Nonlinear optical parameters of colloidal silver at various stages of aggregation

The Z-scan method is used to study the variation in the nonlinear susceptibility χ⁽³⁾ (−gw; ω, ω, −ω) and nonlinear refractive index of colloidal silver with the degree of aggregation under the action of picosecond and nanosecond radiation (λ=1064 nm). The values of χ⁽³⁾ for aggregated silver that are defined by Kerr nonlinearity and thermal self-focusing are found to be −1.5×10⁻¹⁴ and −4.4×10⁻¹¹ esu, respectively. The time evolution of the picosecond pulses passing through the colloid is investigated.     

Large third-order nonlinear optical susceptibility of Au-Al2O3 composite films near the resonant frequency

2 O₃ composite films with high Au concentrations (30%–60% in volume fraction) were prepared by reactive co-sputtering and post rapidly thermal annealing. The structure of the films and the size distributions of the Au nanoclusters were examined by TEM, and the third-order nonlinear optical susceptibility χ⁽³⁾ was measured by degenerated four-wave mixing using a 70-ps pulse laser at 532 nm. The maximum value of the χ⁽³⁾ was about 1.2×10^-6 esu in the annealed films and occurred at around 45% Au concentration. The figure of merit, χ⁽³⁾/α (α is the absorption coefficient), has a value of 7×10^-12 esu cm over a wide range of Au concentrations. Received: 23 July 1997     

Large optical nonlinearity in CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> thin films

CaCu₃Ti₄O₁₂ (CCTO) thin films were successfully prepared on LaAlO₃ substrates by pulsed laser deposition technique. We measured the nonlinear optical susceptibility of the thin films using Z-scan method at a wavelength of 532 nm with pulse durations of 25 ps and 7 ns. The large values of the third-order nonlinear optical susceptibility, χ ⁽³⁾, of the CCTO film were obtained to be 2.79×10⁻⁸ esu and 3.30×10⁻⁶ esu in picosecond and nanosecond time regimes, respectively, which are among the best results of some representative nonlinear optical materials. The origin of optical nonlinearity of CCTO films was discussed. The results indicate that the CCTO films on LaAlO₃ substrates are promising candidate materials for applications in nonlinear optical devices.     

The growth,thermal and nonlinear optical properties of single-crystal GdAl3(BO3)4

The nonlinear optical single-crystals GdAl₃(BO₃)₄ (GAB), space group R32, were grown by the top-seed solution growth (TSSG) method. The thermal properties and the nonlinear optical characteristics have been investigated. The two principal coefficients of thermal expansion along (100) and (001) were measured to be 5.30 × 10⁻⁶, 1.88 × 10⁻⁵ K⁻¹. The transmission spectrum was measured and the second-harmonic generation (SHG) was presented. By way of our designed cross superimposed laser crystal multiplier, the frequency doubling laser at wavelength 457.5 nm was measured to be 18.2 mW and the facular properties indicated the multiplier contributed to the improvement of the frequency doubling conversion efficiency.     

Evolution of a femtosecond laser-induced plasma and energy transfer processes in a SiO<Subscript>2</Subscript> microvolume detected by the third harmonic generation technique

A nonlinear optical method has been proposed for probing the evolution of a plasma and energy transfer processes in the microvolume of a transparent dielectric using the third-harmonic probe signal. The electron self-trapping time has been estimated as t = 150 ± 80 fs. A decrease in the third-harmonic probe signal has been detected at the picosecond time scale; this decrease can be attributed to a change in the third-order susceptibility χ⁽³⁾ of a fused silica sample modified by laser radiation.     

Nonlinear-optical and structural properties of nanocrystalline silicon carbide films

The aim of this study is to investigate the nonlinearity of refraction in nanostructured silicon carbide films depending on their structural features (synthesis conditions for such films, substrate temperature during their deposition, concentration of the crystalline phase in the film, Si/C ratio of atomic concentrations in the film, and size of SiC nanocrystals formed in the film). The corresponding dependences are obtained, as well as the values of nonlinear-optical third-order susceptibility χ⁽³⁾(ω; ω, −ω, ω) for various silicon polytypes (3C, 21R, and 27R) which exceed the value of χ⁽³⁾ in bulk silicon carbide single crystals by four orders of magnitude.     

