Wave Transmission Through a Plasma-Loaded W-Band Photonic Crystal

A microplasma is sustained by millimeter waves (MMWs) at 93.37 GHz within the vacancy of a photonic crystal (PhC). The resonance of the electric field inside the vacancy creates a free-floating spherical plasma with electron density on the order of $10^{20}~\text{m}^{-3}$ . The steady-state plasma is less than 1 mm in diameter and requires only 100–600 mW of power. By probing the microplasma using an auxiliary, low-power source from 92–96 GHz, the spectrum of power absorption by the plasma is reported. The power absorption spectra shift upward in frequency due to the relative permittivity of the plasma being less than one. In combination with electromagnetic modeling, the upward frequency shift is used to extract the electron density in the central core of the plasma. The absorption spectra show a secondary absorption peak that is attributed to electron plasma resonance heating of the discharge when the plasma frequency is approximately 93 GHz.