Highly transparent polyimide/nanocrystalline-titania hybrid optical materials for antireflective applications

This study prepared polyimide/titania hybrid thin films of, poly(3,3′,4,4′-Benzophenone tetra carboxylic dianhydride)–(4,4-Diaminodiphenyl Ether)/nanocrystalline titania (BTDA-ODA/TiO₂), with a high refractive index. FTIR analysis confirmed the formation of a polyimide and titania matrix. TGA and DSC analysis gauged the decomposition temperature in the range of 455–532 °C, indicating that the addition of titania could increase the glass transition temperature of hybrid films. XRD results indicate the formation of nanocrystalline titania domains of approximately 8–11 nm in the hybrid films. AFM, SEM, TEM, and XRD results show the formation of well-dispersed nanocrystalline titania. The refractive index was measured by ellipsometry, demonstrating an increase in the refractive index of the prepared hybrid thin films from 1.657 to 1.958, when the titania content was increased from 0 to 90 wt.%. UV–vis analysis gauged the cutoff wavelength in the range of 288–357 nm. The prepared polyimide/titania hybrid thin films were used to develop a three layer antireflective (AR) coating on a glass substrate. The average reflectance of the AR was 0.5% for the PT20/PT90/F-siloxane layer and 0.6% for the PT20/PT90/porous silica. Transparency at 550 nm exceeded 90% for both AR coatings.     

Evanescent wave optical-fiber sensing (temperature, relative humidity, and pH sensors)

Sensitive and versatile evanescent wave-sensing systems featuring polished optical fiber-based sensor designs with low-cost light sources have been developed for temperature, relative humidity, and pH measurements. The work herein contained describes the fabrication of three types of sensors based on standard silica, single-mode fibers previously subjected to a lateral polishing of the cladding. Temperature sensing through oils whose refractive index varied linearly with temperature showed applicability with up to 5 dB//spl deg/C for a 5/spl deg/ range. Polyvinyl alcohol films on the fibers showed almost 10-dB linear variation from 70% to 90% relative humidity. Sol-gel trapped dyes as thin films on the polished surface were capable of performing 15-dB output variation (although not linearly) for pH ranging from 2 to 11.     

Y0.9 Er0.1 Al3(BO3)4 thin films prepared by the polymeric precursor method for integrated optics

This work reports on the optimization of Yo.9 Er0.1 Al3(BO3)4 thin films for integrated optics. The films were deposited on silica and silicon substrates using the spin-coating technique involving solutions previously prepared by the polymeric precursor method. These deposits, 400-800 nm thick, were prepared by a 5-10 multi-layer process and heat treatments at different temperatures from glass transition to crystallization temperature, using heating rates of 2 or 5 degrees C/min. The structural characterizations were performed using grazing incidence X-ray diffraction and Fourier transform infrared spectroscopy (FT-IR). Water and/or hydroxyl contents were also evaluated from FT-IR spectra. Microstructural evolution in term of annealing temperatures was analyzed by high resolution scanning electronic microscopy and atomic force microscopy. Optical transmission spectra were used to determine the refractive index and thickness through the envelope method of the films. Finally, the film guiding and optical properties were studied by m-line spectroscopy. The best film showed a good waveguiding with high light-coupling efficiency close to the theoretical limit.     

Optical properties and sensing applications of lithium iron phosphate thin films

A composite optical waveguide sensor, consisting of lithium iron phosphate (LiFePO₄, LFP) as the sensing material, was constructed and utilized for the detection of volatile organic compound gases. Nano-LFP powder was prepared via the hydrothermal method and was subsequently utilized in a dip-coating procedure for the fabrication of LFP thin films. The effect of heat treating temperature on the refractive index of the thin films was studied. A glass optical waveguide gas sensor was fabricated by coating an LFP thin film on the surface of single-mode tin-diffused glass optical waveguide. The sensor was found to exhibit a linear response to xylene in the range of 50-1000 ppm, with response times of less than 5 s.     

