Numerical modeling of femtosecond laser induced nanostructuring in glass
Résumé
Femtosecond laser nanoprocessing allows imprinting periodic subwavelength polarization-dependent nanostructures in fused silica and few other glasses [1]. The mechanism of their organization is still far from being completely understood [2]. Recently, transient nanoplasmonics model has been proposed to explain the growth of nanoplasmas perpendicular to the laser polarization due to localized inhomogeneous multiphoton ionization [3]. Numerical modeling showed that initial nanometric inhomogeneities were required to start the process [4]. However, the influence of laser parameters such as irradiation wavelength, laser pulse energy, number of pulses and laser polarization on the nanostructure characteristics has not been investigated previously
Figure 1 Electron density snapshot at the end of the pulse duration 240 fs (FWHM) shows quasi-periodic nanogratings elongated perpendicular to the laser polarization from initially random inhomogeneities. Corresponding Fourier Transform (FT) of the image reveals the nanograting periodicity close to half the laser wavelength in glass. The pulse energy is fixed to E = 500 nJ. The irradiation wavelength is 800 nm in air. The electron density is normalized to its critical value at 800 nm.
We underline that the self-organization of volume nanogratings can be explained by full-vectorial 3D-Maxwell based approach coupled with the electron density equation to describe the dynamics of ultrashort laser interaction with randomly distributed inhomogeneities embedded in glass. We compare the results of numerical modeling with available experimental data. Particularly, we show that the periodicity strongly depends on the irradiation wavelength and the orientation is defined by the electric field polarization. In terms of laser pulse energy, three regimes of modification experimentally investigated by Taylor et al. [5] are identified. Finally, an explanation of the pulse number effect on the nanograting periodicity is proposed basing on the mechanism of the nonlinear ionization memory [3] and correlation between the concentration of inhomogeneities and the number of pulses.
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