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The effects of axial ligands in Sn(IV)-substituted 5,10,15,20-tetrakis [4-(carboethoxymethyleneoxy)phenyl]porphyrin and divalent transition metal substitution in 5,10,15,20-tetraphenylporphyrin on the second molecular hyperpolarizability (γ) at 802 nm were studied using the Z-scan technique. A significant increase in γ for divalent metal ions is observed with decreasing d-shell occupancy. In the case of the tetravalent metal-substituted porphyrin a dramatic enhancement of γ is seen when a strongly electronegative axial ligand such as I₂ is attached to the metal ion. A plausible explanation for the observed trend of γ has been sought in terms of metal-ligand interaction.

Optical self-defocusing, characterized by the non-linear refractive index n₂, was investigated by the Z-scan technique in water solutions of two porphyrins (PPhs), negatively charged meso-tetrakis(p-sulfonatophenyl)porphyrin (TPPS₄) and positively charged meso-tetrakis(4-N-methyl-pyridiniumyl)porphyrin (TMPyP), in their free base forms and as Fe(III) and Mn(III) complexes. Significant n₂ values were observed only for the TMPyP metal complexes, while for the other porphyrins the n₂ values were negligible. The effect is explained by the reorientation of the porphyrin molecule due to interaction of its permanent dipole moment perpendicular to the molecular ring plane with the electromagnetic field of the exciting light pulse. The permanent dipole moment is due to the shift of the metal atom out of the molecular plane. The electrostatic interaction between the metal atom and charged substituents increases (repulsion) or decreases (attraction) the shift of the metal atom and consequently affects the dipole moment value. Ligand binding to the metal atoms also increases the out-of-plane metal shift and hence the dipole moment and n₂ value. pH changes were shown to modify the Fe(III)-ligand structure, thus changing n₂. The experimental data correlate well with the metal shift and dipole moment values calculated for simplified PPh structures by the ZINDO method.

We report here, our experimental results and theoretical analysis of the dispersion data of nonlinear absorption in zinc meso-tetra-(p-methoxyphenyl)tetrabenzoporphyrin (ZnmpTBP). Using a ns optical parametric oscillator (OPO) we obtain the open aperture Z-scan data over the visible region starting from 480 nm to 600 nm. We use a more general five-level model for evaluating the excited state parameters like excited state absorption coefficients, two-photon absorption coefficient, etc. Due to its large excited state absorption cross-sections this material acts as a very good candidate for a broadband optical limiter in the visible region.

Nano-particle materials with large optical nonlinearities are in great need for the development of nonlinear optical technology and are being studied intensively in recent years. ZnO nanoparticles have been synthesized by precipitation from zinc acetate in a series of n-alkanols from ethanol to 1-hexanol as a function of temperature. The formation and microstructure of ZnO particles were monitored by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis spectrum and z-scan technique. We studied the optical limiting effect of ZnO nano-particles. Using a CW He-Ne laser at 632.8 nm. The UV-Visible spectrum of this material shows a 373.5 nm peak. Using the UV-Visible, the band gap of this nano-particle is obtained about 3.31 eV. Also the XRD spectroscopy indicates that the structure is wurtzite hexagonal. Average particle size obtained from SEM analysis about 43nm. Furthermore the nonlinear refractive index, n₂, and nonlinear absorption coefficient, β, were measured using close and open aperture z-scan respectively, at peak laser irradiation of 1.743 kW/cm². Then the nonlinear refraction index of this nano-particle was measured in order of 10^-7 (cm²/W) with negative sign and the nonlinear absorption coefficient was obtained in order of 10^-3 (cm/W).

The single beam Z-scan technique was used to determine the nonlinear optical properties of the organic dye Nile Blue chloride in the solvent ethanol. The experiments were performed with a He-Ne laser with a wavelength of 632.8 nm. The negative nonlinear refractive index and two-photon absorption coefficient were observed in this dye. The intensity-dependent nonlinear refractive index was investigated. The result shows that the dye exhibits a great nonlinear response with the real and imaginary parts of the third-order nonlinear optical susceptibility χ⁽³⁾ being -4.12×10^-5 esu and 1.35×10^-6 esu, respectively. These results show that the Nile Blue chloride dye has potential applications in nonlinear optics.

The optical nonlinearity of an organic dye from a triphenylmethane group is measured using the Z-scan technique. The sample under study is found to exhibit the negative nature of nonlinearity, which is mainly due to the thermally induced refractive index change. Since this thermally induced refractive index change can lead to self-diffraction, the self-diffraction properties of the sample is studied by using thin films of a dye-sensitized gelatin. A He-Ne laser (633 nm) is used for both the Z-scan and self-diffraction studies. In the self-diffraction studies, it is also found that the different diffracted orders exhibit varying phase differences.

