Researchers from the Fraunhofer Institute for Applied Solid State Physics (Fraunhofer IAF) demonstrated the principle of laser threshold magnetometry for the first time, they said. The Fraunhofer team is working to develop diamond-based laser threshold magnetometry — a practice that enables measurement of even the smallest magnetic fields. The application can be used in medical care, where measures of the typically weak magnetic fields of heart and brain activity, for example, are gauged as images in magnetic resonance imaging (MRI) systems to detect diseases at an early stage. The team achieved measurement of magnetic-field-dependent stimulated emission and set a record for contrast. Currently, only a few highly sensitive magnetic field sensors achieve the necessary precision for these magnetic field measurements. According to the researchers, each of these sensors presents technical obstacles for clinical application. Superconducting quantum interference device (SQUID) sensors require complex cryogenic cooling. Other magnetometers, such as vapor cell magnetometers, which are optically pumped, require absolute shielding of all background fields, including Earth's magnetic field. This places structural requirements on the rooms and buildings in which they are to be used. As a result, electric measurements continue to be common in everyday clinical practice, even though they are less accurate than the optical pump approach. The Fraunhofer researchers aim to develop an extremely sensitive magnetic field sensor that works both at room temperature as well as in the presence of background fields, making it applicable for clinical use. As part of project “NV-doped CVD diamond for ultra-sensitive laser threshold magnetometry” (DiLaMag), the researchers — led by Fraunhofer IAF project manager Jan Jeske — used diamond in a laser system to enable considerably more precise magnetic field measurements. “Due to its material properties, diamond with a high density of NV centers can vastly improve measurement precision when used as a laser medium,” Jeske said. 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 high sensitivity. The diamond sample has a high NV concentration after irradiation, which results in a pink color. Fraunhofer IAF researchers used diamond with a high density of NV centers as the lasing medium to demonstrate laser threshold magnetometry, which supports the ability of medical personnel to measure the magnetic fields of organ activity to make medical diagnostics. Courtesy of Fraunhofer IAF. After achieving the measurement of magnetic-field-dependent stimulated emission, the researchers observed the absorption of red light induced by green laser irradiation. By using an NV diamond as a laser medium, they achieved a 64% amplification of the signal power by stimulated emission. The team reported magnetic-field-dependent emission that showed a contrast of 33% and a maximum output power in the milliwatt regime — the contrast record in magnetometry with NV ensembles. The researchers said the concept of laser threshold magnetometry works only if the diamond has a very high density of NV centers while retaining strong optical properties. As a result, the work has necessitated producing diamond by chemical vapor deposition (CVD), as well as post-processing by electron irradiation and temperature treatment to increase NV density. This facet of the work enabled the team to produce a CVD diamond with a high density of NV centers of a high quality, which they said is a prerequisite to the development of diamond-based laser threshold magnetometry for the measurement of extremely small magnetic fields. RMIT University in Australia, the National Institutes for Quantum and Radiological Science and Technology in Japan, and the College of Staten Island contributed to the to the scientific publication. The German Ministry for Education and research supports the five-year project. The research was published in Science Advances (www.doi.org/10.1126/sciadv.abn719).