Femtosecond Z-scan measurement of third-order optical nonlinearities in anatase TiO2 thin films

Anatase phase TiO₂ films have been grown on fused silica substrate by pulsed laser deposition technique at substrate temperature of 750 °C under the oxygen pressure of 5 Pa. From the transmission spectra, the optical band gap and linear refractive index of the TiO₂ films were determined. The third-order optical nonlinearities of the films were measured by Z-scan method using a femtosecond laser (50 fs) at the wavelength of 800 nm. The real and imaginary parts of third-order nonlinear susceptibility χ⁽³⁾ were determined to be −7.1 × 10⁻¹¹esu and −4.42 × 10⁻¹²esu, respectively. The figure of merit, T, defined by T=βλ/n₂, was calculated to be 0.8, which meets the requirement of all-optical switching devices. The results show that the anatase TiO₂ films have great potential applications for nonlinear optical devices.     

Nonlinear optical properties in SrTiO3 thin films by pulsed laser deposition

Well-crystallized 250 nm-thick SrTiO₃ thin films on fused-quartz substrate were prepared by pulsed laser deposition. The band-gap of SrTiO₃ thin film by transmittance spectra is equal to 3.50 eV, larger than 3.22 eV for the bulk crystal. The nonlinear optical properties of the films were examined with picosecond pulses at 1.064 μm excitation. A large two-photon absorption (TPA) with absorption coefficient of 87.7 cm/GW was obtained, larger than 51.7 cm/GW for BaTiO₃ thin films. The nonlinear refractive index n₂ is equal to 5.7×10⁻¹⁰ esu with a negative sign, larger than 0.267×10⁻¹¹ esu for bulk SrTiO₃. The large TPA is attributed to intermediate energy levels introduced by the grain boundaries, and the optical limiting behaviors stemming from both TPA and negative nonlinear refraction were also discussed.     

Nonlinear optical properties of Au/ZnO nanoparticle arrays

The triangular-shaped Au/ZnO nanoparticle arrays were fabricated on fused quartz substrate using nanosphere lithography. The structural characterization of the Au/ZnO nanoparticle arrays was investigated by atomic force microscopy. The absorption peak due to the surface plasmon resonance of Au particles at the wavelength of about 570 nm was observed. The nonlinear optical properties of the nanoparticle arrays were measured using the z-scan method at a wavelength of 532 nm with pulse duration of 10 ns. The real and imaginary part of third-order nonlinear optical susceptibility, Re χ⁽³⁾ and Im χ⁽³⁾, were determined to be 1.15 × 10⁻⁶ and −5.36 × 10⁻⁷ esu, respectively. The results show that the Au/ZnO nanoparticle arrays have great potential for future optical devices.     

Third-order optical nonlinearities of lead-free (Na1−xKx)0.5Bi0.5TiO3 thin films

(Na_1−xK_x)_0.5Bi_0.5TiO₃ (NKBT) (x = 0.1, 0.2, and 0.3) thin films with good surface morphology and rhombohedral perovskite structure were fabricated on quartz substrates by a sol-gel process. The fundamental optical constants (the band gaps, linear refractive indices and absorption coefficients) of the films were obtained through optical transmittance measurements. The nonlinear optical properties were investigated by Z-scan technique performed at 532 nm with a picosecond laser. A two-photon absorption effect closely related with potassium-doping content was found in thin films, and the nonlinear refractive index n₂ increases evidently with potassium-doping. The real part of the third-order nonlinear susceptibility χ⁽³⁾ is much larger than its imaginary part, indicating that the third-order optical nonlinear response of the NKBT films is dominated by the optical nonlinear refractive behavior. These results show that NKBT thin films have potential applications in nonlinear optics.     

Enhanced ferroelectric properties of (Pb0.90La0.10)Ti0.975O3 multilayered thin films prepared by RF magnetron sputtering

The (Pb_0.90La_0.10)Ti_0.975O₃/PbTiO₃ (PLT/PT), PbTiO₃/(Pb_0.90La_0.10)Ti_0.975O₃/PbTiO₃ (PT/PLT/PT) multilayered thin films with a PbO_x buffer layer were in situ deposited by RF magnetron sputtering at the substrate temperature of 600 °C. With this method, highly (1 0 0)-oriented PLT/PT and PT/PLT/PT multilayered thin films were obtained. The PbO_x buffer layer leads to the (1 0 0) orientation of the films. The dielectric, ferroelectric and pyroelectric properties of the PLT multilayered thin films were investigated. It is found that highly (1 0 0)-oriented PT/PLT/PT multilayered thin films possess higher remnant polarization 2P_r (44.1 μC/cm²) and better pyroelectric coefficient at room temperature p (p = 2.425 × 10⁻⁸ C/cm² K) than these of PLT and PLT/PT thin films. These results indicate that the design of the PT/PLT/PT multilayered thin films with a PbO_x buffer layer should be an effective way to enhance the dielectric, ferroelectric and pyroelectric properties. The mechanism of the enhanced ferroelectric properties was also discussed.     