A novel dmit^2- complex: [(C₂H₅)₄N][Co(dmit)²], abbreviated as EtCo, was synthesized and its third-order optical nonlinearities were characterized by Z-scan technique at the wavelength of 1064 nm with laser duration of 40 ps. Z-scan curves revealed a negative nonlinear refraction at 1064 nm and a positive absorption was observed. The nonlinear refraction coefficient n₂, the nonlinear absorption coefficient β and the molecular second hyperpolarizability γ have been determined to be as large as 9.51×10^-19m²/W, 2.19×10^-12 m/W and 5.41×10^-31 esu respectively, suggesting that this material has potential applications in optical limiting.

We present our results on nonlinear optical (NLO) of chicago sky blue 6B doped polyvinyl alcohol (PVA) film. This film was studied at 633 nm and 532 nm using a continuous wave laser. We have evaluated the sign and magnitude of the third-order nonlinearity from the closed aperture Z-scan data while the nonlinear absorption properties were assessed using the open aperture data. The chicago sky blue 6B doped PVA film exhibited nonlinear saturated absorption and strong self-defocusing effect. The limiting effect of the sample was studied and the results indicate that the film possesses good characteristic of optical limiting.

Erythrosine B based poly (methacrylate) films with varying even number of methylene spacers have been prepared by the free radical polymerization method. All optical switching and beam deflection effects of polymer films have been investigated by using Nd:YAG laser (532 nm) as a pump beam and a He–Ne laser (632.8 nm) as a probe beam. The nonlinear refractive indices of polymer films are determined using the Z-scan technique. The optical switching and beam deflection effects are due to the change of the refractive index of polymer under the pump beam. The experimental results show that erythrosine B based poly (methacrylate) films have potential application in optical switching.

A series of polymeric thin films with BFDT (BFDT = 4,5-bis(foroylsulfanyl)-1,3-dithiole-2-thione) doped in PMMA (polymethylmethacrylate) were fabricated by means of spin-coating on quartz substrate. The third-order nonlinear optical properties of the films were investigated by Z-scan technique at 532 nm wavelength with 20 ps pulse width. The influences of doping concentration for third-order nonlinearity were also studied. A self-defocusing effect was observed from the Z-scan curves and the nonlinear refractive index of the film increases with the increase in doping concentration. Our results suggest that considerable nonlinear optical properties were found in BFDT. In addition, it was found that the nonlinear coefficient of the BFDT-doped PMMA thin film was about two orders of magnitude larger than that of homologous materials with organic solvents. By analysis, we can conclude that the material is a potential candidate for applications of nonlinear optics and can be considered in the fabrication of all-optical switching devices, etc.

A novel dmit^2- complex: [C₇H₇(C₂H₅)₃N][Ni(dmit)₂], abbreviated as BTEANDT, was synthesized and its third-order nonlinear optical properties were investigated by Z-scan technique at the wavelength of 1064 nm with laser duration of 20 ps. A self-focusing effect and a saturable absorption were observed. The nonlinear refraction coefficient n₂ and the nonlinear absorption coefficient β have been found to be 7.311 × 10^-18m²/W and -6.064 cm/GW when I₀=3.025 GW/cm². The intensity dependence curve of β was also studied and the result reveals that β is proportional to I₀. We explained the result with the theory of absorption cross-section.

The nonlinear optical properties of a nanoparticle polyaniline/polyacrylonitrile (PAn/PANr) copolymer newly prepared via experimental and theoretical findings are reported. Diffraction ring patterns result in the far field when a CW, visible, 473 nm, low power laser beam traverses the solution of PAn/PANr in the solvent ethanol. The diffraction ring patterns are usually used to estimate the nonlinear refractive index while the Z-scan is used to measure both nonlinear refractive index and its sign and nonlinear absorption coefficient. The experimentally observed ring patterns are numerically reproduced via the use of the Fresnel–Kirchhoff procedure. Good agreement between experimental findings and the numerical ones are obtained. The obtained diffraction ring patterns suffer modification in the vertical direction due to convection current as a result of local heating of the sample and the presence of gravity.

We describe a modified Z-scan technique for the case when nonlinear refraction is accompanied by nonlinear absorption to separately evaluate the nonlinear refraction and the nonlinear absorption by performing straightforward measurements with the aperture removed. We divide the closed aperture (S < 1) normalized Z-scan by a scan with an open aperture (S = 1). With nanosecond scale at a 532 nm wavelength, we first measured the nonlinear optical absorption and refraction coefficients of a solution of Ni(Ac)₂[N-(8-quinolyl) pyridine-2-carboxamide] · 4H₂O by using the modified Z-scan technique. The nonlinear absorption originates from the near resonant two-photon absorption while the mechanism of the nonlinear refraction is the near resonant two-photon absorption transition enhancement. The linear increasing dependences of the optical nonlinearities on the incident irradiance arise from the population redistribution due to the near resonant two-photon absorption.

