**3. Conclusion**

The medium with the high polarizability compared with the polarizability of the dye in the solutions arises as a result of the dye thin solid film formation. This creates the condition for the appearance, as well the high negative value of the non-linear cubic susceptibility in resonance as the change of the sign on the dispersion curve of the cubic susceptibility and appearance the darkening on the long wavelength slope of the J-peak absorption. In the medium with high polarizability the considering of the four-level system for the Jaggregates excited state description is more appropriate compared with the two-level system typically considered for the J-aggregates. The depopulation of J-aggregate Frank-Condon excited state due to the appearance of lowered relaxed state leads to giant nonlinear bleaching of J-aggregate peak and the excitation of the relaxed state leads to the nonlinear absorption appearance. The value of non-linear response strongly depends on the relaxation time. The decrease of the relaxation time leads to the growth of non-linear response.

The increase of the medium polarizability enhances the non-linear response and is reflected in the increasing of the inhomogeneous broadening in the absorption and luminescence of the J-aggregates' spectra. The formation of the medium with high polarizability is clear seen from the spectral shift of the dye absorption during spin-coating and helps to explain the Jpeak growth at the solid film formation. The inhomogeneous broadening in the J-peak absorption has the connection with aggregate thermal decay energy activation distribution and spectral inequivalent in the kinetics of the thermal decay of the J-aggregate absorption counter. The medium polarization influence on the optical and non-linear optical properties is important for the aggregated or supramolecular state of different dyes and nanostructures in the condensed phase.

The number of facts allow us to consider the properties of the pseudoisocyanine Jaggregates in the thin solid films as the properties of the strongly coupled dimers with inhomogeneous broadened spectral contour: the existence of the isobestic point at the Jaggregates' thermal conversion to monomer dye in the thin solid films or in the polymer films, the absence of the any hypsochromic spectral shift at the transition of the J-aggregate to monomer in the thin films at the addition of the octadecylquinolinium iodide, the high stabilization of the J-aggregate at the addition of the dipole closo-hydrodecaborate anion, the batochromic spectral shift of the J-peak at heating of the dye thin film, the narrowing of the J-peak in the thin solid films in the presence of organic cations and others.

The practical use of the obtained results on the non-linear properties is in the possible application of cyanine dyes' J-aggregates as the elements for teraherz demultiplexing of light signals and in schemes of ultra-short laser pulse stabilization (Plekhanov et al., 2004). The radiation stability of the J-aggregates is at a level of 0.5-1 MW/cm2. This is useful to increase the radiation stability of the J-aggregates by 5-10 times. For safer J-aggregated films, application is enough to increase the thermal stability to 100-150oC. However, the search for new highly polarized aggregates of dyes with high absorption coefficients in the visible and infra-red spectral range that possess high thermal and photochemical stability remains a basic and practical task.
