Spectroscopic diagnostics of continuous and transient microplasma formed in a millimeter wave photonic crystal

We present spectroscopic measurements of a microplasma formed within a photonic crystal (PhC) at argon gas pressures up to 750 Torr. Continuous and pulsed millimeter waves (MMW) generate plasma at a vacancy defect within the crystal. Rotational gas temperatures of 300–1050 K and electron densities of 0.3–1.5 × 10²⁰ m⁻³ are obtained from the CH rotational emission spectrum and from Stark broadening of the $H_{\beta }$ atomic transition. Electron density is found to be weakly dependent on MMW power because the plasma expands within the defect volume at higher power. Pulsed MMWs slightly increase the peak electron density compared to continuous waves with the same peak power. With MMW pulses, a rapid decrease of wave transmission through the PhC is observed at the beginning of plasma formation. During plasma ignition two distinct plasmas are formed, one on the conducting wall of the PhC near the optical access port and a second within the center of the PhC defect. With longer periods between pulses, the weak wall plasma is observed to form before the principal plasma in the defect, which aids in initiating the main plasma. (paper)