Effects of optical excitation on the Ramsey magnetometry of ensemble NV centers in diamond
Y. Aoki, T. Sekiguchi, T. Shibata, T. Taniguchi, M. Miyakawa, C. Shinei, Onoda Shinobu, Saiki Seiichi, Ishii Shuya, T. Teraji, Ohshima Takeshi, Hatano Mutsuko, T. Iwasaki
Nitrogen-vacancy (NV) centers in diamond have found broad applications owing to optical spin initialization and readout as well as long spin coherence time at room temperature.1 Recently, we have reported the construction of a pulsed optically detected magnetic resonance (ODMR) system based on double quantum (DQ) Ramsey magnetometry with lock-in (LI) detection towards DC magnetic field sensitivity sufficient for magnetoencephalography.2 The LI detection of fluorescence allowed reduction of the noise floor, removal of the signal offset, and real-time monitoring of the magnetic field. Beyond the CW-ODMR, where spectral broadening and contrast reduction are induced by high-power excitation, the DQ-Ramsey is expected to have highersensitivity, as it allows extension of spin dephasing time T2 * and increase of contrast and fluorescence rates.2 In practice, however, the magnetic sensitivity depends also on how the optical excitation is applied. In this study, we report the excitation power stabilization by reducing the phase noise and the effect of the excitation power on the fluorescence saturation behavior.
Nitrogen-vacancy (NV) centers in diamond have found broad applications owing to optical spin initialization and readout as well as long spin coherence time at room temperature.1 Recently, we have reported the construction of a pulsed optically detected magnetic resonance (ODMR) system based on double quantum (DQ) Ramsey magnetometry with lock-in (LI) detection towards DC magnetic field sensitivity sufficient for magnetoencephalography.2 The LI detection of fluorescence allowed reduction of the noise floor, removal of the signal offset, and real-time monitoring of the magnetic field. Beyond the CW-ODMR, where spectral broadening and contrast reduction are induced by high-power excitation, the DQ-Ramsey is expected to have highersensitivity, as it allows extension of spin dephasing time T2 * and increase of contrast and fluorescence rates.2 In practice, however, the magnetic sensitivity depends also on how the optical excitation is applied. In this study, we report the excitation power stabilization by reducing the phase noise and the effect of the excitation power on the fluorescence saturation behavior.
The spin dynamics of the negatively charged nitrogen-vacancy (NV⁻) centers in diamond are highly sensitive to quantum electromagnetic fluctuations arising from optical excitation, chemical interactions, and thermal processes. Here we investigated the optical and spin properties of ensembles of NV⁻ centers in diamond under continuous-wave (CW) and nanosecond pulsed laser excitation. Optical detection of magnetic resonance (ODMR) is employed to monitor laser induced shifts of the zero-field splitting parameter D under ambient and high-vacuum conditions. Increasing CW laser power results in a systematic downshift of the resonance frequency due to laser-induced heating. The effect is significantly enhanced under vacuum, where the shift in D reaches up to ~17 MHz, compared to ~5 MHz in air, emphasizing the role of environmental thermal dissipation. Under nanosecond pulsed excitation, increasing the pulse repetition rate similarly leads to resonance shifts and reduced ODMR contrast, consistent with cumulative thermal effects. Visible and infrared spectroscopy confirm power dependent modifications of NV emission. Time resolved photoluminescence measurements showed that the excited-state lifetime (~6 ns) remains nearly constant across the investigated excitation range, indicating that in this regime the dominant limitation is thermal loading of the spin resonance rather than a change in intrinsic radiative decay dynamics. These results provide a comprehensive characterization of thermal effects in high power optical excitation of NV ensembles and are directly relevant for high intensity NV-based sensing and spectroscopy, particularly in vacuum environments.
Magnetometry with nitrogen-vacancy (NV) centers in diamond has so far been measured via emission of light from NV centers or via absorption at the singlet transition at 1042 nm. Here, we demonstrate a phenomenon of broadband optical absorption by the NV centers starting in the emission wavelength and reaching up to 1000 nm. The measurements are enabled by a high-finesse cavity, which is used for room-temperature continuous wave pump-probe experiments. The red to infrared probe beam shows the typical optically detected magnetic resonance (ODMR) signal of the NV spin with contrasts up to 42 %. This broadband optical absorption has not yet been reported in terms of NV magnetometry. We argue that the lower level of the absorbing transition could be the energetically lower NV singlet state, based on the increased optical absorption for a resonant microwave field and the spectral behavior. Investigations of the photon-shot-noise-limited sensitivity show improvements with increasing probe wavelength, reaching an optimum of 7.5pT/Hz. The results show significantly improved ODMR contrast compared to emission-based magnetometry. This opens a new detection wavelength regime with coherent laser signal detection for high-sensitivity NV magnetometry.
