CC BY
22LMI-I-23
Ionization-field instabilities and nanograting formation in optical breakdown processes
V. Gildenburg12,1. Pavlichenko12
institute of Applied Physics RAS, Plasma physics department, Nizhny Novgorod, Russian Federation
2University of Nizhny Novgorod, Radiophysics department, Nizhny Novgorod, Russian Federation
The ionization-field instability developing due to the mutual enhancement of the electric field and plasma density perturbations in the processes of the optical and microwave breakdown of different media can lead, as is well known, to formation of small-scale periodic ionization structures instead the homogeneous plasma seemingly corresponding to the initially homogeneous ionizing radiation [1-5]. During the last decade, the interest in such structures has been renewed in connection with their growing applications for the production of the subwavelength nanogratings in the optical materials, in particular, in the fused silica, by the series of repeated laser pulses [6-8]. The creation of such nanogratings is considered now as a promising technique of the material optical properties modification and compact writing and storage of the optical information. Nevertheless, the physical mechanisms predetermining these structures appearance are not revealed completely and remain the subject of discussion.
In the presented report, the spatiotemporal evolution of the field and plasma in the optical breakdown induced in the volume of transparent dielectric (fused silica) by fs laser pulse is studied under condition of the so-called plasma-resonance-induced ionization instability that results in the deep small-scale periodic modulation of the breakdown plasma parameters in the direction of the laser polarization. We address two essentially different models of optical discharge created (i) in the homogeneous dielectric with a slight (modeling the seed perturbation) periodic modulation of the permittivity [9,10] and (ii) in the dielectric with multiple randomly placed and easily ionized ellipsoidal inclusions (modeling small voids and atomic structure defects) [11-13]. For the second model we calculate the effective permittivity of the nano-dispersive media based on the Maxwell Garnett theory. The optical electric field is calculated with allowance for the effects influencing both its long-scale structure (the beam focusing accounted for in the given-ray-tube approximation) and the small-scale one (quasi-static enhancement in the plasma resonance regions). The plasma density evolution is described by the rate equation taking into account the photoionization, avalanche ionization, and ambipolar diffusion. Based on the fulfilled numerical calculations, we have described the main types of the plasma-field structures formed in one laser pulse and have found the laser pulse intensity range where the instability evolving from seed perturbations leads to the formation of the contrast subwavelength periodic structure containing the number of the narrow zones with overcritical plasma density and enhanced energy deposition. The latter allows us to consider this structure formed in one pulse as underlying the nanograting formation observed experimentally in the fused silica irradiated by series of repeated fs pulses.
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