Third-order nonlinear optical properties of Bi2S3 and Sb2S3 nanorods studied by the Z-scan technique

Bismuth sulfide (Bi₂S₃) and antimony sulfide (Sb₂S₃) nanorods were synthesized by hydrothermal method. The products were characterized by UV-vis spectrophotometer, X-ray powder diffraction (XRD) and transmission electron microscope (TEM). Bi₂S₃ and Sb₂S₃ nanorods were measured by Z-scan technique to investigate the third-order nonlinear optical (NLO) properties. The result of NLO measurements shows that the Bi₂S₃ and Sb₂S₃ nanorods have the behaviors of the third-order NLO properties of both NLO absorption and NLO refraction with self-focusing effects. The third-order NLO coefficient χ⁽³⁾ of the Bi₂S₃ and Sb₂S₃ nanorods are 6.25×10⁻¹¹ esu and 4.55×10⁻¹¹ esu, respectively. The Sb₂S₃ and Bi₂S₃ nanorods with large third-order NLO coefficient are promising materials for applications in optical devices.     

Ultrafast non-linear optical properties of Ge20As25Se55 chalcogenide films

The non-resonant third-order non-linear optical properties of amorphous Ge₂₀As₂₅Se₅₅ films were studied experimentally by the method of the femtosecond optical heterodyne detection of optical Kerr effect. The real and imaginary parts of complex third-order optical non-linearity could be effectively separated and their values and signs could be also determined, which were 6.6 × 10⁻¹² and −2.4 × 10⁻¹² esu, respectively. Amorphous Ge₂₀As₂₅Se₅₅ films showed a very fast response in the range of 200 fs under ultrafast excitation. The ultrafast response and large third-order non-linearity are attributed to the ultrafast distortion of the electron orbitals surrounding the average positions of the nucleus of Ge, As and Se atoms. The high third-order susceptibility and a fast response time of amorphous Ge₂₀As₂₅Se₅₅ films makes it a promising material for application in advanced techniques especially in optical switching.     

Femtosecond optical Kerr effect study of Ge10As40S30Se20 film

The results of the femtosecond optical heterodyne detection of optical Kerr effect at 805 nm with the 80 fs ultrafast pulses in amorphous Ge₁₀As₄₀S₃₀Se₂₀ film is reported in this paper. The film shows an optical non-linear response of 200 fs under ultrafast 80 fs-pulse excitation, and the values of real and imaginary parts of non-linear susceptibility χ⁽³⁾ were 9.0×10⁻¹² and −4.0×10⁻¹² esu, respectively. The large third-order non-linearity and ultrafast response are attributed to the ultrafast distortion of the electron orbits surrounding the average positions of the nucleus of Ge, As, S and Se atoms. This Ge₁₀As₄₀S₃₀Se₂₀ chalcogenide glass would be expected as a promising material for optical switching technique.     

Nonlinear optical properties of periodic gold nanoparticle arrays

Periodic Au nanoparticle arrays were fabricated on silica substrates using nanosphere lithography. The identical single-layer masks were prepared by self-assembly of polystyrene nanospheres with radius R = 350 nm. The structural characterization of nanosphere masks and periodic particle arrays was investigated by atomic force microscopy. The nonlinear optical properties of the Au nanoparticle arrays were determined using a single beam z-scan method at a wavelength of 532 nm with laser duration of 55 ps. The results show that periodic Au nanoparticle arrays exhibit a fast third-order nonlinear optical response with the nonlinear refractive index and nonlinear absorption coefficient being n₂ = 6.09 × 10⁻⁶ cm²/kW and β = −1.87 × 10⁻⁶ m/W, respectively.     

Enhanced ferroelectric property of (Pb0.95Ca0.05)(Nb0.02Zr0.80Ti0.20)O3 thin films prepared by RF magnetron sputtering

(Pb_0.95Ca_0.05)(Nb_0.02Zr_0.80Ti_0.20)O₃ [PCNZT] thin films were deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering with and without a LaNiO₃ [LNO] buffer layer. Ca and Nb elements in PZT films enhance the ferroelectric property, LaNiO₃ buffer layer improves the crystal quality of the PCNZT thin films. PCNZT thin films possess better ferroelectric property than that of PZT films for Ca and Nb ion substitution, moreover, PCNZT thin films with a LNO buffer layer possess (1 0 0) orientation and good ferroelectric properties with high remnant polarization (P_r = 38.1 μC/cm²), and low coercive field (E_c = 65 kV/cm), which is also better than that of PCNZT thin films without a LNO buffer layer (P_r = 27.9 μC/cm², E_c = 74 kV/cm). The result shows that enhanced ferroelectric property of PZT films can be obtained by ion substitution and buffer layer.     