The nitrogen-vacancy (NV) center in diamond has demonstrated a potential of biomagnetic sensing.1 Realization of magnetoencepharlography (MEG) using NV centers requires further improvement of magnetic sensitivity at a target frequency ranges of 0 to 200 Hz. Based on the Ramsey magnetometry, which can in principle reach the standard quantum limit. So far, a sensitivity of 460 fT/VHZ has been reported above 2 kHz.2 However, a sensitivity better than 1 pT/vHz in the DC region below 200 Hz has not yet been realized. To enhance the sensitivity for MEG, it is necessary to increase the signal response to magnetic field variations (Slope) while suppressing the noise in such a low-frequency range. The slope is increased with the Ramsey contrast, the free precession time, and the number of detected fluorescence photons. On the other hand, "intensity" noises caused by the laser and acousto-optic modulator may be suppressed by lock-in (LI) detection. However, the conventional LI detection extracts only the principal component at its demodulation frequency, so a part of the transient fluorescence signal is also filtered out. In this study, we focus on enhancing the contrast. Our core idea is to acquire higher-order harmonic signals by introducing boxcar averaging.
The nitrogen-vacancy (NV) centers in diamond have the advantages of stable triaxial structure, ultra-long electron spin coherence time and simple optical readout at room temperature. A nitrogen atom in the diamond crystal replaces a carbon atom and a vacancy is generated at the adjacent position, forming a point defect in the C3v space group structure. Its ground state and excited state are both spin triplet states. It is the key to achieving efficient preparation of optical initial state and extracting NV color center’s information in the researches of highly sensitive sensing magnetic detection, temperature detection, biological imaging, quantum computing, etc. However, there was no systematic study on relevant parameters of laser for high-concentration NV color center’s samples in previous experimental studies. Based on a high concentration diamond NV ensemble, we use pulsed optical detection magnetic resonance (ODMR) technology to systematically study the relationship among laser initial polarization time, information reading time and laser power, and the influence of laser incident polarization angle on the accuracy of sensing information. The effects of various laser parameters on the NV1 peak of ODMR on the [111] axis of the NVs of diamond are also investigated. The contrast of ODMR increases firstly with a sigmoid function and then decreases with an e-exponential function as the information reading time increases. The incident polarization angle of the laser is sinusoidal, with a period of 90°. According to the above experimental results, we finally choose the appropriate experimental parameters at 45.8 W/cm2 (300 μs of polarization, 700 ns, reading time, laser incident angle is 220°) for ODMR test. Compared with previous experimental parameters (polarization time was 50 us, read the time of 3000 ns, laser incident angle was 250°), the experimental results show that the contrast of ODMR increases from 2.1% to 4.6%, and the typical magnetic sensitivity is improved from 21.6 nT/Hz1/2 to 5.6 nT/Hz1/2. The optimization of the optical control of NVs in solid diamond is realized. The above results provide an effective support for the detection of high-sensitivity manipulation sensing based on high-concentration NV ensemble.
Ensembles of nitrogen-vacancy (NV) centers in diamond are a well-established platform for quantum magnetometry under ambient conditions. One challenge arises from the hyperfine structure of the NV, which, for the common 14N isotope, results in a threefold reduction of contrast and thus sensitivity. By addressing each of the NV hyperfine transitions individually, triple-tone microwave (MW) control can mitigate this sensitivity loss. Here, we experimentally and theoretically investigate the regimes in which triple-tone excitation offers an advantage over standard single-tone MW control for two DC magnetometry protocols: pulsed optically detected magnetic resonance (ODMR) and Ramsey interferometry. We validate a master equation model of the NV dynamics against ensemble NV measurements, and use the model to explore triple-tone vs single-tone sensitivity for different MW powers and NV dephasing rates. For pulsed ODMR, triple-tone driving improves sensitivity by up to a factor of three in the low-dephasing regime, with diminishing gains when dephasing rates approach the hyperfine splitting. In contrast, for Ramsey interferometry, triple-tone excitation only improves sensitivity if MW power is limited. Our results delineate the operating regimes where triple-tone control provides a practical strategy for enhancing NV ensemble magnetometry in portable and power-limited sensors.
The nitrogen-vacancy (NV) center in diamond is a leading solid-state platform for room-temperature quantum magnetometry owing to its long spin coherence times, optical spin initialization and readout, and high sensitivity to magnetic, electric, and thermal perturbations. As NV-based optically detected magnetic resonance (ODMR) systems transition from controlled laboratory environments toward portable and field-deployable sensors, a detailed understanding of realistic noise sources and experimental imperfections becomes essential for optimizing performance and sensitivity. In this work, we present a comprehensive simulation framework, i.e., a digital twin, for continuous-wave wide-field ODMR in NV-center ensembles. The model is built upon a physically consistent seven-level description of the NV center and incorporates a broad range of experimentally relevant noise and imperfection mechanisms as modular, parameterized components. These include laser and microwave amplitude fluctuations, microwave phase noise, uncertainty in the NV gyromagnetic ratio, spin dephasing, temperature-induced shifts of the ground-state zero-field splitting, surface-induced magnetic field perturbations, and photon shot noise. Power broadening and contrast degradation arising from optical and microwave driving are captured self-consistently through linewidth calculations. Also, the spatial inhomogeneity is modeled via a Gaussian laser intensity profile across the sensing region...