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SPIE Quantum Information Science, Sensing, and Computation

Creating a compact low-power laser threshold magnetometer utilizing the NV center in diamond

Gary A. Sevison, Michael S. Wolf, Robert F. Bedford, Michael A. Slocum

Published May 30, 2025• 0 citations
Publisher

Abstract

Laser threshold magnetometry (LTM) promises a path towards significant reductions in optical power and volume for high sensitivity diamond nitrogen-vacancy (NV) magnetometers. LTM utilizes the difference in laser threshold due to intra-cavity diamond absorption while driving the RF spin resonance. Power consumption is directly related to the sizes of the laser spot on the gain chip and the diamond. By confining the spot size, we can drastically reduce the laser threshold pump power needed (up to ~34x for proposed linear cavity design). By physically connecting all the pieces of our sensor, we also reduce laser noise allowing us to approach closer to threshold, therefore increasing our contrast and decreasing our photon shot noise limit.

DOI
10.1117/12.3053199

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Laser threshold magnetometry (LTM) promises a path towards significant reductions in optical power and volume for high sensitivity diamond nitrogen-vacancy (NV) magnetometers. LTM utilizes the difference in laser threshold due to intra-cavity diamond absorption while driving the RF spin resonance. Power consumption is directly related to the sizes of the laser spot on the gain chip and the diamond. By confining the spot size, we can drastically reduce the laser threshold pump power needed (up to ~34x for proposed linear cavity design). By physically connecting all the pieces of our sensor, we also reduce laser noise allowing us to approach closer to threshold, therefore increasing our contrast and decreasing our photon shot noise limit.
We propose a new type of sensor, which uses diamond containing the optically active nitrogen-vacancy (NV − ) centres as a laser medium. The magnetometer can be operated at room-temperature and generates light that can be readily fibre coupled, thereby permitting use in industrial applications and remote sensing. By combining laser pumping with a radio-frequency Rabi-drive field, an external magnetic field changes the fluorescence of the NV − centres. We use this change in fluorescence level to push the laser above threshold, turning it on with an intensity controlled by the external magnetic field, which provides a coherent amplification of the readout signal with very high contrast. This mechanism is qualitatively different from conventional NV − –based magnetometers which use fluorescence measurements, based on incoherent photon emission. We term our approach laser threshold magnetometer (LTM). We predict that an NV − –based LTM with a volume of 1 mm 3 can achieve shot-noise limited dc sensitivity of 1.86 fT and ac sensitivity of 3.97 fT .
Nitrogen-vacancy (NV) centers have considerable promise as high sensitivity magnetometers; however, they are commonly limited by inefficient collection and low contrasts. Laser threshold magnetometry (LTM) enables efficient collection and high contrasts, providing a path toward higher sensitivity magnetometry. We demonstrate an infrared LTM using an ensemble of NV centers in a single-crystal diamond plate integrated into a vertical external cavity surface emitting laser. The laser was tuned to the spin-dependent absorption line of the NV centers, allowing for optical readout by monitoring the laser output power. We demonstrate a magnetic sensitivity of 7.5 nT/Hz in the frequency range between 10 and 50 Hz. Furthermore, the contrast and the projected photon shot noise limited (PSNL) sensitivity are shown to improve significantly by operating close to the lasing threshold, achieving 18.4% and 26.6 pT/Hz near the threshold. In addition, an unexpected saturable absorption phenomenon was observed near the threshold, which enhanced the contrast and projected PSNL sensitivity.
Nitrogen-vacancy (NV) centers in diamond can be used as a highly sensitive sensor for magnetic fields. One concept to reach the fT/sqrt(Hz) regime is laser threshold magnetometry (LTM). In this talk, we present a self-sustainable, magnetic field-dependent cw NV laser system. To achieve this, we place the NV-diamond within the external cavity of a red VECSEL. When the NV-diamond is pumped, we observe a decrease of the pump power at laser threshold of the VECSEL while the cavity output increases by up to 10%. We demonstrate a strong magnetic-field dependence of the laser threshold as a function of the VECSEL pump power. Our results demonstrate a practical concept for self-sustainable LTM devices based on NV-gain.
The negatively charged nitrogen-vacancy center (NV−^-−) in diamond is a versatile platform for quantum magnetometry under ambient conditions. Recently, laser threshold magnetometry (LTM) has been proposed as a means to significantly enhance the sensitivity of NV-based magnetometers by incorporating a diamond hosting NV−^-− centers within a laser cavity and operating near threshold. While demonstrations have validated the concept, practical implementations remain technically demanding, requiring high pump powers and precise alignment of free-space cavities. It remains unclear whether the benefits of operating near threshold will outpace increased laser noise. In this work, we integrate an NV-diamond with a high NV−^-− content into a compact external cavity diode laser and demonstrate singlet infrared absorption optically detected magnetic resonance (ODMR). The system exhibits exceptional threshold current stability, enabling ODMR using the threshold current as the read-out parameter. We report a five-fold enhancement in the ODMR contrast by operating near threshold. The best magnetic field sensitivity of 7.6 nT/Hz7.6~\mathrm{nT/\sqrt{Hz}}7.6 nT/Hz​ (DC-500 Hz) is achieved well above threshold, while near threshold sensitivity is limited by increased probe laser noise. These results establish a compact and mechanically robust platform for singlet absorption-based NV−^-− magnetometry and highlight key trade-offs between contrast enhancement and laser noise near threshold.
Nitrogen-vacancy centers (NV) in diamond are promising quantum systems for magnetic field sensing. The sensitivity and the linearity of such a quantum sensor can be greatly improved by using stimulated emission of the NV centers in the concept of laser threshold magnetometry (LTM), which is projected to reach the fT/sqrt(Hz) regime. Previous implementations of NV centers in optical cavities relied on external seed lasers, pulsed operation, or sensing via NVabsorption. In our work, we combine the NV centers with a second gain medium, a laser diode, within the same cavity, achieving self-sustainable continuous-wave lasing. This approach compensates the intrinsic losses of the cavity and the diamond, through a fixed additional gain below the threshold of the laser system. A continuous-wave laser threshold and a linewidth narrowing is observed with increasing pump power on the NV centers of the combined laser system. An improved laser system shows a magnetic field-dependent laser threshold, which is the basis for improved-sensitivity NV magnetometry via LTM.
Nitrogen-vacancy (NV) centers in diamond are a promising platform for high-precision magnetometry. In contrast to the use of spontaneous emission in a number of NV-magnetometers, laser threshold magnetometry exploits stimulated emission of NV centers by placing an NV-doped diamond inside an optical cavity. The NV laser system is predicted to reach a high magnetic-field-dependent contrast and coherent signal strength, leading to an improved magnetic field sensitivity combined with high linearity. Here, we consider a two-media setup where the cavity additionally includes a vertical external cavity surface emitting a laser. This optically active material compensates cavity losses at 750 nm while still allowing for magnetic-field-dependent effects from the NV-diamond. We demonstrate a magnetic-field-dependent laser threshold and investigate the effects of pump laser induced absorption of the diamond. The experimental data are supported by an analytical simulation based on a rate model. Furthermore, we derive a generalized formula to compute the shot-noise-limited magnetic field sensitivity in the regime of high contrast, yielding 49.07(0.33)pT/Hz for the present setup. Simulations with an optimized NV-diamond suggest that values down to 4.9fT/Hz are possible.