Microfilamentation in Optical-Field-Induced Ionization Process

Abstract

A plasma-resonance field-ionization instability of uniform gas breakdown produced by intense laser fields via tunneling ionization of atoms is studied theoretically and by computer simulation. The field amplitude and produced plasma are found to be unstable relative to spatial modulation in the direction of electric field with the spatial period shorter than the wavelength. In a dense gas the process, at the nonlinear stage of instability, becomes explosive and leads to the formation of thin resonance layers and sharp peaks of the field amplitude.[S0031-9007 (97)02954-2] PACS numbers: 52.40.Nk, 52.50.JmIt is well known that a powerful electromagnetic wave beam in a medium with positive (focusing) nonlinearity is subject to filamentation instability with a characteristic transverse scale large compared to the wavelength. This instability was predicted about thirty years ago based on the paraxial approximation for the scalar wave field [1]. Its manifestations were repeatedly observed in experiments with powerful laser and microwave pulses (see, for example, [2,3] and references therein).A less known fact is the existence of "vector" smallscale instability of the wave in a transparent medium with a "defocusing" (ionization-type) nonlinearity. This instability was originally described based on the vector wave equation [4], and studied theoretically and experimentally [5-7] as one of a wider class of ionizationfield (or electrodynamic) instabilities of high-frequency and microwave discharges in gases. Further, we will use the term "plasma-resonance ionization (PRI) instability," revealing the underlying physical mechanism of its generation. The PRI instability results in the filamentation of the wave and produced plasma with density gradients parallel to the wave electric field and with the spatial period shorter than the wavelength. So, it may be considered as an ionization analog of the known modulation instability of the field in a collisionless plasma with positive (ponderomotive-force-induced) nonlinearity. However, unlike the latter, it evolves not only in a narrow plasma resonance region (near the critical-density surface) but covers the entire transmission medium and affects significantly its electrodynamic characteristics.The PRI instability, probably, has not yet been observed in experiments with powerful ionizing laser pulses, since the majority of the experiments realized the electronimpact (avalanche-type) mechanism of gas ionization; the characteristic time of instability for this mechanism turns out to be longer than the time of the avalanche itself or the time of gas heating. However, advances in the generation of powerful laser pulses with field amplitudes comparable to atomic fields have stimulated interest in studies on the dynamics of the laser breakdown determined by opticalfield-induced (tunneling) ionization of gas atoms [8][9][10][11][12][13][14].The growth rate of the PRI instability with this ionization mechanism, as we find below, can be high enough so that, at the final stage of the breakdo...

Ionization Induced Scattering of Short Intense Laser Pulses

“…With the terawatt lasers now being used in a wide range of plasma experiments this intensity threshold can be achieved well before the beam reaches its focus. This ionization process leads to a number of interesting nonlinear phenomena, including frequency upshifting of the laser radiation by the moving ionization front [1-7], refractive defocusing of the laser pulse due to the radial inhomogeneity of the plasma electron density [8,9], and harmonic generation due to the nonlinear dependence of the ionization rate on the field amplitude [8].An additional effect that has received only a small amount of attention is the possible scattering of the radiation by the collective amplification of modulations of the electron density transverse to the initial direction of propagation of the laser pulse [10][11][12]. Such a transversely modulated density appears in the presence of transverse modulations of the laser amplitude due to the dependence of the ionization rate on field amplitude.…”

Ionization-induced dynamics of ultrashort laser pulses focused in a dense gas

“…The situation changes dramatically for high density gas, when plasma density approaches N cr . The transversal field in plasma inhomogeneities rapidly increases due to plasma resonance, 15 in turn leading to enhanced ionization, so, in fact, overdense plasma inhomogeneities for the TM polarization develop much faster than for the TE case ( Fig. 6(second row)), they are better localized in space and have sharper boundaries (Fig.…”

“…(20) may be high enough, it may still be insufficient in systems with small interaction volume (i.e., characteristic spatial scales are comparable with laser wavelength) for the instability to be well developed because of the convectional nature of the ionization scattering instability. 1 We also have to consider the spatial growth rate within dispersion equation (15) assuming γ = iΩ and the wavenumber κ to be a function of real Ω (frequency shift of the scattered waves). Since the inequality ωΩ kc 2 κ cos θ is readily satisfied in a broad range of parameters, except the case of forward scattering with large-scale density perturbations, solution of Eq.…”

Investigations on terahertz radiation generated by two superposed femtosecond laser pulses

“…In this context, Leemans et al 27 have shown that the tunnel ionization rate is exceptionally dependent on the laser field, which leads to steep plasma density gradients in the transverse or radial direction. Moreover, in the process of optical-field induced ionization, Gil'denburg et al 29 have reported that the field and plasma cannot stay homogeneous even on the small scales compared with the wavelength. In view of all these facts, it is necessary to consider the inhomogeneity in the plasma density in order to understand the more realistic situation.…”