Third-order nonlinear optical studies of new anthraquinone derivatives

Four new donor-acceptor-donor anthraquinone derivatives were synthesized and characterized by UV, IR, ¹H NMR and elemental analysis. Their off-resonant third-order nonlinear optical properties were measured using femtosecond laser and degenerate four-wave mixing technique. The third-order nonlinear optical susceptibilities χ⁽³⁾ were 3.36-3.76 × 10⁻¹³ esu. The nonlinear refractive indexes n₂ were 6.19-6.91 × 10⁻¹² esu. The second-order hyperpolarizabilities γ of the molecules were 3.36-3.76 × 10⁻³¹ esu. The response times were 101-115 fs. The results show that these compounds have potential nonlinear optical applications.     

Nonlinear optical properties of Cu nanoclusters by ion implantation in silicate glass

Metal nanocluster composite glass prepared by 180 keV Cu ions into silica with dose of 1 × 10¹⁷ ions/cm² has been studied. The microstructural properties of the nanoclusters were analysed by optical absorption spectra and transmission electron microscopy (TEM). Third-order nonlinear optical properties of the nanoclusters were measured at 1064 nm and 532 nm excitations using Z-scan technique. The nonlinear refraction index, nonlinear absorption coefficient, and the real and imaginary parts of the third-order nonlinear susceptibility were deduced. The mechanisms responsible for the nonlinear response were discussed. Absolute third-order nonlinear susceptibility χ⁽³⁾ of this kind of sample was determined to be 2.1 × 10⁻⁷ esu at 532 nm and 1.2 × 10⁻⁷ esu at 1064 nm, respectively.     

Structure dependence of ultrafast third-order optical nonlinearity for GeS2-In2S3-CsI chalcohalide glasses

Ultrafast third-order nonlinear optical responses of GeS₂-In₂S₃-CsI chalcohalide glasses have been measured by using the femtosecond time-resolved optical Kerr effect (OKE) technique at a wavelength of 820 nm. The third-order nonlinear susceptibility was estimated to be as large as 5.12×10⁻¹³ esu. The full width at half maximum of the Kerr signal was 120 fs and its response was dominantly assigned to the ultrafast distortion of the electron cloud. The relationship between the structural units and the third-order nonlinear optical responses was analysed by Raman spectra. It is suggested that the covalent bonds of S-Ge or S-In constituting the tetrahedral units [GeS_4/2] or [InS_4−xI_x], respectively, play an important role in the ultrafast third-order nonlinear optical responses of these chalcohalide glasses.     

Spectral properties of Nd:Ca2Nb2O7 crystal

This paper reports the spectral properties of Nd³⁺:Ca₂Nb₂O₇. The spectral parameters of Nd³⁺ in Nd³⁺:Ca₂Nb₂O₇ crystal have been investigated based on Judd-Ofelt theory. The spectral parameters were obtained. The parameters of line strengths Ω_λ are Ω₂=4.967×10⁻²⁰ cm², Ω₄=5.431×10⁻²⁰ cm², Ω₆=5.693×10⁻²⁰ cm². The radiative lifetime, the fluorescence lifetime and the quantum efficiency are 122 μs, 103 μs and 84.4%, respectively. The fluorescence branch ratios calculated: β₁=0.425, β₂=0.479, β₃=0.091, β₄=0.004. The emission cross section at 1068 nm is 6.204×10⁻²⁰ cm².     

Refractive index, oscillator parameters and temperature-tuned energy band gap of Tl4In3GaS8-layered single crystals

Transmission and reflection measurements in the wavelength region 450-1100 nm were carried out on Tl₄In₃GaS₈-layered single crystals. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.32 and 2.52 eV, respectively. The rate of change of the indirect band gap with temperature dE_gi/dT=-6.0×10⁻⁴ eV/K was determined from transmission measurements in the temperature range of 10-300 K. The absolute zero value of the band gap energy was obtained as E_gi(0)=2.44 eV. The dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 4.87 eV, 26.77 eV, 8.48×10¹³ m⁻² and 2.55, respectively.     

Growth and spectral properties of Nd:GdVO4 laser crystal

In this paper, the Czochralski growth, absorption spectra, and photoluminescence spectra of Nd:GdVO₄ crystals are studied. From its absorption spectra, Nd:GdVO₄ is found to exhibit an anisotropic optical absorption effect, and its effective Judd-Ofelt parameters are calculated: Ω₂=10.281×10⁻²⁰ cm², Ω₄=5.426×10⁻²⁰ cm² and Ω₆=9.943×10⁻²⁰ cm². By these parameters, the absorption oscillator strengths, emission oscillator strengths, transition probabilities, fluorescence branch ratios, energy lifetimes, and integrated emission cross-sections are also derived. The photoluminescence spectra of Nd:GdVO₄ crystal consist of a wide emission band of host and the characteristic emission bands of Nd³⁺. Based on the excitation spectrum, both the two evident peaks locating at 345 and 371 nm are ascribed to the characteristic excitation of Nd³⁺, and an energy transfer from the host to its doping Nd³⁺ ions is indicated.     

