A 263GHz 32-Channel EPR-on-a-Chip Injection-Locked VCO-Array

Electron paramagnetic resonance (EPR) spectroscopy is a powerful technique that uses the spin of an unpaired electron as a nanoscopic probe inside a molecule to extract information about its chemical structure and composition as well as its surroundings via small changes in its resonance frequency, see Fig. 21.4.1 (left). EPR has applications in studying and monitoring a wide range of materials, starting from defects in semiconductors over free radicals in the blood to metal catalysts in hydrogen fuel production. EPR measurements are typically performed in moderate static magnetic fields around 0.3T, corresponding to EPR frequencies around
$9\mathsf{GHz}$
. This combination of field and frequency is commonly used due to the relative ease of generation of 0
$3\mathsf{T}$
magnetic fields with sufficient homogeneity as well as the required
$9\mathsf{GHz}$
frequency signal. However, access to higher frequencies and fields is strongly desirable due to higher spin polarization (with corresponding increased signal amplitude), higher spectral resolution, which allows, e.g., the determination of electronic and geometric structures of active centers in enzymes, and access to higher energy electronic transitions, such as those found in metal complexes or materials interesting for antiferromagnetic spintronics, explaining the ongoing research towards high-frequency EPR (HFEPR) with operating frequencies beyond
$90\mathsf{GHz}$
. Here, another strong driver of HFEPR is the field of dynamic nuclear polarization (DNP), a method that can be used to boost the relatively poor sensitivity of nuclear magnetic resonance (NMR) by transferring the intrinsically higher electron polarization to the nuclear spins. Today, there are only a few commercial HFEPR-based DNP spectrometers on the market, with the most popular ones operating at an EPR frequency of
$263\mathsf{GHz}(9.4\mathsf{T})$
, corresponding to a proton NMR frequency of
$400\mathsf{MHz}$
. These instruments use gyrotron as the source for the mm-wave magnetic field (
$\mathsf{B}_{1}$
field) and cost far beyond 1 million €.