Longitudinal Pulsed Dynamic Nuclear Polarization Transfer via Periodic Optimal Control

TL;DR

This paper introduces a new family of broadband DNP pulse sequences called LOOP, which use optimal control to achieve robust longitudinal polarization transfer with high bandwidth and low microwave power at 0.35 T.

Contribution

The paper develops LOOP sequences that leverage optimal control and Hamiltonian insights to improve broadband DNP transfer, overcoming limitations of previous methods.

Findings

Achieves DNP bandwidths over 100 MHz.

Uses only 32 MHz peak microwave field.

Provides robust polarization transfer with inhomogeneity compensation.

Abstract

Taking inspiration from NMR spectroscopy, periodic irradiation schemes have recently shown remarkable performance when implemented into pulsed dynamic nuclear polarization (DNP) sequences. This has prompted considerable interest in development of broadband pulsed DNP sequences utilizing such schemes. On this background, most efforts have focused on solid-state NMR like transverse spin-locked pulse sequences whose performance in DNP applications may be compromised by the broadband capabilities of the initial excitation pulse. Leveraging the flexibility and robustness of optimal control theory combined with underlying insights from effective Hamiltonian theory, we present a new family of broadband DNP pulse sequences, termed LOOP (Longitudinally Optimized with Overarching Periodicity), that alleviates the excitation-pulse challenge by accomplishing longitudinal polarization transfer. These sequences define robust single-spin effective $z$ rotations, with impressive compensation towards microwave field inhomogeneity, and are capable of delivering DNP transfer with bandwidths exceeding 100 MHz, while employing a peak microwave field amplitude of only 32 MHz, at an external magnetic field of 0.35 T.