Optimization of THz wave coupling into samples in DNP/NMR spectroscopy

High power millimeter wave and terahertz sources are used in DNP/NMR spectroscopy to greatly enhance the NMR signal. A key issue is the efficient coupling of the source power to the sample. We present HFSS calculations showing that the MIT 250 GHz DNP/NMR system provides a B₁ value of 13 μT/W^1/2. The simulation shows the importance of the diffraction of radiation passing through the RF coil, leading to non-uniform illumination of the sample. A simple single pass model yields insight into the origin of the observed B₁ value. Methods of optimizing the B₁ value are described.