Organic thin films based on a dicyanovinyl-quaterthiophene: Influence of electrode configuration on third-order nonlinear optical properties measured by electroabsorption spectroscopy

Electroabsorption (EA) studies at room temperature on organic thin films based on a dicyanovinyl-quaterthiophene 4T-V(CN)₂ are reported. An electric field modulation is applied to the samples for two different electrode geometries, i.e. sandwich and coplanar versus the organic layer. Changes in optical absorption coefficient of 4T-V(CN)₂ based thin films are measured and analyzed to determine the character of the optical transition in the visible range (400-800 nm). Depending on the experimental electrode configuration, magnitude of electroabsorption responses are different, possibly due to different distribution of the externally applied electric field. The results indicate a higher resolution of EA response for the sandwich electrode configuration and confirm the charge transfer exciton character of 4T-V(CN)₂ in contrast to the unsubstituted quaterthiophene 4T. Finally, a third-order nonlinear susceptibility χ⁽³⁾ (−ω; ω, 0, 0) of 16 × 10⁻¹² e.s.u. is obtained.     

Influence of B2O3 to the inhomogeneous broadening and spectroscopic properties of Er/Yb codoped fluorophosphate glasses

In a three-components fluorophosphate glass system, the introduction of H₃BO₃ brings some valuable influence to the spectroscopic and thermal properties of the glasses. With H₃BO₃ increases from 2 to 20 mol%, Ω₆, S_ed4I13/2, FWHM, T_g and fluorescence lifetime change from 3.21×10⁻²⁰ cm², 1.77×10⁻²⁰ cm², 45 nm, 480 °C and 8.8 ms to 4.66×10⁻²⁰ cm², 2.11×10⁻²⁰ cm², 50 nm, 541 °C and 7.4 ms, respectively. σ_abs, σ_emi, FWHM×τ_f×σ_emi has a maximum when H₃BO₃ is 11 mol%. T_g and T_x−T_g increases with H₃BO₃ introduction. Results showed that in fluorophosphate glasses, proper amount of B₂O₃ can be used as a modifier to suppress upconversion and improve spectroscopic properties, broadband property and crystallization stability of the glasses while keeps the fluorescence lifetime relatively high.     

Effect of substrate temperature on structure and electrical resistivity of laser-ablated IrO2 thin films

IrO₂ thin films were prepared on Si(1 0 0) substrates by laser ablation. The effect of substrate temperature (T_sub) on the structure (crystal orientation and surface morphology) and property (electrical resistivity) of the laser-ablated IrO₂ thin films was investigated. Well crystallized and single-phase IrO₂ thin films were obtained at T_sub = 573-773 K in an oxygen partial pressure of 20 Pa. The preferred orientation of the laser-ablated IrO₂ thin films changed from (2 0 0) to (1 1 0) and (1 0 1) depending on T_sub. With the increasing of T_sub, both the surface roughness and crystallite size increased. The room-temperature electrical resistivity of IrO₂ thin films decreased with increasing T_sub, showing a low value of (42 ± 6) × 10⁻⁸ Ω m at T_sub = 773 K.     

EDXRF measurements on gold diffusion-doped Bi1.8Pb0.35Sr1.9Ca2.1Cu3Oy

Gold (Au) diffusion in superconducting Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃O_y was investigated over the temperature range 500-800 °C by the energy dispersive X-ray fluorescence (EDXRF) technique. It is found that the Au diffusion coefficient decreases as the diffusion-annealing temperature decreases. The temperature dependences of Au diffusion coefficient in grains and over grain boundaries are described by the relations D₁=6.7×10⁻⁵exp(−1.19 eV/k_BT) and D₂=9.7×10⁻⁴exp(−1.09 eV/k_BT), respectively. The diffusion doping of Bi-2223 by Au causes a significant increase of the lattice parameter c by about 0.19%. For the Au-diffused samples, dc electrical resistivity and transport critical current density measurements indicated the critical transition temperature increased from 100 to 104 K and the critical current density increased from 40 to 125 A cm⁻², in comparison with those of undoped samples. From scanning electron microscope (SEM) and X-ray diffraction (XRD) measurements it is observed that Au doping of the sample also improved the surface morphology and increased the ratio of the high-T_c phase to the low-T_c phase. The possible reasons for the observed improvement in microstructure and superconducting properties of the samples due to Au diffusion are also discussed.