Value Of Nonlinear Susceptibility Research Articles

Continuous control of third-order optical nonlinearity in charge-transfer-type conjugated polymers

Third-order optical nonlinearity was investigated in two charge-transfer (CT)-type conjugated polymers, poly(aryleneethynylene) and poly(thiophene-alt-thiazole), using the third-harmonic generation (THG) method. Comparison of the results with previously obtained THG data for polythiophene and the strong CT polymer poly(thiophene-alt-quinoxaline) revealed that the ratio of the maximum value of third-order nonlinear susceptibility and the absorption coefficient max∣χ(3)∣∕αmax increased as the degree of CT character increased for the four polymers. This variation in max∣χ(3)∣∕αmax is discussed based on the magnitude of the transition dipole moment between the one-photon-allowed and the one-photon-forbidden excited states.

Second and third order nonlinear optical properties of microrod ZnO films deposited on sapphire substrates by thermal oxidation of metallic zinc

We report the preparation of microcrystalline ZnO thin films on sapphire substrates using a simple method based on the thermal evaporation of metallic Zn in vacuum with further annealing process. The aim of annealing in the oxygen atmosphere in the range of 800–850°C was to obtain the high quality ZnO films. The surface morphology was studied by scanning electron microscopy and atomic force microscopy. The polycrystalline films with ZnO microrods at different stages of their growth were investigated. Second and third harmonic generation measurements were performed by means of the rotational Maker fringe technique using Nd:YAG laser at 1064nm in picosecond regime. The obtained values of second and third order nonlinear susceptibilities were found to be high enough for the potential applications of the investigated materials in the optical switching devices based on refractive index changes.

Second-harmonic generation in the thermal/electrical poling (100− x)GeS 2· x(0.5Ga 2S 3·0.5CdS) chalcogenide glasses

Utilizing Maker fringe (MF) method, second-harmonic generation (SHG) has been observed within the GeS 2–Ga 2S 3–CdS pseudo-ternary glasses through thermal/electrical poling technique. The SHG phenomenon was considered to be the result of breakage of the glassy macroscopic isotropy originated from the reorientations of dipoles during the thermal/electrical poling process. Under the same poling condition conducted with 5 kV and 280 °C for 30 min, the maximum value of second-order nonlinear susceptibility χ (2) of the poled (100− x)GeS 2· x(0.5Ga 2S 3·0.5CdS) glasses was obtained to be ≈4.36 pm/V when the value of x is equal to 30. Nonlinear dependence of χ (2) on compositions of these glasses can be well explained according to the theory related to the reorientation of dipoles.

Ultrafast third-order nonlinearity of silver nanospheres and nanodiscs

We have measured and compared the absolute values of nonlinear susceptibility of colloidalsolutions containing silver nanospheres and nanodiscs at their respective plasmon peaks using afemtosecond laser. The nonlinear process responsible for the laser-induced grating formation inthe sample is determined to be of third order. The ratio between the third-order susceptibility(|χ(3)|) and the linearabsorption coefficient (α) of the nanodiscs at 590 nm is three times than that of the similar ratio for nanospheres at398 nm. Using a randomly oriented ellipsoidal model, we have shown that the increase in|χ(3)|/α for a nanodisc at 590 nm can be attributed to the change in the field enhancement factorwith shape.

Large and Ultrafast Third-Order Nonlinear Optical Propertiesof Ge-S Based Chalcogenide Glasses

We report ultrafast third-order nonlinear optical (NLO) properties ofseveral chalcogenide glasses GeSx (x = 1.8, 2.0, 2.5)measured by femtosecond time-resolved optical Kerr gate technique at820 nm. The third-order nonlinear susceptibility of GeS1.8 glassis determined to be as large as 1.41×10−12 esu, which is themaximum value of the third order nonlinear susceptibility χ(3)for the three compositions investigated. The symmetric Gauss profiles ofoptical Kerr signals reveal the nature of ultrafast nonlinear responseof these samples, which are originated from the ultrafast polarizationof the electron clouds. By detailed microstructural analysis of theseglasses based on the chain-crossing model (CCM) and therandom-covalent-network model (RCNM), it can be concluded thatχ(3) value of GeSx glasses can be enhanced greatly by S–Scovalent bonds or S3Ge–GeS3 ethane-like units.

