Abstract
Microplasma generated within a millimeter wave (MMW) photonic crystal (PhC) is analyzed by direct measurement of the electron density and the wave transmission spectra (S21) of the crystal. A continuous wave (cw) drive frequency of 43.66 GHz maintains plasma, while a low power frequency sweep (43.5–44.1 GHz) simultaneously probes the wave transmission in the presence of microplasma. Rotational gas temperature and electron density are measured from the CH emission spectrum and the Stark broadening of the Hβ atomic transition. The permittivity of the plasma depends on the electron density. As higher cw power and argon gas pressure increase the electron density in the PhC vacancy, the resonant frequency of the PhC shifts upward in accordance with the measured electron density and plasma permittivity. As the PhC obscures the central core of the microplasma, we present a simple diffusion model that approximates the electron density distribution within the microplasma. The diffuse electron density is then used in a numerical model for S21(ω). The measured and modeled transmission spectra can only be reconciled using the diffusion density gradient of the microplasma, thus resolving previous discrepancies observed when using simplified, abrupt-boundary plasma models.
Highlights
Microplasmas have been a growing area of interest due to their attractive features, such as high electron density and discharge stability at gas pressures up to at least 1atm.1,2 Microwave driven microplasma can be ignited and sustained by using a power of less than 1 W.3–6 High electron density (>1019 m−3) is attributed to distinct heating mechanisms for electrons at higher excitation frequencies
Microplasma generated within a millimeter wave (MMW) photonic crystal (PhC) is analyzed by direct measurement of the electron density and the wave transmission spectra (S21) of the crystal
The outline of this paper is as follows: We experimentally demonstrate the manipulation of MMWs by directly measuring the wave transmission spectra (S21) in the presence of microplasma in a photonic crystal
Summary
Microplasmas have been a growing area of interest due to their attractive features, such as high electron density and discharge stability at gas pressures up to at least 1atm. Microwave driven microplasma can be ignited and sustained by using a power of less than 1 W.3–6 High electron density (>1019 m−3) is attributed to distinct heating mechanisms for electrons at higher excitation frequencies. This work focuses on the properties of a plasma that is ignited and sustained by millimeter wave radiation at 43.66 GHz. Using plasma to manipulate the electromagnetic transmission properties of a photonic crystal has been the subject of research for more than a decade. Using plasma to manipulate the electromagnetic transmission properties of a photonic crystal has been the subject of research for more than a decade As plasma properties such as electron density and collision frequency are readily adjustable, the plasma-loaded PhCs possess tunable characteristics with a fixed mechanical structure.. The outline of this paper is as follows: We experimentally demonstrate the manipulation of MMWs by directly measuring the wave transmission spectra (S21) in the presence of microplasma in a photonic crystal. This improved plasma model reconciles the differences between previously reported volume-averaged models and the electron density determined by Stark measurements
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