Sort by
Frequency-swept dynamic nuclear polarization

Dynamic nuclear polarization (DNP) improves the sensitivity of NMR spectroscopy by the transfer of electron polarization to nuclei via irradiation of electron-nuclear transitions with microwaves at the appropriate frequency. For fields > 5 T and using g ∼ 2 electrons as polarizing agents, this requires the availability of microwave sources operating at >140 GHz. Therefore, microwave sources for DNP have generally been continuous-wave (CW) gyrotrons, and more recently solid state, oscillators operating at a fixed frequency and power. This constraint has limited the DNP mechanisms which can be exploited, and stymied the development of new time domain mechanisms. We report here the incorporation of a microwave source enabling facile modulation of frequency, amplitude, and phase at 9 T (250 GHz microwave frequency), and we have used the source for magic-angle spinning (MAS) NMR experiments. The experiments include investigations of CW DNP mechanisms, the advantage of frequency-chirped irradiation, and a demonstration of an Overhauser enhancement of ∼25 with a recently reported water-soluble BDPA radical, highlighting the potential for affordable and compact microwave sources to achieve significant enhancement in aqueous samples, including biological macromolecules. With the development of suitable microwave amplifiers, it should permit exploration of multiple new avenues involving time domain experiments.

Relevant
A Low-Loss Silicon MEMS Phase Shifter Operating in the 550-GHz Band

This article presents a low-loss silicon microelectrical mechanical system (MEMS) phase shifter operating in the 500-600 GHz band. The phase shifter consists of a 30-μm thick perforated silicon slab that is moved in and out of a waveguide in the E-plane with a large deflection MEMS actuator. By implementing different hexagonal patterns in the silicon slab, a stepped permittivity is created to impedance match, and thus, reduce return loss. When the silicon slab is inserted into the waveguide, the phase velocity of the incoming wave is decreased, thus resulting in different phase shifts depending on the position of the slab inside the waveguide. The MEMS phase shifter is fully actuated at around 50 V and can move up to ±95 μm, depending on the applied voltage. The insertion loss, when the maximum phase shift is achieved, is measured to be 1.8 dB, compared to a 1.6-dB insertion loss for a waveguide of equivalent length. The return loss is better than 18 dB for the desired band. The measured phase shift, with the slab fully inserted into the waveguide at 550 GHz was 145 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> . The MEMS phase shifter enables a variety of applications including phased array antenna systems with scanning capability for mapping of planetary surfaces with an electronically steerable antenna.

Relevant