Third order optical characterisation of a π-conjugated polydiacetylene by Maker fringes technique

Conjugated polymers are promising photonic materials due to the high values of non-linear optical coefficients when compared to inorganic compounds. A key point in view of applications is the figure of merit defined as the ratio between the non-linear optical and the absorption coefficients. ort on the NIR measurement of the third order optical susceptibility (χ(3)) by third harmonic generation in polydiacetylene (poly-[1,6-bis-(3,6-dihexadecyl-N-carbazolyl)-2,4-hexadiyne], polyDCHD-HS). Samples are prepared as thin films by spin-coating technique and the χ(3) is evaluated by Maker fringes method in the spectral range from 1200 to 2000 nm. The absorption coefficient has been derived from measurements in CS2 solution. Spectra of χ (3) show typical resonant behaviour due to the multi-photon processes and are strictly related to the absorption spectra in the visible range. Values of χ(3) are in the range of 10−11 esu. The absorption coefficient is as low as 0.25 cm−1 at 1.5 μm showing the good figure of merit of polyDCHD-HS.