We report on the measurement of the nonlinear optical absorption and refraction coefficients of C60-containing polyurethane–urea films on a nanosecond scale at a 532-nm wavelength, using the Z-scan technique. The optical limiting properties were also observed. The results show that the C60 polyurethane–urea is a kind of homogeneous material with a range of C60 content. With the difference in C60 content in polymers, the optical limiting properties are many and the required transmittance and transmitted energy can be altered depending on the C60 content in polymers. We suggest that the third-order nonlinearities and optical limiting behavior are consistent with excited-state absorption (reverse saturable absorption) as a mechanism.

A novel dmit^2- salt: (hexadecyltrimethylammonium)bis(4,5-dithiolato-1,3-dithiole-2-thione)copper, abbreviated as HTCu, is synthesized and characterized for third-order optical nonlinearity by Z-scan technique at a wavelength of 1064 nm with laser duration of 30 ps. Z-scan spectra reveal a negative Kerr coefficient at 1064 nm and no nonlinear absorption was observed under this condition. The nonlinear refraction coefficient n₂ and the second hyperpolarizability γ have been determined to be -1.51 × 10^-11 and 1.36 × 10^-30esu, respectively, suggesting HTCu is a promising material for all-optical switching.

Nonlinear optical (NLO) properties of the aqueous solution of Acid Orange 10 dye have been studied using the closed aperture and open aperture Z-scan technique at different concentrations and various powers of Ar⁺ laser at 488 nm wavelength. The nonlinear refractive index (n₂) shows negative nonlinearity for all concentrations studied, and it is measured to be -13.5 × 10^-9cm²/W at 0.05 mM concentration. Moreover, the nonlinear refractive index is found to vary with intensity and concentration. The nonlinear absorption coefficient (β) is measured to be -3.8 × 10^-4cm/W. Optical limiting behavior has been demonstrated for this dye. These results show that Acid Orange 10 dye has potential applications in nonlinear optics.

Nematic liquid crystal (NLC) is a material that features a large nonlinear optical response. The nonlinearity can be enhanced by doping a dye agent into a NLC host. In this paper, we review the nonlinear phenomena recently observed for dye-doped liquid crystal (DDLC) films measured using the Z-scan technique, which is a simple, yet powerful method for measuring nonlinear refractive indices (optical Kerr constant) n₂ and nonlinear absorption coefficients β. The nonlinear effects of liquid crystals (LCs) include the electronic polarization effect, electrostriction effect, thermal effect and reorientation effect. When LCs are doped with dye agents, the dye-induced orientational effect, photoisomerization effect, and light-induced thermal effect come about to enhance LC nonlinearity. The dominant effect depends strongly upon the spatial, temporal and polarization properties of the excitation laser beam. Generally speaking, the nonlinear effects resulted from the electronic polarization and photoisomerization effects arise from a laser pulse on picosecond duration scale, while electrostriction, thermal and reorientation effects occur on nanosecond ~ microsecond, nanosecond ~ millisecond and millisecond ~ second duration scales, respectively. In this paper, the causation of nonlinear refractive index n₂ into various different experimental conditions is summarized, including the changes of the external electric field, external optical field, polarization of pump beam, temperature and the use of deuterated materials. The mechanisms, which influence the nonlinear effect in dye-doped liquid crystal films, are also discussed.

The third-order nonlinearity of a novel ET[bis(ethylenedithio)-tetrathiafulvalene] complex (ET ⋅ BF₃) has been measured by using the Z-scan technique with a laser pulse width of 30 ps at 1064 nm. The third-order nonlinear refractive index (n₂), nonlinear absorption coefficient (β) and nonlinear susceptibilities (χ⁽³⁾) of a sample acetonitrile solution are 3.5 × 10^-11 esu, 1.65 cm/GW and 5.4 × 10^-12 esu, respectively. The nonresonant second-order hyperpolarizability (γ) for the molecule has been estimated to be 2.77 × 10^-30 esu. The two merit factors for all-optical switching are W ≫ 1 and T = 1.49, respectively.

In this work, we report preliminary results obtained for methacrylic polymers incorporating azobenzene side-group as nonlinear optical (NLO) active molecule. The trans-cis isomerization properties are discussed. The third-order non-resonant nonlinear refractive index (n₂) and nonlinear absorption coefficient (β) are measured using the Z-scan technique at 1064 nm in the picosecond regime. The influence of different electron-acceptor groups in azobenzene moieties on the nonlinear properties is investigated.

In this paper, we are dedicated to exemplifying a two parameter curve fitting method and developing a Matlab-based simulation program to extract the nonlinear refractive index and nonlinear absorption coefficient from closed-aperture Z-scan or R(z) data without the need for performing open-aperture Z-scan measurement. It should be noted, however, that both approaches can only be applicable to a case for which the on-axis phase shift at the focus is small. In this way, we not only determine the nonlinear parameters quickly with reasonable accuracy, as well as save time, efforts and equipment in the process of Z-scan implementation, but also obtain an initial estimate in order to compare with the results of the open-aperture Z-scan measurement when needed.