Assignment of the NV${}^0$ 575-nm zero-phonon line in diamond to a ${}^{2}E$-${}^2{A}_2$ transition

The time-averaged emission spectrum of single nitrogen-vacancy defects in diamond gives zero-phonon lines of both the negative charge state at 637 nm (1.945 eV) and the neutral charge state at 575 nm (2.156 eV). This occurs through photoconversion cycling between the two charge states. Due to strain in the diamond the zero-phonon lines of both charge states are split and it is found that the splitting and polarization of the two zero-phonon lines are the same. From this observation and consideration of the electronic structure of the nitrogen-vacancy center it is concluded that the excited state of the neutral center has $A_2$ orbital symmetry. The assignment of the 575-nm transition to a ${}^{2}E$-${}^2{A}_2$ transition has not been established previously.

Figure 1

Emission spectra of single NV centers averaged over 100 s. In all cases there are ZPLs associated with NV${}^{-}$ at 635.6 nm (red) and NV${}^0$ at 574.9 nm (orange). For a single site the splittings of the NV${}^{-}$ and NV${}^0$ ZPLs are the same although the magnitude can vary from site to site.

Figure 2

Polarization of NV ZPLs for single center shown in the upper figure of Fig. 1. Angle of polarizer for NV${}^0$ is shown in left panel (a) and that of NV${}^{-}$ is shown in the right panel (b). Experimental data are given in magenta (high energy line) and green (low energy line). The lines are cosine fits to the data.

Figure 3

Energy levels of the lowest energy configurations of NV${}^{-}$. The diagram depicts configuration levels (left-hand side) and first-order (central) and second-order (right-hand side) corrections due to two-electron Coulomb repulsion. Splitting of the orbital $E$ levels that can occur with strain are also indicated on the far right. The vertical arrow on the left-hand side denotes the difference in energy $\epsilon$ of the $e$ and $a_1$ one-electron orbitals. The vertical arrow on the right indicates the spin-allowed optical transition from the ground state. The configurations are given in terms of four electrons. The two additional electrons in the valence band can be ignored.

Figure 4

Energy levels of the lowest energy configurations of NV${}^0$. The diagram depicts configuration levels (left-hand side) and first-order (central) and second-order (right-hand side) corrections due to two-electron Coulomb repulsion. Splitting of the orbital $E$ states that can occur with strain are also indicated on the far right. The vertical arrow on the left-hand side denotes the difference in energy $\epsilon$ of the $e$ and $a_1$ one-electron orbitals. The vertical arrow on the right indicates the spin-allowed optical transition from the ground state. The configurations are given in terms of three electrons. The two additional electrons in the valence band can be ignored. Note that the ordering of the ${}^{2}A$${}_2$, ${}^{2}E$, and ${}^{2}A$${}_1$ levels of the $a_1$ $e^2$ configuration are unknown without experimental investigation.

Figure 5

Summary of splitting due to stress for the $\sim$575 nm NV${}^0$ and $\sim$637 nm NV${}^{-}$ transitions. A transition with a common $X$ polarization is shown and in both cases is displaced to higher energy. The transition to the $E$${}_X$ state is $Y$ polarized and displaced to lower energy but, for clarity, is not shown.