Complex formation by bismuth and boron with fullerene (C60): A reaction that opens up a novel route for synthesis of C60–inorganic hybrid composites

A dark red colored composite of C60 and (zinc, bismuth) borate glass was synthesized by solid-state reaction between C60 and frits of the glass at 650–700 °C in an argon atmosphere completely free from oxygen and moisture. An unusual change in color and the absorption spectrum of the glass upon composite formation with C60 indicated that the incorporated C60 underwent some sort of interaction with the glass. Such fullerene molecules bonded to relatively smaller units of bismuth-boron network, were extracted out by eluting a powdered sample with toluene. The composite itself and the toluene extract there of, were then characterized by ultraviolet–visible–near-infrared, infrared, and mass spectral studies. The results showed that the bismuth ions of the bismuth-boron network were bonded with the C60 cages through direct donation of their lone pair of electrons to the latter. The phenomenon of addition of boron to C60 via an “oxygen bridge,” which was observed in our earlier work, was also detected in this case. Studies on the nonlinear optical properties of the composite exhibited a moderate value of third order nonlinear susceptibility χ(3) (1.5 × 10−11 esu) and optical limiting properties of the composite. The result showed that the material had the prospect of being used in nonlinear optical devices.

Large optical third-order nonlinearity of composite thin film of carbon nanotubes and BaTiO 3

Composite thin films of multiwalled carbon nanotubes and BaTiO 3 prepared by pulsed-laser deposition technique are investigated, which have large optical third-order nonlinearity. We characterized the films by using scanning electron microscopy, Raman spectroscopy, UV–vis absorption and X-ray diffraction. The observation reveals that the multiwalled carbon nanotubes are coated by the amorphous BaTiO 3. Nonlinear optical properties of the composite films are investigated by z-scan technique. The result shows that the value of third-order nonlinear susceptibility χ (3) has achieved as high as 10 −6 esu, which is more than three orders of magnitude larger than that of carbon nanotubes in suspension.

Characterization of nonlinear optical parameters of KDP, LiNbO 3 and BBO crystals

Third-order nonlinear susceptibilities ( χ (3)) and nonlinear refractive indices ( n 2) of KDP, BBO and LiNbO 3 crystals are measured at the wavelengths of 1064 nm and 532 nm by Z-scan technique. The measurements were carried out at different pulse energies, focusing conditions, and crystals’ lengths. It was shown that the increase of phase-matching angle leads to the decreasing of χ (3) of KDP crystals. The parameters of multi-photon absorption in KDP, BBO and LiNbO 3 crystals are determined. The analysis and comparison of experimental and theoretical values of nonlinear susceptibilities are presented.

Thin films of Sb2S3 doped by Sm3+ ions

Pure and Sm3+ doped amorphous thin films of Sb2S3 were prepared by thermal co-evaporation of Sb2S3 and Sm metal. The prepared films contained a slight over-stoichiometric excess of Sb. From transmittance measurement, the absorption edge, index of refraction, n, thickness, d, optical gap, Egopt, relative permittivity, εr, single oscillator energy, Eo, dispersion energy, Ed, and the values of third-order nonlinear susceptibility χ(3) were evaluated. The values of χ(3) increase with Sm doping. Some of above-mentioned values are slightly influenced by exposure of films to a UV lamp. The Raman spectra revealed two main structural units in the films: SbS3 pyramids and Sb–Sb bonds in S2Sb–SbS2 structural units. Three luminescence bands were found. The first one with maximum near 1130 nm can be assigned to 6F9/2→6H5/2 and/or 6F11/2→6H7/2 electron transitions, which are overlapping. The second band can be assigned to the electron transition 6F11/2→6H9/2 or 6F9/2→6H7/2 or 6F7/2→6H5/2. The third luminescence band with maximum near 1500 nm can be probably assigned to the electron transition 6F11/2→6H11/2.

Tunable room temperature second harmonic generation in glasses doped with CuCl nanocrystalline quantum dots

Two-photon excited emission centered at 379–426 nm in photodarkening borosilicate glass doped with CuCl nanocrystalline quantum dots at room temperature has been observed. The emission is detected in the direction of the fundamental near-infrared beam. Time- and frequency-resolved measurements at room temperature and 77 K indicate that the emission is largely coherent light characteristic of second harmonic generation (SHG). An average conversion efficiency of ∼10 −10 is obtained for a 2 mm thick sample. The observed SHG can originate in the individual noncentrosymmetric nanocrystals, leading to a bulk-like contribution, and at the nanocrystal-glass interface, leading to a surface contribution. The bulk-like conversion efficiency is estimated using previously reported values of coherence length ( 5 μm ) and bulk nonlinear susceptibility. This bulk-like conversion efficiency estimate is found to be smaller than the measured value, suggesting a more prominent surface contribution.

