Bipolaronic-enhanced third-order nonlinearity in organic ladder polymers

Third order nonlinear optical properties of organic ladder copolymer (POL) system is studied using degenerate four-wave mixing with picosecond laser pulse. Both the real and imaginary part of the third order nonlinear susceptibility (3) were determined by a new phase conjugate interferometric method over the wavelength range of 532 - 720 nm. From the space symmetry and wavelength dependence of -) we attribute the observed nonlinearity to the nonlinear photoexcitation of bipolarion states in this ladder copolymer system.