Low-field nuclear magnetic resonance (NMR) spectroscopy ($<1\mathrm{T}$) often suffers from reduced sensitivity due to low nuclear spin polarization, magnetic field inhomogeneities, and poor chemical shift resolution, compared to high-field systems, and the low Larmor frequency at low field limits the achievable quality factor ($Q$) of NMR detection circuits. These factors may result in poor detection fidelity and broad spectral linewidths that obscure details about the chemical environment being analyzed. Radiofrequency amplification by stimulated emission of radiation (RASER) is a radiation damping feedback mechanism between a strongly coupled nuclear spin system and a high-$Q$ resonator, which may yield linewidths orders of magnitude narrower than is achievable using conventional acquisition protocols at low magnetic field strengths. Here, we present a simple theoretical description of RASER and practical implementation details for a high-$Q$ resonator. We demonstrate a repeatable acquisition protocol for hyperpolarized ${}^{129}\mathrm{Xe}$ gas RASER at 1.7 mT, achieving sufficiently narrow linewidths to resolve the chemical shift separation between ${}^{129}\mathrm{Xe}$ gas phase and dissolved phase.

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• Masers
• Spontaneous emission
• Noble gases
• Cavity resonators
• Medical magnetic resonance imaging
• Nuclear magnetic resonance
• Optical pumping
• Radio frequency techniques