Preparation and optical properties of barium titanate thin films dispersed with Au nanoparticles

A high doping level of Au nanoparticles was successfully incorporated into sol-gel derived dip-coating amorphous BaTiO 3 thin films. Unique absorption red shift was observed by absorption spectra. The calculated values of third nonlinear susceptibility χ (3) varied between 10 −8 and 10 −9 esu.

Linear optical properties of niobium-based tellurite glasses

10·Na 2O– x·Nb 2O 5–(90− x)·TeO 2 ( x=0, 5, 10 and 20) glasses have been prepared and some of their physical and optical properties are investigated. The refractive index variation with wavelength is measured using a novel technique, white light interferometry. Optical band gaps and Urbach energies are estimated from the optical absorption spectra. Optical parameters viz., dispersion energy, average oscillator energy, Abbe's number and third order nonlinear susceptibility values are estimated from the dispersion of the refractive index. Respective refractive index and estimated third order nonlinear optical susceptibility values are larger than for the tellurite glasses reported earlier. Relatively large refractive index values obtained for the present glasses are attributed to the hyperpolarisability of the Nb–O bands. The present study of the optical band gap and dispersion energies of these glasses suggests the decrease of covalency of the glass with the increase of niobium (Nb) content.

Infrared second-order optical susceptibility in InAs/GaAs self-assembled quantum dots

We have investigated the second-order nonlinear susceptibility in self-assembled quantum dots. The nonlinear susceptibility associated with intraband transitions in the conduction band and in the valence band is theoretically estimated for lens-shaped InAs/GaAs self-assembled quantum dots. The confined energy levels in the dots are calculated in the effective-mass approximation by solving the three-dimensional Schr\"odinger equation. Giant values of nonlinear susceptibility, about six orders of magnitude larger than the bull GaAs susceptibility, are predicted. We show that this enhancement results from three key features: (i) the achievement of the double resonance condition, (ii) the specific polarization selection rules of intraband transitions that allow both in-plane and z-polarized transitions with large dipole matrix elements to be optically active, and (iii) the small homogeneous linewidth of the intraband transitions. The conclusions of the calculations are supported by the measurement of the midinfrared nonlinear susceptibility in the valence band. The measurements have been performed using a picosecond free-electron laser. Both ${\ensuremath{\chi}}_{\mathrm{zzz}}^{(2)}$ and ${\ensuremath{\chi}}_{\mathrm{zxx}}^{(2)}$ components of the susceptibility tensor are observed. A satisfying agreement is found between theoretical and experimental values.

Optical Properties of Gold‐Dispersed Barium Titanate Thin Films Prepared by Sol‐Gel Processing

Gold‐dispersed BaTiO3 thin films were prepared by the sol‐gel process using spin coating, after which the effects of the gold particle size and shape on nonlinear optical properties were investigated. The BaTiO3 matrix was amorphous up to 500°C. Polycrystalline BaTiO3 thin films were obtained by heating at 650°C for 1 h. The mean gold particle size, which was calculated from the Au(111) diffraction peak according to Scherrer's formula, was ∼8‐10 nm in the range of 300° to 550°C, but rapidly increased thereafter with increasing temperature up to 650°C. From 650° to 800°C, however, particle size was almost constant at ∼16 nm. For those gold particles heated to 500°C, the particle‐size distribution was small and centered at ∼10 nm. However, for gold particles heated at 650°C, a broad particle‐size distribution ranging from 10 to 100 nm could be observed in the BaTiO3 thin films from TEM observation. The peak position of the surface plasmon resonance of the gold particles shifted to a longer wavelength as temperature increased. An absorption peak position was calculated that was in very good agreement with Maxwell‐Garnet (MG) theory, using the dielectric constant of the gold particles by Kreibig theory to calculate the absorption coefficient, α, of each distribution (each 2.5 nm) of the gold particle size, and these values were then averaged. BaTiO3 thin films heat‐treated at 650°C for 1 h exhibited third‐order optical nonlinear susceptibility values (chi(3)) of 1.09 × 10‐6 esu and chi(3)/α= 4.31 × 10‐11 esu·cm at a wavelength of 532 nm. The values of chi(3)/α of gold‐dispersed BaTiO3 thin films were 10 times larger than those of gold‐dispersed SiO2 thin films.

