Ultrafast third-order nonlinearities in novel zwitterionic molecules

The results of an experimental study into the third-order nonlinear optical properties of a group of zwitterionic organic molecules based on the chromophore DEMI-3CNQ are described. Utilising a tuneable sub-picosecond laser source, a comprehensive study of the nonlinear response of samples doped with a range of zwitterionic molecules was carried out in order to gain an understanding of the dynamics of the nonlinear response, and to evaluate the potential of the materials for the technological application of ultra-fast all-optical switching. Two regions of material behaviour were studied - on-resonant absorption saturation and off-resonant nonlinear refraction. On-resonance, the absorption saturation effect occurred due to three contributing processes resolvable by their differing lifetimes. In DEMI, the longest lifetime effect, with a lifetime of 40 ps, is attributed to saturation of the dominant transition, the largest measured Τmχ(3) of which is -1.84 x 1014m2V-2 . The medium (3-5 ps) and fast (< 0.5 ps) effects are attributed to the first and second excitations of the dimerised form of the DEMI molecule. The largest measured Τmχ(3) for the medium lifetime is -7.4 x 10-15m2V-2, and the largest effective nonlinearity of the fast response (Τmχeff(3)) was measured to be -1.2 x 10-15m2V-2. Two three-level models were proposed to describe the observed resonant nonlinear effects. Off-resonance, only one of the materials studied, C10, showed a significant non-linearity with an n2 of 4.5 x 10-4 cm2GW-1 and no nonlinear absorption, giving extremely favourable figures-of-merit for optical switching when compared with other commonly quoted organic and inorganic materials. In particular, the minimum switching energy for C10 was calculated to be 0.015 pJ, almost an order of magnitude smaller than has been previously obtained for other materials. (author)