Optical waveguiding and nonlinear optical characterization of 2-docosylamino-5-nitropyridine Langmuir-Blodgett films

Well ordered Y-type Langmuir-Blodgett (LB) multilayers of 2-docosylamino-5-nitropyridine (DCANP) have been characterized with linear and nonlinear optical techniques. The nonlinear optical susceptibilities have been determined for the wavelengths λ = 1064 nm (d₃₃ = (7.8±1) pm/V, d₃₁ = (2.0 ± 0.5) pm/V) and λ = 1318 nm (d₃₃ = (5.6 ± 1) pm/V). Waveguiding experiments with grating couplers allowed the excitation of TE_o and TM_o modes propagating over more than 20 mm. Guided wave attenuation coefficients as low as 12 dB/cm (at the wavelength λ = 632.8 nm) are reported. Coupling experiments allowed the determination of the dispersion of the refractive index n₃ (n₃^{632.8 nm} = 1.598). The refractive index data could be used to calculate phase-matching conditions for frequency doubling with TE modes in DCANP waveguides.     

A New Elaboration Route by Sol-Gel Process for Cerium Doped SrHfO3 Films and Powders

We present for the first time the elaboration via sol gel route of cerium (1 mol%) doped SrHfO₃ powders and films. The sol is elaborated using hafnium and strontium ethoxides as precursors and cerium nitrate as dopant. The structure of powders and films are characterized by convergent methods: Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, Raman spectroscopy and optical measurements conducted by the prism coupling method. The powder crystallises from amorphous to pure SrHfO₃ orthorhombic perovskite phase after a 800°C heat treatment. Nevertheless HfO₂ monoclinic phase coexists with orthorhombic perovskite phase after a 1000°C heat treatment. The film is amorphous for annealing temperatures lower than 700°C and presents good waveguiding performances. The film heat-treated at 700°C exhibits a refractive index of 1.810 ± 0.001 (λ = 543.5 nm) for a thickness around 375 nm. The attenuation coefficient obtained on the 400°C heat-treated film is α = 4.0 ± 0.5 dB/cm (λ = 632.8 nm). The film starts to crystallize at 750°C into the SrHfO₃ orthorhombic phase but HfO₂ monoclinic phase is also detected after a heat treatment at 1000°C. The potentiality of sol gel Ce³⁺:SrHfO₃ powders and films for scintillation applications are investigated.     

Optical anisotropy of polymeric films measured by waveguide propagation mode determination

Extruded thin films of a liquid-crystalline charge-conjugated rigid-rod polymer poly(p-phenylenebenzobisthiazole), PBT, and a semicrystalline thermoplastic polyethylene-terephthalate (Mylar) were fabricated and examined for film thickness, refractive index, and linear attenuation coefficient. Optical waveguide modes were successfully induced on the polymeric films using a prism coupler at λ = 633 and 1300 nm. Highly consistent thickness values were obtained for the polymeric films. In addition, the anisotropic nature of the optical properties was determined using TE and TM propagation modes. A refractive index as high as 2.3 was observed on PBT film. The refractive index data suggested that the PBT and Mylar films were optically anisotropic with refractive indices n? (out-of-plane) invariably smaller than n∥ (in-plane). Large anisotropy was also discovered in the linear attenuation coefficient α, with α_∥/α_? ≈ 50 for the Mylar films. Complementary polarimetric and spectroscopic interference measurements were also applied to investigate the optical anisotropy of the extruded polymeric films. © 1992 John Wiley & Sons, Inc.     

Characteristics of SnO2 thin films prepared by SILAR

SnO₂ thin films were deposited on glass substrates by using Successive Ionic Layer Adsorption and Reaction (SILAR) method at room temperature. The film thickness effect on characteristic parameters such as structural, morphological, optical and electrical properties of the films was studied. Also, the films were annealed in oxygen atmosphere (400 °C, 30 min) and characteristic parameters of the films were investigated. The X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) studies showed that all the films exhibited polycrystalline nature with tetragonal structure and were covered well on glass substrates. After the investigation of the crystalline and surface properties of the films, it was found that they were improving with increasing film thickness. Optical band gap decreased from 3.90 eV to 3.54 eV and electrical conductivity changed between 0.015–0.815 (Ω-cm)⁻¹ as the film thickness increased from 215 to 490 nm. The refractive index (n), optical static and high frequency dielectric constants (ɛ_o, ɛ_∞) values were calculated by using the optical band gap values as a function of the film thickness.     

Optical and other physical characteristics of amorphous Ge15Te85−xCux alloys

Composition dependencies of the optical properties of as- deposited amorphous Ge₁₅Te_85−xCu_x (x = 2, 3, 4, 5 and 6 at %) prepared by thermal evaporation have been studied. The optical transmission and reflection spectra are measured in the wavelength range (200–1200 nm). The optical absorption coefficient are studied for as- deposited samples. The optical absorption edge shift to higher energy range, as the copper content, x, increases in the film. Tauc's relation for the allowed direct transition is successfully describing the mechanism of the optical absorption. The refractive index, n has been determined from the transmission spectra measured at normal incidence. The absorption coefficient, α, therefore extinction coefficient, k, have been determined from the transmission spectra at the strong absorption region. The dispersion of refractive index is discussed in terms of the Single-oscillator Wimple–DiDomenico model. The optical dielectric constant is also estimated. The average heat of atomization and related parameters such as the average coordination number were also calculated from the heat of atomization and coordination number of the used elements. These obtained results were discussed in terms of chemically ordered network model and constraints theory.     

