Category Archives: quasi-optics

Field Guide to Terahertz Sources, Detectors, and Optics

I recently came across this very handy and useful publication. It is helpful to anyone who wants to understand the design of quasi-optical elements, which are for example used in many high-field EPR spectrometers.

O’Sullivan, Créidhe M., and J. Anthony Murphy. Field Guide to Terahertz Sources, Detectors, and Optics. SPIE, 2012.

https://doi.org/10.1117/3.952851.

The region of the electromagnetic spectrum between microwaves and infrared radiation has come to be known as the “THz gap,” mainly due to the lack of readily available laboratory sources and detectors. For many years technology development was driven by astronomers and planetary scientists, but other potential uses, particularly in medical and security applications, have led to increased activity by the mainstream physics and engineering community in recent times. Because diffraction is important at these frequencies, THz systems cannot be successfully designed using traditional optical techniques alone.

The primary objective of this Field Guide is to provide the reader with a concise description of the quasi-optical techniques used at THz frequencies, as well as the basic principles of operation of the most common THz system components in use today. More detailed accounts of specific devices can be found in the bibliography and references therein.

A quasi-optical and corrugated waveguide microwave transmission system for simultaneous dynamic nuclear polarization NMR on two separate 14.1 T spectrometers #DNPNMR

Gyrotrons typically generate much more microwave power than needed in a DNP-NMR experiment. This article describes a very nice way how to share the microwave power generated by a single gyrotron between two NMR experiments.

Dubroca, T., et al., A quasi-optical and corrugated waveguide microwave transmission system for simultaneous dynamic nuclear polarization NMR on two separate 14.1 T spectrometers. J. Magn. Reson., 2018.

https://www.sciencedirect.com/science/article/pii/S1090780718300387

Nuclear magnetic resonance (NMR) is an intrinsically insensitive technique, with Boltzmann distributions of nuclear spin states on the order of parts per million in conventional magnetic fields. To overcome this limitation, dynamic nuclear polarization (DNP) can be used to gain up to three orders of magnitude in signal enhancement, which can decrease experimental time by up to six orders of magnitude. In DNP experiments, nuclear spin polarization is enhanced by transferring the relatively larger electron polarization to NMR active nuclei via microwave irradiation. Here, we describe the design and performance of a quasi-optical system enabling the use of a single 395 GHz gyrotron microwave source to simultaneously perform DNP experiments on two different 14.1 T (1H 600 MHz) NMR spectrometers: one configured for magic angle spinning (MAS) solid state NMR; the other configured for solution state NMR experiments. In particular, we describe how the high power microwave beam is split, transmitted, and manipulated between the two spectrometers. A 13C enhancement of 128 is achieved via the cross effect for alanine, using the nitroxide biradical AMUPol, under MAS-DNP conditions at 110K, while a 31P enhancement of 160 is achieved via the Overhauser effect for triphenylphosphine using the monoradical BDPA under solution NMR conditions at room temperature. The latter result is the first demonstration of Overhauser DNP in the solution state at a field of 14.1 T (1H 600 MHz). Moreover these results have been produced with large sample volumes (∼100 µL, i.e. 3 mm diameter NMR tubes).

Effect of electron spectral diffusion on static dynamic nuclear polarization at 7 Tesla #DNPNMR

Leavesley, A., et al., Effect of electron spectral diffusion on static dynamic nuclear polarization at 7 Tesla. Phys. Chem. Chem. Phys., 2017. 19(5): p. 3596-3605.

https://www.ncbi.nlm.nih.gov/pubmed/28094364

Here, we present an integrated experimental and theoretical study of 1H dynamic nuclear polarization (DNP) of a frozen aqueous glass containing free radicals at 7 T, under static conditions and at temperatures ranging between 4 and 20 K. The DNP studies were performed with a home-built 200 GHz quasi-optics microwave bridge, powered by a tunable solid-state diode source. DNP using monochromatic and continuous wave (cw) irradiation applied to the electron paramagnetic resonance (EPR) spectrum of the radicals induces the transfer of polarization from the electron spins to the surrounding nuclei of the solvent and solutes in the frozen aqueous glass. In our systematic experimental study, the DNP enhanced 1H signals are monitored as a function of microwave frequency, microwave power, radical concentration, and temperature, and are interpreted with the help of electron spin-lattice relaxation times, experimental MW irradiation parameters, and the electron spectral diffusion (eSD) model introduced previously. This comprehensive experimental DNP study with mono-nitroxide radical spin probes was accompanied with theoretical calculations. Our results consistently demonstrate that eSD effects can be significant at 7 T under static DNP conditions, and can be systematically modulated by experimental conditions.

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