Study of Magnetic Phase Transitions in YVO3 by Non-linear Susceptibility

The linear and non-linear ac-susceptibilities of the Mott insulator YVO3 were measured using a Hartshorn mutual inductance bridge. It is found by the temperature dependence of ac-susceptibilities that there are two magnetic phase transitions at 72.1 and 112.0 K. Above 112K, the values of linear and non-linear susceptibilities qualitatively consist with the result of the mean-field theory. It is verified that there is a power-law dependence between linear and non-linear susceptibilities, which is predicted by the mean-field theory.

Chiral polyesters with azobenzene moieties in the main chain, synthesis and evaluation of nonlinear optical properties

The synthesis, characterization and evaluation of nonlinear optical properties of a new series of chiral copolyesters containing push-pull electronic azobenzene mesogenic groups in the main chain are reported. The copolyesters showed liquid crystalline behaviour with high glass transition temperatures (∼200 °C), stable liquid crystalline mesophases (up to 360 °C) and high second harmonic generation (SHG) efficiency. The second order nonlinear susceptibility values were measured as a function of the percentage of the chiral building unit and the temporal stability was found to be significant over a long duration of time.

Optimization of intersubband resonant second-order susceptibility in asymmetric graded AlxGa1−xAs quantum wells using supersymmetric quantum mechanics

A systematic procedure is proposed for the design of ternary alloys based quantum-well structures optimized for double-resonance second-harmonic generation. The method relies on the supersymmetric quantum mechanics as derived here for the case of position-dependent effective mass. Starting from a symmetric, truncated quasiparabolic potential, itself lacking any second-order nonlinearity, we generate a family of asymmetric potentials, fully isospectral with the starting potential, and choose the one which maximizes the product of transition matrix elements relevant for the second-order nonlinearity. Realization of the optimized potential (in an approximate manner) by grading the ternary alloy Al${}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}$As is then described. The best value of nonlinear susceptibility obtained exceeds those reported in the literature.

Optimization of resonant intersubband nonlinear optical susceptibility in semiconductor quantum wells: The coordinate transform approach.

A systematic procedure is proposed for the optimal design of quantum-well structures, which provide maximal resonant second-order susceptibility. The method is based on the coordinate transform. Starting from the linear harmonic-oscillator Hamiltonian, itself lacking any nonlinear susceptibility, it generates a family of asymmetric isospectral Hamiltonians in parametrized form. Their parameters are then varied to maximize the product of matrix elements relevant for nonlinear susceptibility. Practical realization of quantum-well structures having such optimized Hamiltonians is also discussed. Values of nonlinear susceptibility obtained by this method are about equal to the best values reported in current literature. \textcopyright{} 1996 The American Physical Society.

Third-order optical non-linearities of CuCl-doped glasses in a near resonance region

Third-order optical non-linearities of CuCl microcrystallites with sizes of 6.8-11.0 nm in glasses were investigated. Absolute values of third-order non-linear susceptibilities, |χ (3) (-ω 1 : ω 1 , -ω 1 , ω 1 )|, were measured in the resonance region by a degenerate four-wave mixing method and |χ (3) (-ω 2 : ω 1 , -ω 1 , ω 2 )| were measured in the near resonance region by a non-degenerate four-wave mixing method. |χ (3) (-ω 1 : ω 1 , - ω 1 , ω 1 )| was strongly dependent on the frequency, ω 1 , of incident beam, while |χ (3) (-ω 2 : ω 1 , - ω 1 , ω 2 )| was less dependent on the frequency, ω 2 , of incident beam under the resonant excitation condition. In the non-degenerate four-wave mixing configuration in which the pumping frequency is resonant with the confined excitons, the figure of merit, |χ (3) |/α, was enhanced to more than an order of magnitude compared with that in the degenerate four-wave mixing configuration in a near resonance region.

Experimental and theoretical investigation of highly nonlinear optical glasses

Glasses containing hyperpolarizable entities have a potential and scientifical interest for their nonlinear optical properties. Within this scope glasses of the Na 2 O-TiO 2 -P 2 O 5 system have been investigated with a high sensitivity absolute interferometric technique. The values of the third order nonlinear susceptibilities will be discussed from the structural point of view and by extension to glasses of the bond orbital theory or Lines [3]