Optimization of Resonator Quality Factor and SNR for RASER Observation at Ultra-Low Magnetic Fields

Low-field (<50 mT) NMR spectroscopy and MRI have recently gained popularity because of their low instrumentation cost, compatibility with metal implants, and portability. However, spectral resolution and signal-to-noise ratio (SNR) at these fields are limited by reduced chemical shift separation, spectrum polluting RF interference noise sources, and the relatively lower homogeneity achievable with electromagnetic or permanent magnets.

Radio-frequency Amplification by Stimulated Emission of Radiation (RASER) has been proposed as a strategy to recover spectral separation at low magnetic fields. Analogous to a laser, a RASER produces self-sustaining, coherent radio-frequency emission from nuclear spins with an inverted population coupled to a high-quality resonator. This effect drives the ensemble of spins into phase coherence and generates ultra-narrow spectral lines far sharper than those limited by transverse relaxation. The observation of RASER depends on the balance between transverse relaxation, the resonator quality factor (Q), and the degree of spin polarization (M_o​). Because of this, the effect requires high resonator Q factors not typically seen at ultra-low field strengths and calls for improved optimization of circuit components.

The improvement of Q and subsequently SNR is highly dependent on hardware at lower field strengths. To increase the circuit’s performance capability and move towards observing RASER, electronic components were selected and optimized based on material and operating characteristics. Changes to the solenoid wire type and circuit layout were made to existing NMR coils and tested for both quality factor increases and noise level reduction. Here we present the optimization of NMR primary resonators to achieve unprecedented quality factors in excess of 100, and an improved signal-to-noise ratio of >1500. These improved values are a step towards observing RASER at ultra-low field.