Third-order optical nonlinearities in anatase and rutile TiO2 thin films

Titanium dioxide (TiO₂) films have been fabricated on fused quartz and Si(001) substrates by pulsed laser deposition technique and the single-phase anatase and rutile films were obtained under the optimal conditions. The surface images and optical transmission spectra were investigated by scanning electron microscopy and double beam spectrophotometer, respectively. The values of optical band-gap and linear refractive index of the anatase and rutile films were determined. The optical nonlinearities of the films were measured by Z-scan method using a femtosecond laser (50 fs) at the wavelength of 800 nm. Through the open-aperture and closed-aperture Z-scan measurements, the real and imaginary parts of the third-order nonlinear optical susceptibility were calculated and the results show that the anatase phase TiO₂ films exhibit larger nonlinear refractive effects compared with rutile phase. The figure of merit, T, defined by T = βλ/n₂, was calculated to be 0.8 for anatase films, meeting the requirement of T < 1 and showing potential applications in all-optical switching devices.     

Z-scan measurement for the nonlinear absorption of Bi2.55La0.45TiNbO9 thin films

Bi_2.55La_0.45TiNbO₉ (BLTN-0.45) thin films with layered aurivillius structure were fabricated on fused silica substrates by pulsed laser deposition technique. Their structure, fundamental optical constants, and nonlinear absorption characteristics have been studied. The film exhibits a high transmittance (> 60%) in visible-infrared region. The optical band gap energy was found to be 3.44 eV. The optical constant and thickness of the films were characterized using spectroscopic ellipsometric (SE) method. The nonlinear optical absorption properties of the films were investigated by the single-beam Z-scan method at a wavelength of 800 nm laser with a duration of 80 fs. We obtained the nonlinear absorption coefficient β = 4.64 × 10^− 8 m/W. The results show that the BLTN-0.45 thin film is a promising material for applications in absorbing-type optical device.     

Second-order nonlinear optical properties of cross-linked silica films prepared through sol-gel process

In this paper, second-order NLO chromophores containing two reactive sites have been synthesized and characterized by ¹H NMR, FTIR and elemental analysis. The transparent films have silica network of matrix and covalently bonded chromophore of NLO segment were then fabricated via sol-gel process. The SHG coefficients (d₃₃) of the poled films were calculated to be around 50 pm/V by in situ second harmonic generation (SHG) measurement. The thermal stability of the NLO coefficient was investigated by the depoling experiment and temporal decay test, showing that the cross-linked films had a good thermal stability up to 160 °C.     

High thermally stable hybrid nonlinear optical films containing heterocycle chromophores

Two azole heterocycle-contained chromophores, named EBTC and EBOC respectively, were synthesized and then reacted with 3-isocyannatopropyltriethoxysilane (ICTES) to form functionalized alkoxysilane dyes (ASDs) which were named as ASD-EBTC and ASD-EBOC, respectively. Sol solutions were obtained after co-hydrolysis/condensation of ASD and tetraethoxysilane (TEOS). Second-order nonlinear optical (NLO) inorganic-organic hybrid films were successfully prepared via sol-gel process. The second harmonic coefficients (d₃₃) of poled hybrid films were measured by a Maker fringe technique. The d₃₃ values obtained for the EBTC and EBOC films are 23.7 and 9.3 pm/V, respectively. The hybrid film containing benzothiazole exhibited better NLO properties than its analogue containing benzoxazole.     

Preparation of sol-gel films containing thiazole-based chromophore and their nonlinear optical performances

A push-pull thiazolylazo chromophore and alkoxysilane-terminated chromophore have been synthesized. Their structures were verified by elemental analysis, FTIR, UV-Visible spectra and ¹H NMR. Followed by a hydrolysis and copolymerization process of the alkoxysilane with poly(3-(trimethoxysilyl)propyl methacrylate) (PTMSPM), transparent hybrid films were obtained by spin-coating. From TGA thermogram the initial decomposition temperature of the hybrid film was determined to be 231 °C. The molecular hyperpolarizability of thiazolylazo chromophore was evaluated by solvatochromic method and the nonlinear optical coefficient value of the hybrid film was also calculated to be 56.8 pm/V by second harmonic generation (SHG) measurements. The poled film exhibits fairly high stability of optical nonlinearity in depoling experiment, implying its suitability for device applications.     

