We present systematic measurements of longitudinal relaxation rates ($1/T_1$ ) of spin polarization in the ground state of the nitrogen-vacancy (NV^–) color center in synthetic diamond as a function of NV^– concentration and magnetic field B. NV^– centers were created by irradiating a Type 1b single-crystal diamond along the [100] axis with 200 keV electrons from a transmission electron microscope with varying doses to achieve spots of different NV^– center concentrations. Values of ($1/T_1$ ) were measured for each spot as a function of B.

We studied the dynamic nuclear spin polarization of nitrogen in negatively charged nitrogen-vacancy (NV) centers in diamond both experimentally and theoretically over a wide range of magnetic fields from 0–1100 G covering both the excited-state level anticrossing and the ground-state level anticrossing magnetic field regions. Special attention was paid to the less studied ground-state level anticrossing region. The nuclear spin polarization was inferred from measurements of the optically detected magnetic resonance signal. These measurements show that a very large (up to $96\pm 2\%$) nuclear spin polarization of nitrogen can be achieved over a very broad range of magnetic field starting from around 400 G up to magnetic field values substantially exceeding the ground-state level anticrossing at 1024 G. We measured the influence of angular deviations of the magnetic field from the NV axis on the nuclear spin polarization efficiency and found that, in the vicinity of the ground-state level anticrossing, the nuclear spin polarization is more sensitive to this angle than in the vicinity of the excited-state level anticrossing. Indeed, an angle as small as a tenth of a degree of arc can destroy almost completely the spin polarization of a nitrogen nucleus. In addition, we investigated theoretically the influence of strain and optical excitation power on the nuclear spin polarization.

Energy levels of nitrogen-vacancy centers in diamond were investigated using optically detected magnetic-resonance spectroscopy near the electronic ground-state level anticrossing (GSLAC) at an axial magnetic field around 102.4 mT in diamond samples with a nitrogen concentration of 1 and 200 ppm. By applying radio waves in the frequency ranges from 0 to 40 MHz and from 5.6 to 5.9 GHz, we observed transitions that involve energy levels mixed by the hyperfine interaction. We developed a theoretical model that describes the level mixing, transition energies, and transition strengths between the ground-state sublevels, including the coupling to the nuclear spin of the NV centers' ${}^{14}\mathrm{N}$ and ${}^{13}\mathrm{C}$ atoms. The calculations were combined with the experimental results by fitting the ODMR spectral lines based on a theoretical model, which yielded information about the polarization of nuclear spins. This study is important for the optimization of experimental conditions in GSLAC-based applications, e.g., microwave-free magnetometry and microwave-free nuclear-magnetic-resonance probes.

Nonlinear magneto-optical resonances on the hyperfine transitions belonging to the D2 line of rubidium were changed from bright to dark resonances by changing the laser power density of the single exciting laser field or by changing the vapor temperature in the cell. In one set of experiments atoms were excited by linearly polarized light from an extended cavity diode laser with polarization vector perpendicular to the light's propagation direction and magnetic field, and laser-induced fluorescence was observed along the direction of the magnetic field, which was scanned. A low-contrast bright resonance was observed at low laser power densities when the laser was tuned to the Fg=2$\rightarrow${}Fe=3 transition of 87Rb and near to the Fg=3$\rightarrow${}Fe=4 transition of 85Rb. The bright resonance became dark as the laser power density was increased above 0.6 mW/cm2 or 0.8 mW/cm2, respectively. When the Fg=2$\rightarrow${}Fe=3 transition of 87Rb was excited with circularly polarized light in a second set of experiments, a bright resonance was observed, which became dark when the temperature was increased to around 50$\circ${}C. The experimental observations at room temperature could be reproduced with good agreement by calculations based on a theoretical model, although the theoretical model was not able to describe measurements at elevated temperatures, where reabsorption was thought to play a decisive role. The model was derived from the optical Bloch equations and included all nearby hyperfine components, averaging over the Doppler profile, mixing of magnetic sublevels in the external magnetic field, and a treatment of the coherence properties of the exciting radiation field.

In this work, cross relaxation between nitrogen-vacancy (NV) centers and substitutional nitrogen in a diamond crystal is investigated. It is demonstrated that optically detected magnetic resonance signals (ODMR) can be used to probe cross relaxation. ODMR is detected at axial magnetic field values around 51.2 mT in a diamond sample with a relatively high (200 ppm) nitrogen concentration. We observe transitions that involve magnetic sublevels that are split by the hyperfine interaction. Microwaves in the frequency ranges from 1.3 GHz to 1.6 GHz ($m_S=0⟶m_S=-1$ NV transitions) and from 4.1 to 4.6 GHz ($m_S=0⟶m_S=+1$ NV transitions) were used. To understand the cross-relaxation process in more detail and, as a result, reproduce measured signals more accurately, a model is developed that describes the microwave-initiated transitions between hyperfine levels of the NV center at anticrossing that are strongly mixed. Additionally, we simulate the extent to which the microwave radiation driving ODMR in the NV center also induces transitions in the substitutional nitrogen via cross relaxation. The improved understanding of the NV processes in the presence of magnetic field will be useful for designing NV-diamond-based devices for a wide range of applications from implementation of q bits to hyperpolarization of large molecules to various quantum technological applications such as field sensors.