Radiofrequency amplification by stimulated emission of radiation using nuclear spin polarized ¹²⁹Xe

Ultra-low field nuclear magnetic resonance (NMR) spectroscopy is a low-cost and accessible modality for studying nuclear spin interactions and elucidating chemical structure at magnetic field strengths comparable to a refrigerator magnet; however, it suffers from low spectral resolution due to the weak magnetic field strength. Typically, NMR resolution is limited by the linewidth of the detected signal, which is inversely proportional to the dephasing time of the nuclear spins. An emerging solution to improve spectral resolution is radiofrequency amplification by stimulated emission of radiation (RASER). Analogous to LASER, RASER depends on the coupling of an inverted spin population to a high quality-factor (Q) resonator. Under these conditions, the NMR signal can persist for orders of magnitude longer, limited by the spin population lifetime rather than the dephasing time. Using a lab-designed and built high-Q resonator, we observed RASER with hyperpolarized ¹²⁹Xe gas at ultra-low magnetic field (<2 mT). The RASER signal persisted for over 30 s, yielding a 0.5 Hz linewidth, in contrast to our typical NMR signal lifetimes of less than 500 ms with 10 Hz linewidths. This enhancement improves spectral resolution and enables future work to separate previously unresolvable chemical structure at ultra-low field.