Optically detected magnetic resonance of nitrogen-vacancy centers in diamond using two-photon excitation

Abstract: We demonstrate the use of two-photon excitation for observing the ground state optically detected magnetic resonance (ODMR) of nitrogen-vacancy centers in diamonds at room temperature. An ultrafast femtosecond laser at 1040 nm was used for excitation, while fluorescence signal read out was achieved through a combination of a PMT and a lock-in amplifier. The imaging capability of two-photon excitation fluorescence (2PEF) was utilized to map the distribution of NV centers in a bulk diamond and micro-sized diamonds. For the first time, ODMR traces of the nitrogen-vacancy center are observed with two-photon excitation, providing a promising tool for fast 3D quantum sensing and imaging.

• The paper demonstrates the first two-photon excited ODMR of NV centers, achieving a ~7% fluorescence contrast and revealing hyperfine and Zeeman splitting.
• The experimental setup employs a 1040 nm femtosecond laser with multiphoton microscopy, achieving sub-micron lateral and ~4 µm axial resolution for 3D mapping.
• The study shows that two-photon excitation offers superior background suppression and deeper imaging, advancing localized quantum sensing in diamond.

Two-Photon Excitation ODMR in Diamond NV Centers: Methodology, Results, and Implications

Overview

This paper demonstrates, for the first time, optically detected magnetic resonance (ODMR) of diamond nitrogen-vacancy (NV) centers utilizing two-photon excitation (2PE), instead of conventional one-photon (visible) excitation. Employing a home-built 1040 nm femtosecond (fs) Yb fiber laser as the excitation source, in combination with multiphoton microscopy (MPM), the authors achieve depth-resolved, high-resolution imaging and ODMR readout of NV centers in both bulk and micro-diamond samples at room temperature. This approach addresses existing challenges in background suppression, spatial localization, and three-dimensional (3D) mapping of NV centers, and establishes 2PEF-ODMR as a viable route for localized quantum sensing.

Experimental Innovations and Measurement Scheme

The experimental apparatus centers on an in-house MPM system, equipped with a compact 1040 nm fs fiber laser delivering 50 fs pulses at 7.01 MHz. The system achieves ∼0.57 µm lateral and ∼3.95 µm axial two-photon resolution, utilizing a 20X 0.75 NA Nikon objective. The fluorescence (630–950 nm) arising from NV- and NVº centers is epi-collected and detected with PMT/lock-in amplification, which enables sensitive extraction of fluorescence modulations during microwave (MW) frequency sweeps (2.75–2.96 GHz). MW delivery is realized via a custom PCB loop antenna, ensuring high field intensity in close proximity to the diamond samples.

This non-degenerate two-photon process effectively localizes excitation in 3D and allows optical sectioning, which is not achievable with standard one-photon confocal techniques suffering from limited penetration depth and poor background discrimination. Notably, the nonlinear nature of 2PE is firmly established by the quadratic power dependence of emission (measured slope: $2.25 \pm 0.32$).

Key Experimental Results

ODMR with Two-Photon Excitation

• The first ODMR traces under two-photon excitation are reported, with a fluorescence dip of ~7% contrast and a FWHM linewidth of ~26.87 MHz in bulk diamond at zero magnetic field.
• Hyperfine splitting of $\approx 5.64$ MHz is evident, attributable to strain-induced lifting of the $m_s = \pm1$ degeneracy.
• Zeeman splitting is systematically observed by increasing static external fields, with characteristic ODMR spectral patterns (e.g., four-dip features) reflecting the multi-axis projection of the magnetic field in the diamond lattice.
• Measurements in micro-diamonds ($\sim$15 µm) with variable NV center concentrations confirm that ODMR signals are prominent only in crystals with a high density of NV- centers. Microdiamonds dominated by NVº centers do not exhibit measurable ODMR, highlighting the technique's selectivity.
• In microdiamonds, a 2.5% reduction in fluorescence intensity associated with ODMR is reported, with hyperfine splitting of ~8 MHz.

High-Resolution Multiphoton Imaging

• 2PEF imaging provides multichannel discrimination (SHG, THG, 3PEF, 2PEF) over large areas and substantial depths, revealing inhomogeneous, zoned NV center distributions within both bulk and micro-diamond samples.
• The high axial sectioning capability enables mapping of NV center spatial distributions at subcellular scales, overcoming limitations of confocal techniques for thick samples.
• Spectroscopic analysis unambiguously identifies emission from both NV- (637 nm ZPL) and NVº centers (575 nm ZPL), as well as additional nonlinear signals arising from other defects.

Theoretical and Practical Implications

The adoption of 2PE-based ODMR for NV center interrogation directly addresses fundamental challenges in quantum sensing and imaging:

• Locality and Optical Sectioning: 2PE confines excitation to a tightly focused volume, minimizing background from off-focus NV centers and other color centers, and enabling 3D spatial selectivity.
• Deeper Penetration: NIR excitation (~1040 nm) achieves significantly higher sample penetration with reduced scattering and photodamage, thus supporting applications in bulk crystals and subsurface imaging.
• Background Suppression: The photon energy mismatch with visible emission minimizes reabsorption and leakage issues, improving sensitivity for weak ODMR signals in dense or highly scattering samples.
• Versatility: The method is shown to be compatible with ensemble and single-NV experiments, and supports high-throughput screening of NV quality and defect environments.

The robust observation of all core NV ODMR signatures (zero-field, hyperfine, Zeeman splitting) under 2PE consolidates the fundamental viability of this method for quantum metrology and information processing. Notably, the practical realization of fast, high-resolution, 3D ODMR mapping in native and engineered diamonds expands the parameter space for precision magnetometry, temperature sensing, and potentially for integrating NV centers in photonic circuits where spatial addressability and background suppression are critical.

Future Directions

This work opens pathways for:

• Three-dimensional quantum imaging in heterogeneous or biological environments, leveraging the depth and resolution advantages of multiphoton excitation.
• Integration with advanced quantum devices and on-chip photonics, where NIR-excitation could facilitate less invasive readout and reduced spectral crosstalk.
• Extension to other point defect systems (e.g., silicon vacancies, divacancies in SiC) for generalized nonlinear ODMR protocols.
• Enhancements in sensitivity and speed by leveraging advanced detection schemes and higher-repetition-rate sources, as well as cryogenic implementations for reduced phonon broadening.

Conclusion

This work establishes two-photon excitation as a powerful technique for localized ODMR interrogation of NV centers in diamond, combining efficient nonlinear excitation with depth-resolved, high-resolution imaging. The method overcomes key limitations of standard one-photon ODMR, and demonstrates practical applicability to both bulk and micro-diamond systems with high spatiotemporal resolution. The results solidify the foundational utility of 2PE ODMR for next-generation quantum sensing, imaging, and NV-based device engineering.