Models of few optical cycle solitons beyond the slowly varying envelope approximation

In the past years there was a huge interest in experimental and theoretical studies in the area of few-optical-cycle pulses and in the broader fast growing field of the so-called extreme nonlinear optics. This review concentrates on theoretical studies performed in the past decade concerning the description of few optical cycle solitons beyond the slowly varying envelope approximation (SVEA). Here we systematically use the powerful reductive expansion method (alias multiscale analysis) in order to derive simple integrable and nonintegrable evolution models describing both nonlinear wave propagation and interaction of ultrashort (femtosecond) pulses. To this aim we perform the multiple scale analysis on the Maxwell-Bloch equations and the corresponding Schrodinger-von Neumann equation for the density matrix of two-level atoms. We analyze in detail both long-wave and short-wave propagation models. The propagation of ultrashort few-optical-cycle solitons in quadratic and cubic nonlinear media are adequately described by generic integrable and nonintegrable nonlinear evolution equations such as the Korteweg-de Vries equation, the modified Korteweg-de Vries equation, the complex modified Korteweg-de Vries equation, the sine-Gordon equation, the cubic generalized Kadomtsev-Petviashvili equation, and the two-dimensional sine-Gordon equation. Moreover, we consider the propagation of few-cycle optical solitons in both (1 + 1)- and (2 + 1)-dimensional physical settings. A generalized modified Korteweg-de Vries equation is introduced in order to describe robust few-optical-cycle dissipative solitons. We investigate in detail the existence and robustness of both linearly polarized and circularly polarized few-cycle solitons, that is, we also take into account the effect of the vectorial nature of the electric field. Some of these results concerning the systematic use of the reductive expansion method beyond the SVEA can be relatively easily extended to few-cycle solitons in the general case of multilevel atoms. Prospects of the studies overviewed in this work are given in the conclusions. (C) 2012 Elsevier B.V. All rights reserved.