Femtosecond laser irradiation of fused silica with a nanometric inhomogeneity

Abstract : A finite-difference time-domain (FDTD) method based on Maxwell’s equations coupled with time-dependent electron carrier density’s equation is proposed to investigate numerically femtosecond laser irradiation of dielectrics with a small inhomogeneity. The process of plasma generation due to multiphoton absorption near small defect in fused silica is studied in details and local field distribution around the void inhomogeneity is explained by the Rayleigh scattering from it. Further investigation of the laser-matter interaction has shown that the generated plasma doesn’t depend significantly on the size of the inhomogeneity as long as it remains considerably smaller than the irradiation wavelength. However, there are several parameters which are crucial for modeling. The characteristics of the generated plasma are studied in the dependence of the electron collisional frequency and the irradiated wavelength and the changes in the refractive index are explained by the Lorentz-Drude model with time-dependent carrier density. The differences between considering Keldysh theory for multiphoton ionization and taking into account only six-photon ionization are discussed. Temporal changes in behavior of scattering due to generated nanoplasma are observed and analyzed using estimations from Mie scattering theory