Preparation and optical properties of phase-change VO2 thin films

In this work, VO₂ thin films were prepared on three kinds of substrates by the sol-gel dipcoating method followed by heat treatment under vacuum. These thin films were analysed by x-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. The infrared and ultraviolet-visible spectra of the VO₂ thin films were also recorded during heating and cooling between room temperature and 100°C. The experimental results show that VO₂ thin films thus prepared exhibit thermally induced reversible phase transition, and the largest changes in transmittance and reflectivity are approximately 58 and 25%, respectively, in the case of vacuum heat treatment at 400°C and silica glass substrates. The refractive index (n) decreases and the absorption coefficient (k) increases when heating these thin films from room temperature to 100°C, and vice versa for cooling. The reasons why the optical constants and infrared absorption spectra change so remarkably are discussed.     

Refractive indices of textured indium tin oxide and zinc oxide thin films

The refractive indices of textured indium tin oxide (ITO) and zinc oxide (ZnO) thin films were measured and compared. The ITO thin film grown on glass and ZnO buffered glass substrates by sputtering showed distinct differences; the refractive index of ITO on glass was about 0.05 higher than that of ITO on ZnO buffered glass in the whole visible spectrum. The ZnO thin film grown on glass and ITO buffered glass substrates by filtered vacuum arc also showed distinct differences; the refractive index of ZnO on glass was higher than that of ZnO on ITO buffered glass in the red and green region, but lower in the blue region. The largest refractive index difference of ZnO on glass and ITO buffered glass was about 0.1 in the visible spectrum. The refractive index variation was correlated with the crystal quality, surface morphology and conductivity of the thin films.     

Chemical gas sensor application of open-pore mesoporous thin films based on integrated optical polarimetric interferometry

Chemical gas sensors that employ integrated optical polarimetric interferometry were fabricated by the sol-gel synthesis of transparent mesoporous thin films of TiO2-P2O5 nanocomposite on tapered layers of TiO2 sputtered on tin-diffused glass waveguides. Atomic force microscopy images of the mesoporous thin film clearly show the open pore mouths on the film surface that favor rapid diffusion and adsorption of gas-phase analytes within the entire film. Adsorption of gas and vapor induces changes (Deltan) in the refractive index of the mesoporous thin film that lead to shifts in the phase difference between the fundamental transverse electric and magnetic modes simultaneously excited in the glass waveguide via single-beam incidence. Upon exposure to NH3 gas at concentrations as low as 100 ppb in dry air at room temperature, the sensor exhibits a reversible change in the phase difference with the response and recovery times of less than 60 and 90 s, respectively. It is unexpected that the sensor is unresponsive to either NO2 or C6H6 vapor, leading to a somewhat selective sensitivity to NH3. Determination of Deltan was carried out with a combination of the experimental results and the theoretical calculations. The sensor design represents a novel, effective, and easily accessible approach to mesoporous thin-film-based integrated optical chemical sensors.     

Optical properties of SiO2-TiO2 sol-gel thin films

The optical properties of thin SiO₂-TiO₂ sol-gel composite films were investigated using exact optical models and the Forouhi-Bloomer model, (Phys. Rev. B34, 7018 (1986)), which describes the optical dispersion of amorphous dielectrics. Films deposited on glass and silicon substrates, were characterized by optical transmission and reflection measurements. Theoretical spectra have been generated and fitted to the experimental ones via standard regression analysis techniques. The (five) adjustable Forouhi-Bloomer parameters describing the dispersion of the complex refractive index, as well as the film thickness were determined. The refractive index and absorption coefficient of the films were found to depend on the molar contents of the component oxides.     

Characterization of chemically sprayed CdO films on borate and phosphate glass substrates produced by melt-quenching technique

The properties of substrates used to deposit thin films are an important parameter in thin film production. Instead of using a commercial substrate, in this work, borate and phosphate glasses have been obtained by classic melt-quenching technique to be used as substrates for CdO films. Also, a microscope glass substrate has been used to compare the coating properties by other glass substrates. All films have been produced by Ultrasonic Spray Pyrolysis technique. The substrate temperature has been selected as 275 ± 5 °C. Thicknesses and some optical parameters such as refractive index and extinction coefficient have been determined by spectroscopic ellipsometry. Absorbance and transmittance spectra have been taken by UV/VIS spectrophotometer. Four-probe method has been used to determine the electrical resistivity values of the films. XRD investigations have shown that type of the substrate dramatically affects the characteristics of CdO films. CdO film deposited on phosphate glass substrate has the best structural quality. Atomic Force Microscope has been used to investigate the surface properties and roughness values of the films.     

