The inversion in the dependence of ODMR contrast in NDs appears paradoxical, as it would indicate a reduction in NV -spin-polarization with increasing laser power. However, this phenomenon is likely to reflect the different contributions of NV 0 centers in the bulk and ND samples under the various experimental conditions explored here. In fact, we notice that both NV-and NV 0 signals are collected by the broad bandpass filter with a spectral window of 590-800 nm (see Methods). The ODMR contrast is defined as 𝐶 𝑠 = (𝐼 𝑜𝑓𝑓 -𝐼 𝑜𝑛 ) 𝐼 𝑜𝑓𝑓 ⁄ , where 𝐼 𝑜𝑓𝑓 and 𝐼 𝑜𝑛 are the NV -FL intensities with MWs off-and on-resonance, respectively. However, the FL contains a contribution (𝐼 0 ) from the NV 0 centers, which is not modulated by MWs and reduces the contrast by a factor 𝐼 𝑜𝑓𝑓 (𝐼 𝑜𝑓𝑓 + 𝐼 0 ) ⁄ . This reduction factor contrast is very different for bulk and NDs. In the bulk, the vast majority of the NV centers are negative, as shown in Fig. , and remain stable under laser irradiation. Therefore, a stronger laser irradiation results in a better spin initialization and improved ODMR contrast, without impacting on the NV charges (I0 negligible). On the contrary, NDs tend to have higher relative concentrations of NV 0 centers as a result of surface effects (Fig. ). Moreover, under laser irradiation, NV -→ NV 0 photoconversion might be more efficient in NDs due to presence of surface acceptor states that can take a photoexcited electron from NV -. Higher laser powers then result in increasing relative concentrations of NV 0 in NDs, and in a reduction in ODMR contrast. This phenomenon competes with NV -polarization, which increases with laser power, resulting in the non-monotonic behavior shown in Fig. . Therefore, in NDs there is an optimal laser power that should be used to prepare the NV -spin states. Stabilization of the NV -might shift the maximum of curve of Fig. b,c to higher laser power. ) show ODMR spectra at different laser power with fixed MW power. In the ODMR spectra, the NV -central lines (AL and AR) provide a measure of the spin polarization of the |𝑔, 𝑚 𝑠 = 0⟩ ground state. The MW range was set to 2.75-3 GHz to show the NV -central lines and the 13