Longitudinal Pulsed Dynamic Nuclear Polarization Transfer via Periodic Optimal Control

Pulsed dynamic nuclear polarization (DNP) is gradually emerging to enhance the performance of prevailing continuous-wave DNP which boosts NMR sensitivity by transferring the much higher unpaired electron polarization to nuclear spins. In pulsed operation, taking inspiration from NMR spectroscopy, periodic irradiation schemes have recently shown remarkable performance. Most efforts have focused on solid-state NMR sequences within the conventional paradigm of transverse spin-locked Hartmann-Hahn matching, whose performance in DNP applications may be compromised by the initial excitation pulse. Leveraging the flexibility 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 alleviate the excitation-pulse challenge by accomplishing longitudinal polarization transfer. These sequences define robust single-spin effective <i>z</i> rotations, with impressive compensation toward 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.