Transparent high refractive index nanocomposite thin films

This work reports the preparation of acetic acid-modified TiO₂ nanoparticles by sol–gel synthesis method. The nanoparticles can be incorporated directly into the polymer matrix to form transparent high refractive index nanocomposite thin films. The result shows that increasing the titania content in the hybrid nanocomposite thin films can significantly increase the refractive index. Hybrid nanocomposite thin film with refractive index value of 2.38 had been prepared. All prepared films also exhibit excellent optical transparency in the visible region.     

Optical anisotropy,molecular orientations,and internal stresses in thin sulfonated poly(ether ether ketone) films

The thickness, the refractive index, and the optical anisotropy of thin sulfonated poly(ether ether ketone) films, prepared by spin-coating or solvent deposition, have been investigated with spectroscopic ellipsometry. For not too high polymer concentrations (≤5 wt%) and not too low spin speeds (≥2000 rpm), the thicknesses of the films agree well with the scaling predicted by the model of Meyerhofer, when methanol or ethanol are used as solvent. The films exhibit uniaxial optical anisotropy with a higher in-plane refractive index, indicating a preferred orientation of the polymer chains in this in-plane direction. The radial shear forces that occur during the spin-coating process do not affect the refractive index and the extent of anisotropy. The anisotropy is due to internal stresses within the thin confined polymer film that are associated with the preferred orientations of the polymer chains. The internal stresses are reduced in the presence of a plasticizer, such as water or an organic solvent, and increase to their original value upon removal of such a plasticizer.     

硅碳氧薄膜光学性能研究

硅碳氧(SiCO)薄膜是一种三元玻璃状化合物材料,具有热稳定性好、能带宽、折射率大、硬度高等特性,是一种具有潜在应用价值的新颖光学薄膜材料。本文采用射频磁控溅射技术在Si(100)及K9玻璃上制备了硅碳氧薄膜。利用椭圆偏振仪、紫外/可见/近红外光度计及X射线光电子能谱测试表征了薄膜的光学性能及薄膜组分。研究发现,通过改变基片温度、工作压强及溅射功率等工艺参数,所制备的硅碳氧薄膜均具有高折射率(大于1.80),相比之下,K9玻璃基硅碳氧薄膜的折射率有着更大的变化范围(1.84~2.20)。通过对K9玻璃基硅碳氧薄膜的光学透射性能研究表明,以硅碳氧陶瓷作为溅射靶材,采用射频磁控溅射技术在K9玻璃基上可以制备出,在可见光及近红外区域有着较好光学透射性能,平均透过率能到达83%的硅碳氧薄膜。     

Optical switching property of Pd-capped Mg-Ni alloy thin films prepared by magnetron sputtering

Pd-capped Mg-Ni alloy thin films were prepared by magnetron sputtering, on glass substrates, and the dependence of the optical switching property of these films on alloy composition and temperature has been investigated using diluted hydrogen gas and dry air for changing. The transition from the transparent state to the mirror state by dehydrization shows strong dependence on both factors, while the hydrization transition has weak dependence. At higher temperature, the dehydrization is much faster for Mg-rich Mg-Ni thin film whose transition is very slow at room temperature. By using heated air to change from the transparent state to the mirror state, a fast switching can be done for Mg-rich Mg-Ni alloy thin film which has a wide optical modulation range.     

Phase transformations in TiO2/SiO2 sol-gel films as a function of composition and heat-treatment

The refractive indices of titania/silica sol-gel films are known to vary over a wide range with composition. However, little work has been done to investigate the possible variations in refractive index that can be produced for one specific sol-gel composition by varying the heat-treatment time and temperature. In this study, three different titania/silica sol-gel compositions containing 40, 60 and 80 mol% titania were studied after thermal processing at temperatures from 175–1050°C for variable lengths of time. The refractive indices and thicknesses of the sol-gel films spin-coated on to silicon wafers were determined using a combined technique of ellipsometry and reflectance spectroscopy. The microstructural differences in the films were investigated using X-ray diffraction and transmission electron microscopy techniques.     

Ultraviolet photobleaching process of azo dye doped polymer and silica films for fabrication of nonlinear optical waveguides

We describe the effect of UV photobleaching of poled polymer and silica films and the application of UV photobleaching to waveguide-type optical devices. Disperse Red 1-doped poled polymer and silica films with large and stable second-order nonlinearity were used as nonlinear optical materials. We investigated the mechanism of UV photobleaching of poled films by the changes in absorption spectrum and nonlinearity and refractive index. Moreover, simple fabrication of both the channel waveguide and the chi((2)) diffraction grating based on UV photobleaching is demonstrated.     

