Enhanced dynamic nuclear polarization via swept microwave frequency combs

TL;DR

This paper introduces a swept microwave frequency comb technique to significantly enhance dynamic nuclear polarization efficiency, enabling higher nuclear spin polarization levels in magnetic resonance applications.

Contribution

The authors develop a novel swept microwave frequency comb method that overcomes slow sweep limitations in DNP, achieving multiplicative enhancement gains.

Findings

DNP enhancement increased from 30 to 100 in microdiamonds.

Multiplicative gains exceeding an order of magnitude possible with broad linewidth radicals.

Technique scalable with the number of comb frequencies and electron linewidth.

Abstract

Dynamic Nuclear Polarization (DNP) has enabled enormous gains in magnetic resonance signals and led to vastly accelerated NMR/MRI imaging and spectroscopy. Unlike conventional cw-techniques, DNP methods that exploit the full electron spectrum are appealing since they allow direct participation of all electrons in the hyperpolarization process. Such methods typically entail sweeps of microwave radiation over the broad electron linewidth to excite DNP, but are often inefficient because the sweeps, constrained by adiabaticity requirements, are slow. In this paper we develop a technique to overcome the DNP bottlenecks set by the slow sweeps, employing a swept microwave frequency comb that increases the effective number of polarization transfer events while respecting adiabaticity constraints. This allows a multiplicative gain in DNP enhancement, scaling with the number of comb frequencies and limited only by the hyperfine-mediated electron linewidth. We demonstrate the technique for the optical hyperpolarization of 13C nuclei in powdered microdiamonds at low fields, increasing the DNP enhancement from 30 to 100 measured with respect to the thermal signal at 7T. For low concentrations of broad linewidth electron radicals, e.g. TEMPO, these multiplicative gains could exceed an order of magnitude.