state manipulation on nitrogen vacancy (NV) center in diamond, a potential candidate for quantum computation and metrology.

Stimulated emission depletion (STED) microscopy is one area of many optical super-resolution microscopy techniques which have been developed to detect single NV centers. With STED, the electron spin resonance signals of NV centers have been detected with a resolution lower than the diffraction limit.

SUN and his team replaced the stimulated excitation of STED with the dark-state pumping of NV centers. Based on the mechanism of charge-state conversion and super-resolution microscopy, they demonstrated subdiffraction charge-state manipulation of the NV centers in diamond consisting of a substitutional nitrogen atom and an adjacent vacancy. Using Laser beams with different wavelengths and shapes to control and detect the charge state, they reached the best spatial resolution approximately 4.1nm.

Two charge states were usually observed in the NV center in diamond: NV0 and NV−, both of which were stable in the absence of optical excitation. SUN’s team distinguished the two charge states by measuring the NV center fluorescence for their different wavelength. The conversion between these two charge states can be observed as the fluorescence on and off processes, which can be used for reversible saturable optical fluorescence transition microscopy. The most convenient method to control the charge states of NV centers is high-speed optical manipulation. To demonstrate photon-induced charge-state conversion between NV0 and NV−, the NV center ensemble fluorescence emission of both NV0 and NV− charge states was detected.

The scientists outlined the procedure for subdiffraction charge-state depletion (CSD) microscopy imaging using three different laser pulses in the experiment. Using CSD microscopy, they measured the resonance signal and coherent dynamics of a NV center spin state with subdiffraction spatial resolution, which will be potentially used for nanoscale sensing. 

Sun’s team used the CSD method to selectively detect the electron spin-state dynamics of each NV center. Both of the NV centers exhibited short spin coherence times. Their recent experiments have demonstrated that the charge-state conversion of NV centers could induce a detectable change in the local field in diamond, which might affect the dynamics of the spin system that is coupled to the NV center. Different lasers can be used to change the adjacent NV to NV− or NV0 and maintain the charge state of NV at the beam center to be NV−. For the lasers used in this experiment, the charge-state conversion is usually much slower than the NV−spin-state polarization.

"With the charge-state manipulation, we were able to detect the electron spin-state dynamics of NV centers with subdiffraction resolution, which can be used for nanoscale sensing of electromagnetic fields and biological molecules. Because charge-state conversion changes the local field in diamond, the CSD microscopy method can potentially be used for the study of spin-state quantum coherent dynamics and the interaction between coupled spin systems in diamond.” SUN said.

"In the future, we expect that high-resolution NV charge-state manipulation could help to control the spin-state dynamics of NVs and to switch interactions between NV centers,” SUN added.

The research entitled “Subdiffraction optical manipulation of the charge state of nitrogen vacancy center in diamond” has been published on Light.