Multi-resonant photonic band-gap/saddle coil DNP probehead for static solid state NMR of microliter volume samples

• Published in: Journal of magnetic resonance (1997) 2018-12, Vol.297, p.113-123
• Main Authors: Nevzorov, Alexander A., Milikisiyants, Sergey, Marek, Antonin N., Smirnov, Alex I.
• Format: Article
• Language: English
• Subjects: DNP; Dynamic nuclear polarization; Photonic band gap resonator; Quasioptics; Solid-state NMR; Synthetic diamonds
• ISSN: 1090-7807; 1096-0856; 1096-0856
• DOI: 10.1016/j.jmr.2018.10.010

[Display omitted] •A frequency-agile 200 GHz/300 MHz DNP NMR spectrometer was constructed.•The spectrometer is based on solid-state mm-wave devices and quasioptical components.•200 GHz photonic band gap resonator was integrated into double-tuned NMR saddle coil.•13C DNP enhancement of ca. 1500 was obtained in synthetic diamond crystals at room temperature.•Resonator probehead provides the same electronic B1 at up to 11 dB lower incident power. The most critical condition for performing Dynamic Nuclear Polarization (DNP) NMR experiments is achieving sufficiently high electronic B1e fields over the sample at the matched EPR frequencies, which for modern high-resolution NMR instruments fall into the millimeter wave (mmW) range. Typically, mmWs are generated by powerful gyrotrons and/or extended interaction klystrons (EIKs) sources and then focused onto the sample by dielectric lenses. However, further development of DNP methods including new DNP pulse sequences may require B1e fields higher than one could achieve with the current mmW technology. In order to address the challenge of significantly enhancing the mmW field at the sample, we have constructed and tested one-dimensional photonic band-gap (PBG) mmW resonator that was incorporated inside a double-tuned radiofrequency (rf) NMR saddle coil. The photonic crystal is formed by stacking ceramic discs with alternating high and low dielectric constants and thicknesses of λ/4 or 3λ/4, where λ is the wavelength of the incident mmW field in the corresponding dielectric material. When the mmW frequency is within the band gap of the photonic crystal, a defect created in the middle of the crystal confines the mmW energy, thus forming a resonant structure. An aluminum mirror in the middle of the defect has been used to substitute one-half of the structure with its mirror image in order to reduce the resonator size and simplify its tuning. The latter is achieved by adjusting the width of the defect by moving the aluminum mirror with respect to the dielectric stack using a gear mechanism. The 1D PBG resonator was the key element for constructing a multi-resonant integrated DNP/NMR probehead operating at 190–199 GHz EPR/300 MHz 1H/75.5 MHz 13C NMR frequencies. Initial tests of the multi-resonant DNP/NMR probehead were carried out using a quasioptical mmW  bridge and a Bruker Biospin Avance II spectrometer equipped with a standard Bruker 7 T wide-bore 89 mm magnet parked at 300.13 MHz 1H NMR frequency. The mmW bridge bu