Synthesis and NLO properties of hybrid inorganic-organic films containing thiophene ring

A family of second-order nonlinear optical sol-gel films with a pendent thiophene chromophore has been developed. The key step in preparation these hybrids was the syntheses of alkoxysilane dye containing thiophene ring, which was accomplished by utilizing the urethane forming reaction. Molecular structural characterization for the resultant was achieved by FTIR, ¹H NMR and UV-Visible spectra. The microscopic optical nonlinearity of the thiophene chromophore was evaluated to be 7646 × 10^− 30 esu D by solvatochromic method and second harmonic coefficients (d₃₃) of the hybrid film was also calculated to be 41.2 pm/V by second harmonic generation (SHG) measurements, respectively. Furthermore, the stability of optical nonlinearity in poled film was also investigated through a depoling experiment.     

Layer-by-layer self-assembly of dye-polyoxometalate multilayer composite films and their fluorescent properties

The layer-by-layer (LbL) self-assembly technique was successfully applied to the fabrication of dye-polyoxometalate multilayer composite films consisting of two dye molecules Rhodamine B (RB) and Rhodamine 6G (R6G) and a Keggin-type polyoxometalate [α-SiW₁₂O₄₀]⁴⁻ (α-SiW₁₂). The composite films were characterized by UV-vis spectroscopy, scanning electron microscopy (SEM), and fluorescence spectroscopy. UV-vis spectra show that the characteristic absorbance values of the multilayer films increase almost linearly with the number of dye/α-SiW₁₂ bilayers, suggesting that the deposition process is regular and highly reproducible from layer to layer. SEM micrographs indicate that the film surface is a little rough with some individual granular domains. In addition, the fluorescent properties of these composite films were also investigated by fluorescence spectroscopy.     

Optical response and surface morphology of crystalline Nd-doped fluoride films grown on monocrystalline LiYF4 substrates by pulsed laser deposition

The realization of crystalline films of Nd³⁺:YF₃ and Nd³⁺:LiYF₄ on a monocrystalline LiYF₄ substrate by pulsed laser deposition is reported. The films were obtained by laser ablation with 355 nm photons of a bulk LiYF₄ crystal doped with Nd³⁺ ions at 1.5% atomic concentration in the presence of different ablation/deposition parameters. The films optical characteristics, analyzed via laser induced polarized fluorescence spectroscopy upon IR excitation, are presented. Lifetime measurements of the fundamental Nd³⁺ ion transition in the film were also performed. All these results were compared with those obtained in the Nd³⁺:LiYF₄ bulk crystal. The surface morphology of the depositions was analyzed via a scanning electron microscope. When the production of the deposition took place in high vacuum (1 × 10^− 4 Pa) and the substrate temperature was 750 °C, the grown film was Nd³⁺:YF₃. A 1 Pa controlled atmosphere of He in the ablation chamber and a substrate temperature of 650 °C favoured the growth of a Nd³⁺:LiYF₄ film. In the latter case the film showed also a smoother surface.     

A statistical parameter study of indium tin oxide thin films deposited by radio-frequency sputtering

In order to optimize the electrical and optical properties of indium tin oxide (ITO) thin films, a statistical analysis called Taguchi design was employed. It is shown that the sheet resistance and transmittance are inversely proportional to each other as a function of the process parameters. Additionally, the preferred orientation of crystalline ITO film is distinguishably changed with the increase of sputtering temperature and oxygen fraction (O₂/O₂+Ar) in the sputtering ambient. The change in crystallinity results from the content of incorporated oxygen, which significantly affects the electrical and optical properties of ITO films and causes a rearrangement of atoms to form preferred closed-packed plane orientation. Finally, the microstructure of the ITO films becomes denser with the increasing oxygen fraction. As a result of this work, we have successfully achieved low sheet resistance (7.0 Ω/□) and high transmittance (~90%) for 300 nm thick films.     

Transport phenomena in high performance nanocrystalline ZnO:Ga films deposited by plasma-enhanced chemical vapor deposition

Nanocrystalline gallium doped zinc oxide (ZnO:Ga) thin films were synthesized by plasma-enhanced chemical vapor deposition (PECVD). A statistical design of experiments (DOE) was employed to optimize electrical conductivity. A carrier concentration of 5.5×10²⁰/cm³ and a mobility of 15 cm²/V s yielding a resistivity of 7.5×10⁻⁴ Ω cm resulted from the conditions of high pressure, rf power, and electrode gap. X-ray diffraction showed that gallium doping had a profound impact on film orientation. Atomic force microscopy (AFM) revealed that the films were nanostructured, with an average grain size of 80 nm and a surface roughness of ∼2 nm. This unique morphology benefited optical transmission, but limited electrical performance. Average transmission across the visible spectrum was ∼93% as scattering losses were minimized. Temperature dependent Hall and optical transmission measurements demonstrated that structural defects and ionized impurities were equal contributors to electron scattering.     

