Frequency self-upshifting of intense microwave radiation producing ionization in a thin gaseous layer

The mechanism of frequency upshifting of an electromagnetic wave incident on a thin (compared to the wavelength) layer of a time-varying plasma, which was found recently within the linear parametric consideration [Phys. Plasmas 8, 2987 (2001)], is extended to the nonlinear case when ionization in the layer is produced by the incident wave itself due to breakdown process in the gas. The frequency self-upshifting is achieved for the oblique incidence of a powerful p-polarized wave and connected with the transient excitation of free plasma oscillations in the layer at the moment when the arising plasma density passes the critical value and, thus, plasma resonance in the layer occurs. The excited plasma oscillations, whose frequency follows growing plasma frequency, sustain a high value of the electric field in the layer even after the plasma frequency exceeds the frequency of the incident wave. This and the electron temperature inertia provide fast creation of a highly overdense plasma. Gradual radiation of the oscillations gives rise to the upshifted outgoing electromagnetic waves. The phenomenon is considered both analytically and numerically. Concrete estimations of the effect for a cm range microwave dicharge in the air of 0.1–1 Torr pressure are given.