The nonlinear optical property and photoinduced anisotropy of?a?novel azobenzene-containing fluorinated polyimide

A novel azobenzene-containing fluorinated polyimide was synthesized. The nonlinear optical property and photoinduced birefringence of a polyimide thin film were investigated. Large third-order nonlinear refraction (n ₂=−4.49×10⁻¹¹ cm²/W) was observed in the polyimide thin film by carrying out Z-scan measurement. The polyimide thin film exhibited larger nonlinear refraction than that of a mono-azo dye doped PMMA thin film (n ₂=−1.63×10⁻¹² cm²/W). The photoinduced birefringence of the polyimide thin film (▵ n∼10⁻²) under different pump intensities was investigated; it was much larger than that of the mono-azo dye doped PMMA thin film (▵ n∼10⁻³). Moreover, the time constants for birefringence growth and relaxation processes were determined.     

Optical nonlinearities in hyperbranched polyyne studied by?two-photon excited fluorescence and third-harmonic generation spectroscopy

The nonlinear optical properties of a hyperbranched polyyne (hb-Polyyne) have been measured at infrared wavelengths by using femtosecond and nanosecond pulsed excitation. This hyperbranched polyyne exhibited strong and intrinsic (simultaneous) two-photon absorption and upconverted blue fluorescent emission under femtosecond excitation around 800 nm. The hb-Polyyne in chloroform solution is characterized by a large two-photon absorption cross section of 9068 GM (1GM=10⁻⁵⁰ cm⁴ s) and a fluorescence quantum yield of 0.57. On the other hand, by third-harmonic generation (THG) spectroscopy with nanosecond excitation, the measured third-order nonlinear susceptibility χ ⁽³⁾ for solid films of hb-Polyyne ranged from 2.4×10⁻¹¹ to 6.1×10⁻¹¹ esu in the spectral range of 1100–1600 nm, with results comparable to the values exhibited by the well-known conjugated polymer MEH:PPV, but with a much better transparency for visible wavelengths.     

Near IR laser light visualizators using nonlinear GaSe and other layered crystallites

Two methods of preparation of the devices for visualization of pulsed and continuous near-IR (near infrared) are described and the results of conversion of pulsed and continuous IR (800–1360 nm) laser radiation into the visible range of spectra (400–680 nm) by using a transparent substrate covered with the particles (including nanoparticles) of effective nonlinear materials of GaSe _x S_{1 − x} (0.2 ≤ x ≤ 0.8) are presented. Converted light can be detected in transmission or reflection geometry as a visible spot corresponding to the real size of the incident laser beam. Developed device structures can be used for checking if the laser is working or not, for optical adjustment, for visualization of distribution of laser radiation over the cross of the beam and for investigation of the content of the laser radiation. Low energy (power density) limit for visualization of the IR laser pulses with 2–3 ps duration for these device structures are: between 4.6–2.1 μJ (3 × 10⁻⁴−1 × 10⁻⁴ W/cm²) at 1200 nm; between 8.4–2.6 μJ (4.7 × 10⁻⁴−1.5 × 10⁻⁴ W/cm²) at 1300 nm; between 14.4–8.1 μJ (8.2 × 10⁻⁴–4.6 × 10⁻⁴ W/cm²) at 1360 nm. Threshold damage density is more than 10 MW/cm² at λ = 1060 nm, pulse duration τ = 35 ps. The results are compared with commercially existing laser light visualizators.     

Optical rectification in tellurium from 10.6 μm

Optical rectification in single crystals of tellurium produced by infra-red radiation at 10.6 μm has been observed. The value of |χ⁰ ₁₁₁| is found to be 0.9×10⁻⁶ esu. Preliminary experiments to test the usefulness of the effect to monitor mode-locked infra-red laser pulses have been inconclusive.     

Third-harmonic generation in polyvinylpirrolidone-based metallic complexes

The third-order nonlinear susceptibilities of the aqueous solutions of polyvinylpirrolidone doped with iron and zinc were studied by the method of third-harmonic generation with 1064-nm Nd:YAG laser radiation. The efficiencies of transformation into the third harmonic in these metallic complexes were 8×10^?7 and 5×10^?7, respectively. To determine the role of the Kerr effect in the transformation process, the nonlinear refractive index of the iron-doped polyvinylpirrolidone solution was measured by the Z-scan method and a value of n ₂=?6.7×10^?13 cgs unit was obtained.     