Determination of Optical Constants of Solgel-Derived Inhomogeneous TiO(2) Thin Films by Spectroscopic Ellipsometry and Transmission Spectroscopy

Amorphous and nanocrystalline TiO(2) thin films coated on a vitreous silica substrate by a solgel dip coating method are investigated for optical properties by spectroscopic ellipsometry (SE) together with transmission spectroscopy. A method of analysis of SE data to determine the degree of inhomogeneity of TiO(2) films has also been presented. Instead of the refractive index, the volume fraction of void has been assumed to vary along the thickness of the films and an excellent agreement between the experimental and calculated data of SE below the fundamental band gap has been obtained. The transmission spectrum of these samples is inverted to obtain the extinction coefficient k spectrum in the wavelength range of 300-1600 nm by using the refractive indices and parameters of structure determined by SE. The nonzero extinction coefficient below the fundamental band-gap energy (3.2 eV) has been obtained for the nanocrystalline TiO(2) and shows the presence of optical scattering in the film.     

Application of sample-sample two-dimensional correlation spectroscopy to determine the glass transition temperature of poly(ethylene terephthalate) thin films

The glass transition temperatures (Tg) of poly(ethylene terephthalate) (PET) thin films with different thicknesses are determined by analyzing their in situ reflection-absorption infrared (RAIR) spectra measured over a temperature range of 28 to 84 degrees C. The criterion of standard deviation of the covariance matrices is used as a graphical indicator for the determination of the Tg present in the sample-sample two-dimensional (2D) correlation spectra calculated from the temperature-dependent RAIR spectra. After two data pretreatments of the first derivative of the spectral absorbance versus temperature and the mean normalization over the wavenumbers are sequentially carried out on the RAIR spectra, an abrupt change of the first-derivative correlation spectra with respect to temperature is quickly obtained. It reflects the temperature at which the apparent intensity changes in pertinent absorption bands of PET thin films take place due to the dramatic segmental motion of PET chain conformation. The Tg of the thin PET films is accordingly determined. The results reveal that it decreases with a great dependence on the film thickness and that sample-sample 2D correlation spectroscopy enables one to determine the transition temperature of polymer thin films in an easy and valid way.     

Highly transparent, flexible, and thermally stable superhydrophobic ORMOSIL aerogel thin films

We report preparation of highly transparent, flexible, and thermally stable superhydrophobic organically modified silica (ORMOSIL) aerogel thin films from colloidal dispersions at ambient conditions. The prepared dispersions are suitable for large area processing with ease of coating and being directly applicable without requiring any pre- or post-treatment on a variety of surfaces including glass, wood, and plastics. ORMOSIL films exhibit and retain superhydrophobic behavior up to 500 °C and even on bent flexible substrates. The surface of the films can be converted from superhydrophobic (contact angle of 179.9°) to superhydrophilic (contact angle of <5°) by calcination at high temperatures. The wettability of the coatings can be changed by tuning the calcination temperature and duration. The prepared films also exhibit low refractive index and high porosity making them suitable as multifunctional coatings for many application fields including solar cells, flexible electronics, and lab on papers.     

Measurement of thermally induced changes in the refractive index of glass caused by laser processing

The effects of CO2 laser heating of pure fused silica are investigated. Studies show that the laser heating process causes a small volume of glass to be left in an altered microstructural state. To measure the refractive index of this altered region, a process was developed to create a thin film of altered glass. Samples were measured with a prism coupler, and a theoretical model was developed to predict the intensity values collected during the measurement. A least-squares routine was used to determine the refractive index that results in the best fit between the experimental and predicted intensity data. The refractive index in the altered glass was found to increase by approximately 0.07%.     

TiO2-SiO2系统凝胶玻璃薄膜折射率的研究

应用光度法研究了溶胶－凝胶系统组成、热处理温度对iO2-SiO2系统凝胶玻璃薄膜折射率的影响规律。随薄膜中TiO2含量的增加以及热处理温度的升高，薄膜的折射率逐渐增大。通过调整TiO2-SiO2系统的组成及适当的热处理温度，可实现TiO2-SiO2系统薄膜折射率在1．5－2．4之间的连续变化。