Mode-Splitting and Induced Transparency Observed in 3D Plasma Photonic Crystals in the 100-170 GHz Region*

A class of 3D plasma photonic crystals (PPCs) is being developed for communications and sensing applications in the 100-500 GHz spectral region. The dependence of the refractive index of low-temperature plasma on electron and neutral gas density, as well as the ability to switch plasmas at electronic speeds, makes PPCs of interest as tunable electromagnetic filters, phase shifters, interferometers, and other devices. We report mode-splitting and induced transparency observed in the 100-170 GHz interval for PPCs comprising 355 ± 10 µm diameter microplasma columns arranged in a precise, 8-layer “woodpile” structure. Peak electron densities of ~10¹⁶ cm^-3 generated in independently-addressable microcapillaries enable resonant modes of the crystals to be blue-shifted by as much as 1.6 GHz, and Q values to increase to >5000. In a PPC in which one layer comprises metal microcolumns, mode splitting of ~2 GHz has been observed near 138 GHz by generating microplasma in 4 layers of the crystal. The ability to introduce low temperature plasma, metals, and dielectrics and other materials into artificial crystals of widely-differing geometries yields a class of electromagnetic devices not available previously, but well-suited for multichannel communications and millimeter-wave spectroscopy.