The process of femtosecond laser filamentation in air and other gases is an intriguing manifestation of the nonlinear optical properties of the respective media. The process is initiated through the intensity-dependent refractive index (n = n0 + n2 I + n4 I2+�) and accompanies the passage of intense radiation through a medium.Despite the extensive activity employing nonlinear laser-gas interactions, the quadratic nonlinear index of refraction coefficients are not known with adequate precision. Improving the precision of these coefficients is important for calculations of intense laser pulse propagation in gases and particularly, resolving the current higher-order Kerr effect (HOKE) controversy.In this study, the nonlinear coefficients n4 of the time-dependent refractive index for all noble gases are calculated in the optical regime. To that aim, second-order polynomial fitting is applied using an auxiliary electric field approach. The obtained fourth-order hyperpolarizability coefficients can be related to the degenerate-six wave mixing, ?(4) (-?; ?,-?,?,-?,?), by using relations between fourth-hyperpolarizability coefficients that were first put forward in the comprehensive work of Bishop and H�ttig, suggesting ?(n)� (-?s; ?1, �, ?n) = ? (n)� (0) ? 1+A W2 + B W22 + �), where ?s= ??i, W2 = ?2s + ?21 + � ?2n.The calculations are compared to the results of calculations based on the Kramers-Kronig relations; qualitative agreement is found in the visible and infrared spectral range. We found that the higher-order nonlinear refraction indices n4for noble gasses are positive over the wavelengths 300 nm-2000 nm.

Higher-order nonlinearity of refractive index

246th National Meeting (2013)

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