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The negatively charged nitrogen-vacancy defect center, (NV)–, in diamond has been investigated theoretically for its one- and two-photon absorption properties involving the first excited state with the 3A2→3E transition. Time-dependent density functional theory (TD-DFT), configuration interaction with single excitation (CIS), and complete active space self-consistent field (CASSCF) were employed in this investigation along with the 6-31G(d) basis set. Diamond lattice models containing 24–104 carbon atoms were constructed to imitate the local environment of the defect center. TD-DFT calculations in large molecular cluster models (with 85 or more carbon atoms) predicted the vertical excitation energy quite consistent with the experimental absorption maximum. CASSCF calculations were feasible only for small cluster models (less than 50 carbon atoms) but yielded one-photon absorption (OPA) and two-photon absorption (TPA) cross sections somewhat larger than the experimental values obtained with linearly polarized incident light [T.-L. Wee et al., J. Phys. Chem. A 111, 9379 (2007)]. CIS calculations in larger cluster models showed a systematic overestimation of the excitation energy while just slightly underestimated the OPA cross section and overestimated the TPA cross section. The agreements between calculations and measurements suggest that the computational approaches established in this work are applicable to explore the optical properties of related defect centers in diamond as well. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate a simple and robust optical fiber based method to achieve simultaneously efficient excitation and fluorescence collection from Nitrogen-Vacancy (NV) defects containing micro-crystalline diamond. We fabricate a suitable micro-concave mirror that focuses scattered excitation laser light into the diamond located at the focal point of the mirror. At the same instance, the mirror also couples the fluorescence light exiting out of the diamond crystal in the opposite direction of the optical fiber back into the optical fiber within its light acceptance cone. This part of fluorescence would have been otherwise lost from reaching the detector. Our proof-of-principle demonstration achieves a 25 times improvement in fluorescence collection compared to the case of not using any mirrors. The increase in light collection favors getting high signal-to-noise ratio optically detected magnetic resonance signals and hence offers a practical advantage in fiber-based NV quantum sensors. Additionally, we compacted the NV sensor system by replacing some bulky optical elements in the optical path with a 1 × 2 fiber optical coupler in our optical system. This reduces the complexity of the system and provides portability and robustness needed for applications like magnetic endoscopy and remote-magnetic sensing. [ABSTRACT FROM AUTHOR]