Structural characterization of CdS thin film on quartz formed by femtosecond pulsed laser deposition at high temperature

CdS thin films have been grown on quartz substrates using femtosecond pulsed laser deposition. The structural and optical properties of the CdS thin films were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. The results indicate that the compositional segregation of the CdS films could be drawn from the selective evaporation of sulfur from the film surface as a result of heating up the substrates. Growth temperature played an important role on changing crystal structure and optical properties of the CdS films.     

Thickness dependence of structural, electrical and optical properties of sprayed ZnO:Cu films

ZnO:Cu thin films have been deposited by spray pyrolysis techniques within two different (450 °C and 500 °C) substrate temperatures. The structural properties of ZnO:Cu thin films have been investigated by X-ray diffraction techniques. The X-ray diffraction spectra showed that ZnO:Cu thin films are polycrystalline with the hexagonal structure and show a good c-axis orientation perpendicular to the substrate. The most preferential orientation is along the (002) direction for all spray deposited ZnO:Cu films together with orientations in the (100) and (101) planes also being abundant. Some parameters of the films were calculated and correlated with the film thickness for two different substrate temperatures. The optical properties of ZnO:Cu thin films have been investigated by UV/VIS spectrometer and the band gap values were found to be ranging from 3.29 eV to 3.46 eV.     

Structural, optical and electrical properties of undoped polycrystalline hematite thin films produced using filtered arc deposition

Thin films of α-Fe₂O₃ (hematite) were deposited using filtered arc deposition. The structural, optical and electrical properties of the films have been characterized. High-purity hematite films were produced, free from other iron oxide phases and impurities. The films exhibit preferred orientation, with the c-axis of the hexagonal structure aligned perpendicular to the substrate. The films have an upper uncertainty bound of the porosity of 15%, with a microindentation hardness of 17.5 ± 1 GPa and elastic modulus of 1235 ± 5 GPa. The indirect and direct band gap energies were found to be approximately 1.9 eV and 2.7 eV, respectively. The refractive index, and the extinction and absorption coefficients were determined from total reflectance and direct transmittance measurements. The thin films exhibit a high resistivity (≥ 10⁵ Ω cm) which indicates pure α-Fe₂O₃. An activation energy of 0.7 eV was calculated from an Arrhenius plot of the conductivity.     

Low temperature photoluminescence and infrared dielectric functions of pulsed laser deposited ZnO thin films on silicon

C-axis oriented ZnO thin films were grown on silicon (100) and (111) substrates by pulsed laser deposition. Low temperature photoluminescence spectra show besides the peaks of free excitons, of defect bound excitons, and of a donor-acceptor pair transition a new doublet at 3.328/3.332 eV. The doublet seems to originate from the columnar textured ZnO film structure. A corresponding structural dependence of the broadening parameter of the infrared dielectric functions was derived from spectroscopic ellipsometry in the spectral range from 380 to 1200 cm^− 1. The wave numbers of the E₁ transverse optical and A₁ longitudinal optical phonon modes of the ZnO films on silicon are determined to be 406 and 573 cm^− 1, respectively. These values are slightly smaller than those of single-crystalline ZnO thin films on sapphire.     

Preparation of strain-included rutile titanium oxide thin films and influence of the strain upon optical properties

Rutile type titanium dioxide (TiO₂) thin films are prepared by pulse laser deposition (PLD) with controlled oxygen pressure. Transmission electron microscopy (TEM) analysis clarified that crystalline structures, surface structures and electric states of the TiO₂ films almost correspond to those of the bulk rutile. We observed crystalline domain including strain in the PLD film. The strain of the TiO₂ film was measured by nano-beam diffraction. The strain-included domains affected properties of the film as optical absorption. The obtained optical band gap energy value was 3.30 eV, which was larger than that of bulk. TEM results such as crystalline grain sizes and distribution were used to consider quantum size effect in order to explain the larger band gap value. Moreover the influence of strain in rutile crystalline grains upon optical properties was suggested in the present study. The difference of band gap energy between experimental and theoretically calculated ones was considered to come from the strain effects.     

