World’s first measurement of magnetic field-dependent stimulated emission

Demonstrator of a laser threshold magnetometer. The perspective shows the cavity where the scientists measured the amplification and the magnetic field-dependent stimulated emission. Credit: Fraunhofer-Institut für Angewandte Festkörperphysik IAF

In medical care, the magnetic fields of heart and brain activity are measured to detect disease at an early stage. To measure even the smallest magnetic fields, Fraunhofer IAF researchers are working on a new approach: diamond-based laser threshold magnetometry. The idea is to use diamond with a high density of nitrogen vacancy centers in a laser system. Today, the researchers succeeded in reaching an important milestone: they were able to demonstrate the world’s first measurement of magnetic field-dependent stimulated emission and even set a new contrast record. The results were published in the journal Scientists progress.

In medical diagnostics, sensitive sensors are needed to measure, for example, the weak magnetic fields of heart and brain activity (MCG, MEG) of the human body and to create images of the body via magnetic resonance imaging (MRI). ), which allows the detection of diseases at an early stage. However, only a few highly sensitive magnetic field sensors achieve the necessary accuracy and each of them presents major technical hurdles for clinical application.

Already established SQUID sensors require complex cryogenic cooling of around -270°C. Vapor cell magnetometers (OPM) are another option. Although these achieve the highest sensitivities even without cryogenic cooling, they have the disadvantage of requiring absolute shielding from all background fields, including the earth’s magnetic field, and thus impose massive structural demands on parts and to buildings. For this reason, the most inaccurate electrical measurements (ECG, EEG) continue to be common in daily clinical practice.

At the Fraunhofer Institute for Applied Solid State Physics IAF in Freiburg, a project team is already researching a more suitable alternative: “Our goal is to develop an extremely sensitive magnetic field sensor that operates at room temperature as well as in the presence of background fields and is therefore useful for clinical implementations,” says Dr. Jan Jeske, project manager at Fraunhofer IAF.

Measure the smallest magnetic fields with diamond and laser

In the project “NV-doped CVD Diamond for Ultra-Sensitive Laser Threshold Magnetometry” (abbreviation “DiLaMag”), Jeske and his team are researching a globally unique approach to highly sensitive quantum magnetic field sensors. For the first time, they use diamond in a laser system, allowing much more precise magnetic field measurements.

For the project, the diamond is doped with a high density of nitrogen vacancy centers (NV centers). “Due to its material properties, diamond with a high density of NV centers can significantly improve measurement accuracy when used as a laser medium,” says Jeske. NV centers in diamond are atomic systems consisting of a nitrogen atom and a carbon defect. They absorb green light and emit red light. Since the fluorescence of these atomically small NV centers depends on the strength of an external magnetic field, they can be used to measure magnetic fields with high local resolution and good sensitivity.

World's first measurement of magnetic field-dependent stimulated emission

The diamond sample has a high NV concentration after irradiation, which is responsible for the pink color. Credit: Fraunhofer-Institut für Angewandte Festkörperphysik IAF

First experimental demonstration of laser threshold magnetometry

After several years of research effort, Jeske’s team has reached an important milestone: they have demonstrated the world’s first measurement of magnetic field-dependent stimulated emission. In the process, the researchers made an interesting discovery: “We observed a very relevant and previously unknown physical process in NV diamond: the absorption of red light induced by green laser irradiation,” reports Jeske.

By using NV diamond as the laser medium, they not only achieved a 64% amplification of signal power by stimulated emission. The project team was even able to set a world record: the magnetic field-dependent emission shows a contrast of 33% and maximum output power in the mW regime. This is a new contrast record in magnetometry with NV sets.

Stimulated emission is responsible for this. “We were able to show that this record would not have been possible with spontaneous emission. Thus, we have experimentally demonstrated for the first time the theoretical principle of laser threshold magnetometry”, emphasizes Jeske.

These results also show the advantages of diamond-based laser threshold magnetometry over conventional methods and prove that it is possible to measure the smallest magnetic fields.

Great progress in the production of NV diamonds

The concept of laser threshold magnetometry only works if the diamond has a very high density of NV centers while maintaining good optical properties. For this reason, the project team did extensive material work to optimize the diamond accordingly. This work includes, on the one hand, diamond production by CVD (chemical vapor deposition) and, on the other hand, post-processing by electron irradiation and heat treatment for an increase in NV density.

During diamond growth by CVD, which allows a very precise and controlled integration of the NV centers, the researchers have already been able to achieve a strong nitrogen doping. Using electron irradiation, they then determined an optimal fluence for nitrogen density, which resulted in a 20- to 70-fold increase in NV density. The absorption spectra allowed them to follow the formation of NV centers live.

During the characterization, they established the correlations between three crucial factors for optimal NV sets and optimized them: high NV density, high conversion of substituted nitrogen using high fluence irradiation and charge stability. high. As a result of these detailed studies, the Fraunhofer IAF team succeeded for the first time in producing CVD diamond with a high density of NV centers and good quality, thus creating the prerequisite for the development of laser threshold magnetometry. diamond-based for measuring extremely small magnetic fields.

Toward Superior Nanoscale Detection and Imaging with Optimized Diamond Probes

More information:
Felix A. Hahl et al, Magnetic field-dependent stimulated emission from nitrogen vacancy centers in diamond, Scientists progress (2022). DOI: 10.1126/sciadv.abn7192

T Luo et al, Creation of Nitrogen Void Centers in Chemical Vapor Deposition Diamond for Sensing Applications, New Journal of Physics (2022). DOI: 10.1088/1367-2630/ac58b6

Julia Langer et al, Manipulation of the In Situ Nitrogen-Vacancy Doping Efficiency in CVD-Grown Diamond, solid physical state (a) (2022). DOI: 10.1002/pssa.202100756

Provided by Fraunhofer-Institut für Angewandte Festkörperphysik IAF

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