Soft Report - Dipartimento di Fisica - Sapienza

Nonlinear Optics in Soft-MatterLight propagation in soft-matter such as colloidalsystems can be affected via a feedback mechanismby light induced structural changes, owing to theirhigh responsivity to external stimuli. In spite of suchattractive feature and the low powers needed toobserve nonlinear phenomena, the optical nonlinearresponse of soft matter was generally overlookedwith the exception of metal colloids (whosenonlinearity is mediated by surface polaritons) andliquid crystals that display a giant reorientationalnonlinearity. We have considered the case of acolloidal suspension of dielectric particles dispersedin a solvent, characterized by tunable interactionsthat can be responsible for long range correlationsand even phase transitions such as gelation. The roleof the structure of these materials in nonlinearoptical processes is essentially unexplored.Large scale ordered structures can indeed affectorientational, electrostrictive, and thermophoreticmechanisms. For isotropic particles and negligiblethermal gradients due to light absorption, theleading optical nonlinear mechanism is expected tobe electrostriction: the particles are subject to forcesinduced by light intensity gradients, thus moving inthe region with higher or lower intensity dependingon the difference between their refractive index andthat of the host medium. In both cases, the opticalbeam experiences self-focusing, a process which hasbeen described by a local Kerr law or intensitydependentrefractive index variation. Recently,however, we have shown that the nonlinear responseof such materials is mediated by the static structurefactor S(q) of the material, turning out to be stronglynonlocal. While the strength of the nonlinearity ispredominantly affected by the materialcompressibility, S(q) is strongly affected by thewhole structure of the soft-material phase, e.g. bythe presence of fractal aggregates which has impacton the nonlinear susceptibilities. [1]Thus, we have shown that specific nonlinear opticalprocesses, and in particular we have predicted theexistence of self-trapped optical beams, or “spatialsolutions” (SS). [1] These are light filaments whichcan propagate without diffraction, due to nonlinearresponse of the material. We have found a strictFig. 2: Numerical simulations of laser beampropagation in a soft material in the presence ofaggregates with fractal dimensions D=1.3 (a) andD=2.3 (b). The laser intensity distribution at theoutput plane of a soft-matter sample with lengthcomparable to the Rayleigh length of a beam withwaist w 0 profile is shown in a pseudo-color plot. Thenumber and the distribution of the laser filamentsgenerated from a Gaussian beam is strongly affectedby the fractal dimension.relation between the SS features, like the existencecurve relating the beam waist and power, withspecific structural quantities, like the cluster spatialextension, or their fractal dimensions. We have alsopredicted the existence of sub-wavelength ultra-thinSS than can propagate without diffraction, and thatcan have relevant applications for opticalcommunication devices or laser surgery (figure 1).We have also considered the role of disorder, e.g.due to density material fluctuactions, in thepropagation of a laser beam in a soft-material,including nonlinear effects. Specifically, we haveinvestigated the condition for the beam breakup intoa multitude of filaments, which propagate andinteract in due to the non local nonlinear response ofthe materials. A strong link with the materialstructure factor and these laser filamentation processcan be estabilished. (figure 2).[2]For what concern the experimental activity, weimplemented the z-scan techniques to measure thestatic and dynamic properties of the nonlinear opticalsusceptibility. We have reported specific evidences ofthe aging of the nonlinear susceptibility in Laponite,whose nonlinear optical response was augmented byadding a dye to the solution. [3]References[1] C. Conti, G. Ruocco, S. Trillo, Phys. Rev. Lett.96, 065702 (2006);[2] C. Conti, N. Ghofraniha, G. Ruocco, S. Trillo,submitted;[3] N. Ghofraniha, C. Conti, G. Ruocco, submitted.See the related chapter in this book.Fig. 1: Numerical simulation of the propagation of aself-trapped Gaussian beam in a soft-material withfractal dimension D=2.5, for two different inputpowers in the paraxial regime (a and b) and beyond(c and d), z 0 and w 0 are the diffraction length and thebeam waist respectively; z is the propagationdistance and r the transverse coordinate. [1].AuthorsC. Conti (a,b), N. Ghofraniha (c), G. Ruocco (b,d), S.Trillo (b,e).(a) Research Center “Enrico Fermi”, Rome, Italy.(b) CRS SOFT-INFM-CNR, Rome, Italy.(c) CRS SMC-INFM-CNR, Rome, Italy.(d) Universita’ di Roma, Rome, Italy.(e) Universita’ di Ferrara, Ferrara, Italy.67SOFT Scientific Report 2004-06