Nd-doped fluoride film grown on LiYF4 substrate by pulsed laser deposition

The realization of a monocrystalline nanofilm of Nd³⁺-doped fluoride on LiYF₄ substrates by pulsed laser deposition is reported. The film was obtained by laser ablation with 355 nm photons of a bulk LiYF₄ crystal doped with Nd³⁺ ions at 1.5% atomic concentration. The measurements of the sample thickness obtained by an in situ interferometric technique, and the film optical characteristics analyzed via laser induced fluorescence spectroscopy upon UV and IR excitation, are presented. Lifetime measurements of the fundamental Nd³⁺ ion transition in the film were also performed. All the results were compared with those obtained in a Nd³⁺:LiYF₄ bulk crystal. The emission spectra of the deposited film following IR excitation seem to indicate the obtained deposit is Nd³⁺:YF₃.     

Optical constants and band edge of amorphous zinc oxide thin films

The optical characteristics of amorphous zinc oxide (a-ZnO) thin films grown by radio frequency reactive magnetron sputtering on various substrates at temperature < 325 K have been investigated in the spectral range 340-1600 nm. The amorphous nature of the a-ZnO films was verified by X-ray diffraction and the optical constants were obtained by analysis of the measured ellipsometric spectra using the Cauchy-Urbach model. Refractive indices and extinction coefficients of the films were determined to be in the range 1.67-1.93 and 3.9 × 10^− 8-0.32, respectively. The band edge of the films on Si (100) and quartz has been determined by spectroscopic ellipsometry (3.39 ± 0.05 eV) and spectrophotometric (3.35 ± 0.05 eV) methods, respectively. From the angle dependence of the p-polarized reflectivity we deduce a Brewster angle of 60.5°. Measurement of the polarized optical properties shows a high transmissivity (81%-99%) and low absorptivity (< 5%) in the visible and near infrared regions at different angles of incidence. Also, we found that there was a higher absorptivity for wavelength < 370 nm. This wavelength, ∼ 370 nm, therefore indicated that the band edge for a-ZnO thin films is about 3.35 eV.     

Deposition of particulate-free thin films by two synchronised laser sources: effects of ambient gas pressure and laser fluence

The micrometer and sub-micrometer sized particulates present both on the surface and inside of pulsed laser deposited thin films and structures stand for the main drawback of the method in view of technological applications. We applied a two-laser system in order to withdraw the particulates in case of Ta and TaO_x thin films. The Ta targets were irradiated by the first UV laser, while the second IR laser was directed parallel to the target surface, aiming to heat and evaporate the particulates. The morphology of the obtained thin films was studied by scanning electron microscopy. For the TaO_x films, the ambient gas pressure influences, besides the size and density of particulates, their propagation velocity. This in turn results in the variation of the optimum delay time between the ablating UV and the second IR laser pulse. For the Ta films we found that a threshold fluence of the IR laser pulse exists, above which completely particulate-free films were deposited.     

Irreversible photoinduced changes in As48S52 amorphous thin films prepared by pulsed laser deposition

Thin amorphous As-S films were prepared using pulsed laser deposition. Raman scattering spectroscopy, variable angle spectroscopic ellipsometry, and optical transmittance spectra revealed irreversible photostructural effects, significant photoinduced changes of refractive index, and optical band gap energy in the films. Observed effects are discussed in terms of structural transformations of basic structural units.     

Effects of deposition time and temperature on the optical properties of air-annealed chemical bath deposited CdS films

CdS films of over 1-μm thickness were deposited onto glass substrates by chemical bath deposition (CBD). Deposition temperature and time were varied from 40 °C to 60 °C and from 30 min to 4 h, respectively. The highest deposition rate, 6.39 nm/min, was obtained with samples deposited for 90 min at 60 °C. The films deposited at 60 °C for 4 h were found to have the best adhesion and without defects. The optical properties, in particular the optical band gap, depended on film thickness, the deposition and annealing temperatures. Annealing in air resulted in a shift of the absorption edge towards higher wavelengths, i.e., a decrease in the gap value from 2.45 eV to 2.38 eV. The optical band edges of the films were not constant but depended on the annealing temperature. The refractive index, calculated by applying the envelope method on the transmission of the films in wavelengths from 550 nm to 850 nm, was in the range 1.95-2.26. The resistivity determined from dark conductivity measurement, as a function of the annealing temperature, was found to be in the order of 10⁵ Ω cm for samples annealed in air at 250 °C, 3 h, and the activation energy was about 0.22 eV.