Femtosecond laser plasma in CaF<Subscript>2</Subscript> crystal microchannel and effective generation of characteristic X-ray radiation

The dependence of the characteristic X-ray radiation yield from CaF₂ crystal on the formed microchannel depth under highly intensive (I ∼ 3 × 10¹⁵ W/cm²) laser pulses with different contrast was obtained. The maximum of the characteristic X-ray radiation yield at these experimental conditions corresponded to the microchannel depth of 30–50 μm. The efficiency of the laser radiation conversion to the characteristic X-ray radiation increased from 6 × 10⁻⁸ for the surface up to 10⁻⁷ in the microchannel. The dependence of the characteristic X-ray radiation yield on the viewing angle showed that the source of X-ray radiation was located near the surface inside the microchannel.     

A comparative study of ion-induced damages in C60 and C70 fullerenes

The stability of fullerenes (C₆₀ and C₇₀) under swift heavy ion irradiation is investigated. C₆₀ and C₇₀ thin films were irradiated with 120 MeV Ag ions at fluences from 1×10¹² to 3×10¹³ ions/cm². The damage cross-section and radius of damaged cylindrical zone were found to be higher for C₆₀ than C₇₀ as evaluated by Raman spectroscopy, which shows that the C₇₀ molecule is more stable under energetic ion impact. The higher damage cross-section of the C₆₀ molecule compared with that of the C₇₀ molecule is explained on the basis of thermal conductivity in the framework of the thermal spike model. The surface morphology of pristine C₆₀ and C₇₀ films is studied by atomic force microscopy. UV-visible absorption studies revealed that band gap for C₆₀ and C₇₀ fullerenes thin films decreases with increasing ion fluence. Resistivity of C₆₀ and C₇₀ thin films decreases with increasing ion fluence but the decrease is faster for C₆₀ than C₇₀, indicating higher damage in C₆₀. Irradiation at a fluence of 3×10¹³ ions/cm² results in complete damage of fullerenes (C₆₀ and C₇₀) into amorphous carbon.     

Nonlinear optical characterization of LaEr(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> thin films using the <Emphasis Type="BoldItalic">Z</Emphasis> -scan technique

The nonlinear optical properties of thin films of LaEr(MoO₄)₃ were studied using a ∼30 ps Nd:YAG laser at 532 nm with a repetition rate of 250 Hz. Closed aperture Z-scan measurement revealed a negative nonlinearity in the LaEr(MoO₄)₃. The nonlinear refractive index γ=1.38×10^-10 cm²/W and nonlinear absorption coefficient β=16.8×10^-6 cm/W were calculated from the Z-scan data. The fluorescent upconversion spectra were recorded with 980 nm excitation. An optical switching mechanism based on nonlinear absorption is also presented experimentally. PACS 81.15.Fg; 77.84.Bw; 33.50.Dq; 42.70.Mp     

Possible manifestation of spin fluctuations in the temperature behavior of the resistivity of Sm<Subscript>1.85</Subscript>Ce<Subscript>0.15</Subscript>CuO<Subscript>4</Subscript> thin films

A pronounced step-like (kink) behavior in the temperature dependence of resistivity ρ(T) is observed in the optimally doped Sm_1.85Ce_0.15CuO₄ thin films around T _sf = 87 K and attributed to the manifestation of strong-spin fluctuations induced by Sm³⁺ moments with the energy ħω_sf = k _B T _sf ≃ 7 meV. The experimental data are found to be well fitted by the residual (zero-temperature) ρ_res, electron-phonon ρ_e-ph(T) = AT, and electron-electron ρ_e-e(T) = BT ² contributions in addition to the fluctuation-induced contribution ρ_sf(T) due to thermal broadening effects (of the width ω_sf). According to the best fit, the plasmon frequency, impurity scattering rate, electron-phonon coupling constant, and Fermi energy are estimated as ω_p = 2.1 meV, τ ₀ ⁻¹ = 9.5 × 10⁻¹⁴ s⁻¹, λ = 1.2, and E _F = 0.2 eV, respectively. The text was submitted by the authors in English.     

Laser deposition of amorphous diamond films from liquid aromatic hydrocarbons

6 H₅CH₃, C₆H₆, and C₆H₅CH(CH₃)₂) to pulsed visible laser radiation of a copper vapor laser (λ=510.6 nm). The X-ray Auger electron spectroscopy (XAES), reflection high energy electron diffraction (RHEED), and Raman analysis are employed to characterize the deposited films. The sp³ fraction in deposited films amounts to 60–70% and depends on the precursor. The average film thickness on a glass substrate is about 100 nm. The films show excellent adherence, are transparent in the visible and have microhardness of 50–70 GPa, as measured by nanoindentor. Received: 28 September 1998 / Accepted: 13 January 1999     

Continuous frequency tuning of a picosecond neodymium-glass laser in the UV and VUV

