EUROISMAR 2019

Water and other polar molecules are known to absorb electromagnetic radiation and the absorption is particularly strong in the mm-Wave (mmW) range. Metal surfaces are also becoming increasingly lossy. These high dielectric losses represent the major challenge for constructing EPR and also DNP NMR probeheads suitable for accommodating samples with the maximum volume. While large samples can be fitted into non-resonant mmW structures, resonator cavities offer significantly higher mm-wave B1 fields – an essential condition for DNP. High-Q resonators also provide the best EPR concentration sensitivity at X- and Q-band. Last year we described a radically new line of EPR resonators that are based on one-dimensional photonic band gap (PBG) dielectric crystals. PBG crystals were assembled from λ/4 low-loss dielectric layers with alternating dielectric constants and demonstrated experimental Q≈520 at 94.3 GHz. Anodic aluminum oxide nanoporous disc of 50 μm in thickness was employed as an aqueous sample holder allowing for ca. 2-3 μl sample volume. Here we report on significant improvements of PBG EPR resonators in both sample volume and experimental Q-factors while minimizing dielectric losses even for liquid aqueous samples. A series of smooth and corrugated 95 GHz transitions were tested to improve flatness of the mmW front. The resonator Q-factor was further improved by increasing the sample diameter from 12 to 36 mm yielding 9-fold sample volume increase. The best experimental Q≈3,300 has been observed for 8 alternating λ/4 layers of alumina and air. Experimental tests of the new resonators for aqueous and thin film samples are also reported. Finesse of 200 GHz PBG resonator for 300 MHz (1H) DNP was improved by forming photonic crystals from dielectric layers with high ε_{1/ε2 ratio. Supported by NIH R21EB024110 and R01GM130821.}