Chen Pin-Hui, Gao Chukun, Barnes Alexander B
Department of Chemistry, Washington University in St. Louis, St. Louis, MO 63130, USA; Department of Physics, Washington University in St. Louis, St. Louis, MO 63130, USA.
Department of Chemistry, Washington University in St. Louis, St. Louis, MO 63130, USA.
J Magn Reson. 2019 Nov;308:106518. doi: 10.1016/j.jmr.2019.07.005. Epub 2019 Jul 6.
Continuous wave dynamic nuclear polarization (DNP) increases the sensitivity of NMR, yet intense microwave fields are required to transition magic angle spinning (MAS) DNP to the time domain. Here we describe and analyze Teflon lenses for cylindrical and spherical MAS rotors that focus microwave power and increase the electron Rabi frequency, ν. Using a commercial simulation package, we solve the Maxwell equations and determine the propagation and focusing of millimeter waves (198 GHz). We then calculate the microwave intensity in a time-independent fashion to compute the ν. With a nominal microwave power input of 5 W, the average ν is 0.38 MHz within a 22 μL sample volume in a 3.2 mm outer diameter (OD) cylindrical rotor without a Teflon lens. Decreasing the sample volume to 3 μL and focusing the microwave beam with a Teflon lens increases the ν to 1.5 MHz. Microwave polarization and intensity perturbations associated with diffraction through the radiofrequency coil, losses from penetration through the rotor wall, and mechanical limitations of the separation between the lens and sample are significant challenges to improving microwave coupling in MAS DNP instrumentation. To overcome these issues, we introduce a novel focusing strategy using dielectric microwave lenses installed within spinning rotors. One such 9.5 mm OD cylindrical rotor assembly implements a Teflon focusing lens to increase the ν to 2.7 MHz within a 2 μL sample. Further, to access high spinning frequencies while also increasing ν, we analyze microwave coupling into MAS spheres. For 9.5 mm OD spherical rotors, we compute a ν of 0.36 MHz within a sample volume of 161 μL, and 2.5 MHz within a 3 μL sample placed at the focal point of a novel double lens insert. We conclude with an analysis and discussion of sub-millimeter diamond spherical rotors for time domain DNP at spinning frequencies >100 kHz. Sub-millimeter spherical rotors better overlap a tightly focused microwave beam, resulting in a ν of 2.2 MHz. Lastly, we propose that sub-millimeter dielectric spherical microwave resonators will provide a means to substantially improve electron spin control in the future.
连续波动态核极化（DNP）提高了核磁共振（NMR）的灵敏度，但要将魔角旋转（MAS）DNP转换到时间域需要强微波场。在此，我们描述并分析了用于圆柱形和球形MAS转子的聚四氟乙烯透镜，这些透镜可聚焦微波功率并提高电子拉比频率ν。使用商业模拟软件包，我们求解麦克斯韦方程并确定毫米波（198 GHz）的传播和聚焦情况。然后，我们以与时间无关的方式计算微波强度以得出ν。在没有聚四氟乙烯透镜的外径为3.2毫米的圆柱形转子中，标称微波功率输入为5瓦时，在22微升样品体积内平均ν为0.38兆赫。将样品体积减小到3微升并使用聚四氟乙烯透镜聚焦微波束可将ν提高到1.5兆赫。与通过射频线圈的衍射、穿透转子壁的损耗以及透镜与样品之间分离的机械限制相关的微波极化和强度扰动，是提高MAS DNP仪器中微波耦合的重大挑战。为克服这些问题，我们引入了一种新颖的聚焦策略，即使用安装在旋转转子内的介电微波透镜。一个这样的外径为9.5毫米的圆柱形转子组件采用聚四氟乙烯聚焦透镜，可在2微升样品内将ν提高到2.7兆赫。此外，为了在提高ν的同时获得高旋转频率，我们分析了微波与MAS球体的耦合情况。对于外径为9.5毫米的球形转子，我们计算出在161微升样品体积内ν为0.36兆赫，在置于新型双透镜插入件焦点处的3微升样品内ν为2.5兆赫。我们最后分析并讨论了用于旋转频率>100千赫的时间域DNP的亚毫米金刚石球形转子。亚毫米球形转子能更好地与紧密聚焦的微波束重叠，从而产生2.2兆赫的ν。最后，我们提出亚毫米介电球形微波谐振器将为未来大幅改善电子自旋控制提供一种手段。