The results of an investigation of continuous frequency tuning of a neodymium laser in the UV and VUV ranges are reported. Generation of the sum frequency of second harmonic radiation and the radiation from a parametric light generator in the UV region (338–366 nm) is achieved. The optimal conditions for tuning UV radiation in the range 113.5–117.0 nm in third-harmonic generation processes in xenon and its mixtures with other gases are investigated. A third-harmonic generation efficiency of ∼5×10⁻⁴ and a tuning range >2600 cm⁻¹ are obtained in the VUV range investigated. Zh. Tekh. Fiz. 68, 82–89 (May 1998)     

Measurement of wurtzite ZnO/rutile TiO2 heterojunction band offsets by x-ray photoelectron spectroscopy

Four novel dmit complexes: [(C₂H₅)₄N][Ni (dmit)₂], [(C₃H₇)₄N][Ni(dmit)₂], [(C₂H₅)₄N][Au(dmit)₂] and [(C₃H₇)₄N][Au(dmit)₂], abbreviated as EtNi, PrNi, EtAu, and PrAu, were synthesized. The third-order nonlinear optical properties of them in acetonitrile solutions were investigated by using the Z-scan technique with 20 ps pulses width at 1064 nm. When the on-axis irradiance at focus I ₀ was 5.025 GW/cm², the nonlinear refraction coefficient n ₂, the third-order nonlinear susceptibility χ ⁽³⁾, the molecular second-order hyperpolarizability γ of the four types of material were obtained with subject to Z-scan curves, and these indexes were with the magnitudes of 10⁻¹⁸ m²/W, 10⁻¹³ esu, and 10⁻³¹ esu, respectively. The nonlinear absorption coefficient β of Ni samples had the 10⁻¹² m/W scale. The impact of different metals and cations on the third-order nonlinear optical properties of materials was analyzed. Through the derivation, the result suggests that these dmit complexes are promising candidates for applications to nonlinear optical devices manufacture in the near-infrared waveband.     

Investigation of radiation resistance of fullerenes under irradiation with fast neutrons

Fullerenes C₆₀ and C₇₀ synthesized by the electric arc method and fractionated (purity grades of 99.99 and 99.90 wt %, respectively) were irradiated in a solid phase in the WWR-M reactor (Konstantinov Petersburg Nuclear Physics Institute, National Research Centre “Kurchatov Institute,” Gatchina, Russia) with the aim of determining the survivability in the range of fast neutron fluences Φ = 4 × 10¹⁵?3 × 10¹⁷ n/cm². The irradiated samples were dissolved in carbon disulfide, and intact fullerenes were extracted. With an increase in the fluence, their weight fraction in the samples S(Φ), a measure of radiation resistance of molecules, decreased, to a first approximation, exponentially: S(Φ) = exp(?Φ/Φ _D ). The estimated characteristic fluences were Φ _D = 2.4 × 10¹⁷ and 4.0 × 10¹⁷ n/cm² for C₆₀ and C₇₀, respectively.     

Investigation of third-order nonlinear optical properties of NNDC-doped PMMA thin films by Z-scan technique

A novel chalcone derivative, (2E)-1-(2,4-di- chloro-5-fluorophenyl)-3-[4-dimethylamino)phenyl]prop-2-en-1-one, abbreviated as NNDC, was prepared and characterized by elemental analyses, infrared (IR) and proton nuclear magnetic resonance (¹H NMR) spectrum, and thermal analyses. The NNDC-doped poly(methyl methacrylate) (PMMA) thin films with five different doping concentrations by weight were prepared by using a spin-coating method. Their linear optical properties were investigated by using a prism coupling measuring system. The third-order nonlinear optical properties of NNDC in 1,2-dichloroethane (NNDC/1,2-dichloroethane) solution and NNDC-doped PMMA (NNDC/PMMA) films were investigated by using the laser Z-scan technique with 20 ps pulses at 532 nm. A self-focusing effect was observed from the Z-scan curves for solution and thin films and the nonlinear refractive index of the film increases with the increase of the doping concentration. In addition, nonlinear absorption was negligible for all samples. The magnitude of third-order nonlinear refraction index n ₂ and the third-order nonlinear susceptibility χ ⁽³⁾ for thin films were 10⁻¹⁵ m²/W and 10⁻⁹ esu, respectively, which are about three orders larger than that of NNDC/1,2-dichloroethane solution. Some necessary analyses were presented. The results show that this material is a promising candidate for application in the nonlinear optical devices at 532 nm.