Spectroscopy of photoionization from the
1E singlet state in nitrogen
−vacancy centers in diamond
The
1E−1A1 singlet manifold of the negatively charged nitrogen vacancy
(NV−) center in diamond plays a central role in the quantum information and quantum sensing applications of the
NV− center. However, the energy of this manifold within the diamond bandgap and with respect to the
3A2−3E triplet manifold has not been measured directly. Using field-quenching effects on photoluminescence (PL) spectra, we report on the energy gap between the
1E−1A1 singlet manifold and the
3A2 and
3E ground and excited triplet states of the
NV− as a function of excitation wavelength and power, temperature, and applied magnetic field in a heavily nitrogen-doped sample. Increased PL and decreased zero-phonon line width from the
NV0 were observed in the presence of an applied magnetic field, indicating ionization from the long-lived
1E singlet state. A temperature-dependent ionization threshold between 532 nm and 550 nm was found, locating the singlet states within the diamond band gap.
The
1E--
1A1 singlet manifold of the negatively charged nitrogen vacancy ($\mathrm{N}{V}^{\ensuremath{-}}$) center in diamond plays a central role in its quantum-information and quantum-sensing applications. However, the energy gap between the
1E singlet state and diamond band edges or the
3A2--
3E triplet manifold has not been measured directly. Using the magnetic-field quenching effect on the $\mathrm{N}{V}^{\ensuremath{-}}$ photoluminescence (PL) spectra, we measure these energy gaps as a function of temperature, applied magnetic field, excitation wavelength, and excitation power in a heavily nitrogen-doped sample. Increased PL and decreased zero-phonon line width from the
NV0 center were observed in the presence of an applied magnetic field, indicating ionization from the long-lived
1E singlet state. A temperature-dependent ionization threshold between 532 and 550 nm was found, locating the singlet states within the diamond band gap.
The singlet states of the negatively charged nitrogen-vacancy centers in diamond play a key role in its optical spin control and readout. In this work, the hitherto unknown ionization energy of the singlet is measured experimentally and found to be in the range 1.91--2.25 eV. This is obtained by analyzing photoluminescence measurements incorporating spin control and nitrogen-vacancy charge state differentiation, along with simulations based on the nitrogen-vacancy's master equation. This work establishes a protocol for a more accurate estimate of this ionization energy, which can possibly lead to improved readout methods.
The nitrogen vacancy (NV) center in diamond is promising as an electron spin qubit due to its long-lived coherence and optical addressability. The ground state is a spin triplet with two levels $({m}_{s}=\ifmmode\pm\else\textpm\fi{}1)$ degenerate at zero magnetic field. Polarization-selective microwave excitation is an attractive method to address the spin transitions independently since this allows operation down to zero magnetic field. Using a resonator designed to produce circularly polarized microwaves, we have investigated the polarization selection rules of the NV center. We first apply this technique to NV ensembles in [100]- and [111]-oriented samples. Next, we demonstrate an imaging technique, based on optical polarization dependence, which allows rapid identification of the orientations of many single NV centers. Finally, we test the microwave polarization selection rules of individual NV centers of known orientation.