Abstract

In general the phase shift that a wave propagating trough a nonlinear medium obtains is described by the intensity dependent refractive index n·n₀ + n₂|A|² + n₄|A|⁴+…. Recently it was shown [1] that n₂ is in fact $n_2^{dir}+n_2^{case}$. The second term arises only in quadratic nonlinear media and when the mismatch of the second-order process is small (Δk < 2π) but not zero. Here we show that in the case of slightly mismatched process of third harmonic generation (THG) n₄ can be presented in the same way:$n_4=n_4^{dir}+n_4^{case}$. The analysis of the role of the cascaded third-order process on the nonlinear phase shift (NLPS) of the fundamental beam is made by investigation of THG in a medium where the phase matching condition k₃ = 3k₁ can be fulfilled by use of natural birefringence or so called quasi-phase matched techniques.

© 1996 IEEE