Scattering indicatrices of the investigated films: light solid line – freely propagated 532-nm laser beam, dashes – PMMA with 1-% dye concentration, dots À 50-% concentration, dark solid line – condensed dye film (100-% concentration). 

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... materials for nonlinear optical (NLO) applications rely on a noncentrosymmetric ordering of dye molecules [1–3]. A great attention has been paid to donor-acceptor conjugated compounds because of their highly polarizable p -electron systems and a strong correlation between their structure and optical properties [1,2]. It was shown that a combination of these materials with different polymer matrices can improve and purposefully modify their NLO behavior [4]. Such ‘‘guest-host’’ systems can be useful for the frequency conversion, data storage, image processing, optical switching, and optical limiting. For instance, third-order NLO photoinduced changes in the refractive index are fundamental to all-optical switching and computing. The dye under investigation is of the merocyanine group with donor and acceptor residuals that define its large NLO properties [1]. It is well known that such structures can aggregate and self-organize into microstructures of different types depending on growth conditions, substrate properties, solvents, etc. [2,5,6]. Their THz absorbance spectra are sensitive to the length of their p -conjugated chain [7]. These and other photochemical features make these dyes interesting for various investigations. Polymethyl methacrylate (PMMA) is one of the most versatile polymeric materials convenient for applications in various technological areas including optics and nonlinear optics [4]. In this work, we study the linear and nonlinear optical properties of thin films ( $ 20–100 nm) of the investigated dye ‘‘guest-host’’ compounds with PMMA as a matrix, as well as of a solid dye film condensed on a glass substrate. The dependences of the NLO response on the dye concentration in films give possibility to judge about the aggregation effects in samples [8]. To analyze the third-order NLO coefficients of nanosized films with different concentrations of the dye, as well as those of condensed dye films, we performed measurements of the on-axis transmittance under a single pulse excitation with the second harmonic (532 nm) of a mode-locked Nd:YAG laser. The NLO properties of samples under a continuous wave (CW) laser excitation are presented. An impact of dye nanoclusters on the NLO response, linear absorption spectra, and optical scattering indicatrices is observed. The investigated dye is derived from the class of malonodinitrile dyes. It was synthe- sized via the condensation of dimethylacetal dimethylformamide and 2-(3,5,5- trimethylcyclohex-2-enyliden)-malonodinitrile in acetonitrile. Its composition and THz properties were studied in [7] (dye No. 3). Its chemical formula is shown in Figure. 1(a). The nanosized ‘‘guest-host’’ compound films with different wt. concentrations (0%, 0.1%, 1%, 10%, and 50%) of the studied dye in PMMA were prepared, by using the spin-coating method. Solutions of the polymer matrix mixed with the dye at certain concentrations were dried on a spin dryer at 200 C to produce the nanosized films. The solid dye film was obtained via the sublimation in vacuum, transfer of molecular compounds in the form of a molecular beam, and following condensation on a glass substrate in vacuum (1–3) Á 10 À 4 mmHg with a residual pressure not worse than 1 Á 10 À 5 mmHg [6]. The film mass and the molecular beam density were measured in situ using the quartz crystal microbalance technique. A morphology of the obtained ultrathin films was examined by means of optical microscopy, scanning microinterferometry, and atomic force microscopy (AFM). In Figure. 1(c), an AFM image of the condensed dye film is presented, and aggregated molecular nanoclusters are clearly seen on the surface. The thicknesses of films were evaluated using a contact-free interferometric automatic profilometer ‘‘Micron- Alpha’’ (Ukraine) with an accuracy of 4 nm. They turn out to be 92 nm, 47 nm, 46 nm, and 16 nm for PMMA films with dye concentrations of 0.1%, 1%, 10%, and 50%, respectively; 47 nm for pure PMMA and 40 nm for the condensed dye film. The scattering indicatrices of all investigated samples were measured using the experimental technique described in [9]. To study NLO refractive index variations, the far-field spatial profile analysis [1,10] is used. Photoinduced changes in the total transmittance correspond to certain NLO absorption coefficients [1,11]. The NLO properties of samples are studied under two types of excitation: CW (532 and 633 nm) and the second harmonic of a mode-locked Nd:YAG pulse laser (532 nm, 30 ps FWHM). In organic molecular solids, the dye molecules can form aggregates due to specific and nonspecific interactions. Their structure essentially depends on both the kinetics of the formation of ultrathin films and the nature of molecular interactions at interfaces. In the approximation of a weak dipole-dipole interaction between dye molecules, the first excited level is split into three components shown in Figure. 1(b) [12]. These components manifest themselves in the absorption spectrum of any compound containing the dye. The contribution of each component, i.e., its relative intensity, is a function of the mutual orientation of dipoles in aggregates, and the spectral splitting value is a characteristic of their interaction energy. In condensed molecular media, the inhomogeneous broadening and a small shift of spectral lines are present due to a configura- tional disorder in the system and due to the dielectric environment of aggregates. In Figure. 2, the optical density is plotted for the different dye solutions (a) and for thin PMMA films with different dye concentrations (b). In Figure. 2(a), the positions of spectral components are clearly shown. The type of a solvent significantly affects the nature of the dye aggregation [12]. In acetonitrile, aggregates with the prevailing motive ‘‘head-to-tail’’ are quite significant at dye concentrations of 10 À 5 mol = liter and higher. The same type of aggregates is prevalent for solutions in acetone and isopropyl alcohol. However, the prevalent type of aggregates in aromatic solvents is a ‘‘stack’’ that is manifested by the domi- nance of high-energy absorption components in the spectrum. The investigated dye is soluble in toluene and limitedly soluble in benzene, precipitating as a separate phase at concentrations of order of 10 À 4 mol = liter. For solid films based on PMMA, the predominant type of aggregates and the phase composition are also significantly dependent on the dye concentration. While spectra of a 1-% solid film are similar to those of solutions in acetone, an increase of the dye concentration leads to the growth of high-energy components and to significant inhomogeneous broadening. 50-% concentration film is most likely a two-phase system. The presence of the second phase was observed using an inter- ference microscope with 400 zoom. The significant scattering and the spectral broadening argue for the same result. The aggregation effects can stimulate phase precipitation and therefore a rapid growth in the intensity of light scattered by a sample with increase in the dye concentration in samples. We measured the optical scattering indicatrices of the films under study, and some of them are presented in Figure 3. It is clearly seen that the condensed film scatters much more light than other samples. Scattering losses for all the investigated samples were calculated by the integration of the light power dP scattered into a solid angle d X (normalized to the incident power P 0 ) over a part of the forward hemisphere that corresponds to the non-ballistic propagation of ...