Soft Report - Dipartimento di Fisica - Sapienza

Aging of the Nonlinear Optical SusceptibilityNonlinear optics susceptibility of liquids e. g. organicdye solutions have been widely studied by examiningnonlinear refraction of thermal origin and nonlinearabsorption of electronic origin. However in ourknowledge optical nonlinear response variations inpresence of structural dynamics slowing down(aging) in soft materials such as colloidalsuspensions have not been considered. With thisaim, we investigated on a system of Laponite claypowder dispersed in dye Rhodamine-B aqueoussolution, displaying high thermal nonlinear opticalresponse and the gelation process characteristic ofthe clay suspension [1].We performed Z-scan [2] and dynamic lightscattering (DLS) measurements to show that opticalnonlinearities exhibit the same aging behaviour ofstructural dynamics of a system.We present, as an example, in Fig. 1 Z-scan curves(A) of 0.6mM RhB aqueous solution at 2.2 wt % claydispersion at increasing waiting time t wcorrespondingly to the correlation functions (B),from which is evident the dynamics slowing down ofthe system for increasing aging time. In Fig. 1A isalso reported the 0.6 mM RhB solution transmittancecurve (open circles) and it is worth noticing thatoptical nonlinearities are influenced by Laponitepresence and more interestingly dye nonlinearsusceptibility does vary in the clay matrix undergoingaging. The transmittance curves in Fig. 1A are typicalof beam defocusing nonlinear systems in this casewith thermal lens-like behaviour, in fact, as it is wellknown, dye molecules are characterized by stronglight absorption giving rise to refractive indexgradient due to the thermal effect. MoreoverRhodamines are characterized by nonlinear saturableabsorption as a result of excited-state or two photonabsorption well described in [3]. We used theanalytical description of Z-scan method reported in[4] to estimate both nonlinear refractive index andabsorption coefficient of the 0.6 mM RhB- Laponitedispersions at different clay concentrations. In Fig 2Aand Fig 3A mean relaxation times τ m given by fittingDLS data and nonlinear absorption coefficients fromZ-scan measurements are reported vs. tw. Thespecific t w dependence of τ m(t w) and α 2(t w) leads toconsider a scaling law, that makes all data collapseFig. 1: Z-scan spectra (A) and DLS correlationfunctions (B) of Laponite 2.2 wt % dispersion inRhB 0.6 mM water solution for different waitingtimes t w and corresponding fits (full lines). In A isalso reported RhB 0.6 mM scan (open circles).Fig. 2: Mean relaxation time τ m vs. waiting time t w(A) and vs. the scaled variable µ τ t w (B) in loglinearscale at various clay concentrations; full linesin (A) are the fitted curves.on a single master curve. This curves are reportedrespectively in Fig. 2B and Fig. 3B as functions ofµ τ t w and µ α t w, being µ τ and µ α fitting parameters [5].From our analysis it emerges that the two timescales µ τ -1 and µ α -1 are comparable showing that thenonlinear optical susceptibility does age andmoreover it exhibits the same aging behaviour ofstructural dynamics.Fig. 3: Nonlinear absorption coefficient α 2 vs. waitingtime t w (A) and vs. the scaled variable µ α t w (B) atvarious clay concentrations; full lines in (A) are thefitted curves.References[1] B. Ruzicka, L. Zulian and G. Ruocco, Phys. Rev.Lett. 93, 258301 (2004);[2] M. Sheik-Bahae, A. A. Said, Tai-Huei Wei, D. J.Hagan and E. W. Van Stryland, IEEE J. Quant. El. 26(4), 760 (1990);[3] N. K. M. Naga Scrivinas, S. Venugopal Rao andD. Narayana Rao, J. Opt. Soc. Am. B 20 (12), 2470(2003);[4] R. E. Samad and N. D. Vieira, J. Opt. Soc. Am. B15(11) (1998);[5] N. Ghofraniha, C. Conti, G. Ruocco, preprint.AuthorsN. Ghofraniha (a), C. Conti (b,c), G. Ruocco (d,c).(a) SMC-INFM-CNR, Università di Roma, Italy.(b) Research Center “Enrico Fermi”.(c) SOFT INFM-CNR, Università di Roma, Italy(d) Dipartimento di Fisica, Università di Roma, Italy.71SOFT Scientific Report 2004-06