Optical Chopper for LOngitudinal-Detected (LOD) EPR

Dynamic nuclear polarization (DNP) is a nuclear magnetic resonance (NMR) hyperpolarization technique that mediates polarization transfer from unpaired electrons to nuclear spins. DNP performance can vary significantly depending on the types of polarizing agents employed, and the criteria for optimum DNP efficiency are not fully understood. Thus, a better understanding of the structure, electron paramagnetic resonance (EPR) linewidths, and relaxation properties would aid in designing more efficient DNP polarizing agents. However, EPR characterizations of the polarizing agents are typically performed in different environments (e.g., strength of magnetic field and microwave power) than typical DNP experiments. Here, we demonstrate a low-cost and home-built setup that enables in-situ EPR detection in a dual resonance DNP-NMR/EPR probe using an optical chopper. The chopper modulates the microwave irradiation, thereby modulating the longitudinal magnetization (Mz) of the electron spins. Our results of DNP and EPR spectra on TEMPOL using a solid-state microwave source at 6.7 T / 188 GHz and 4.2 K showed a good agreement. In principle, an optical chopper should be compatible with a wide range of microwave sources, including gyrotrons that output high-power microwaves. To verify this, we placed an optical chopper in between the waveguides of a 527 GHz gyrotron and successfully reproduced a DNP field profile similar to the case without a chopper. Hence, our work provides a proof-of-principle setup that could enable a gyrotron-based EPR spectrometer in the future.