Experimental observation of a radio frequency plasmonic gradient resonance in water

Electromagnetic properties of water and water interfaces in the radio frequency range have fundamental importance in such diverse fields of science and technology as physics, chemistry, life sciences and wireless communications. Despite their importance, many aspects of these properties remain controversial or unresolved, especially where it concerns water response to radio frequency electromagnetic fields inside biological matter or near biological interfaces. Here we note that in the radio frequency range water belongs to a class of electromagnetic materials in which the real part of their complex dielectric permittivity-squared approximately equals zero. We demonstrate that surface electromagnetic waves in such materials may exhibit a "plasmonic gradient resonance", which leads to highly unusual propagation properties of radio fields. Based on our computations, the plasmonic gradient resonance in water may be observed in the frequency range from 2 MHz to 20 GHz, depending on the interface properties and water salinity. Our detailed radio communication experiments underwater indeed reveal such a resonant field behavior at the frequency of 50 MHz at 0.25 S/m water conductivity. The anomalous propagation properties of surface electromagnetic waves at resonance result in extremely large penetration distances and depths of radio fields underwater, reaching hundreds and in some cases thousands of skin depths. Similar resonances should be observable in a very broad range of materials and electromagnetic frequencies, from radio communication underground and underwater, to super-resolution radio frequency biosensing and bioimaging, and silicon ultraviolet nanophotonics.