Chain orientation and anisotropies in optical and dielectric properties in thin films of stiff polyimides

Thin films of poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA), prepared by thermal imidization of the precursor poly(amic acid) on substrates, have been investigated by optical waveguide, ultraviolet-visible (UV-VIS), infrared (IR), and dielectric spectroscopies. The polyimide films exhibit an extraordinarily large anisotropy in the refractive indices with the in-plane index n_∥ = 1.806 and the out-of-plane index n_⊥ = 1.589 at 1064 nm wavelength. No discernible effect of the film thickness on this optical anisotropy is found between films of ca. 2.1 and ca. 7.8 μm thickness. This large birefringence is attributed to the preferential orientation of the biphenyltetracarboximide moieties with their planes parallel to the film surface, coupled with the strong preference of BPDA-PDA chains to align along the film plane. The frequency dispersion of the in-plane refractive index n_∥ is consistent with the results calculated by the Lorentz–Lorenz equation from the UV-visible spectrum exhibiting several absorption bands in the 170–500 nm region. The contribution from the IR absorption in the range 7000–400 cm,^?1 computed by the Spitzer-Kleinmann dispersion relations from the measured spectra, adds ca. 0.046 to the in-plane refractive index n_∥. Tilt-angle–dependent polarized IR results indicate nearly the same increase for the out-of-plane index n_⊥. Application of the Maxwell relation then leads to the out-of-plane dielectric constant ε^⊥ ? 2.7 at 1.2 × 10¹³ Hz, as compared with the measured value of ca. 3.0 at 10⁶ Hz. Assuming this small difference to remain the same for the in-plane dielectric constants ε_∥, we obtain a very large anisotropy in the dielectric properties of these polyimide films with the estimated in-plane dielectric constant ε_∥ ? 3.4 at 1.2 × 10¹³ Hz, and ε_∥ ? 3.7 at 10⁶ Hz. © 1992 John Wiley & Sons, Inc.     

Thickness dependent physical properties of close space evaporated In2S3 films

In recent years, In₂S₃ is considered as a promising buffer layer in the fabrication of heterojunction solar cells. Film thickness is one of the important parameters that alters the physical characteristics of the grown layers significantly. The effect of film thickness on the structural, morphological, optical and electrical properties of close space evaporated In₂S₃ layers has been studied. In₂S₃ thin films with different thicknesses in the range, 100–700 nm were deposited on Corning glass substrates at a constant substrate temperature of 300 °C. The films were polycrystalline exhibiting strong crystallographic orientation along the (103) plane. The deposited films showed mixed phases of both cubic and tetragonal structures up to a thickness of 300 nm. On further increasing the film thickness, the layers showed only tetragonal phase. With increase of film thickness, both the crystallite size and surface roughness in the films were found to be increased. The optical constants such as refractive index and extinction coefficient of the as-grown layers have been calculated from the optical transmittance data in the wavelength range, 300–2500 nm. The optical transmittance of the films was decreased from 82% to 64% and the band gap varied in the range, 2.65–2.31 eV with increase of film thickness. The electrical resistivity as well as the activation energy was evaluated and found to decrease with film thickness. The detailed study of these results was presented and discussed.     

Influence of surfactant and annealing temperature on optical properties of sol–gel derived nano-crystalline TiO2 thin films

Titanium dioxide thin films have been synthesized by sol–gel spin coating technique on glass and silicon substrates with and without surfactant polyethylene glycol (PEG). XRD and SEM results confirm the presence of nano-crystalline (anatase) phase at an annealing temperature of 300 °C. The influence of surfactant and annealing temperature on optical properties of TiO₂ thin films has been studied. Optical constants and film thickness were estimated by Swanepoel's (envelope) method and by ellipsometric measurements in the visible spectral range. The optical transmittance and reflectance were found to decrease with an increase in PEG percentage. Refractive index of the films decreased and film thickness increased with the increase in percentage of surfactant. The refractive index of the un-doped TiO₂ films was estimated at different annealing temperatures and it has increased with the increasing annealing temperature. The optical band gap of pure TiO₂ films was estimated by Tauc's method at different annealing temperature.     

TiO2 nanoparticles synthesized in an aprotic solvent and applied to prepare high-refractive-index TiO2-polyimide hybrid thin films

In this study, very small (2–5 nm) TiO₂ nanoparticles were synthesized in an aprotic solvent, N,N-dimethylacetamide, via hydrolysis and condensation of titanium alkoxide at room temperature. The synthesized TiO₂ sol showed 30 days of storage stability and could be used to prepare high-refractive-index TiO₂-polyimide hybrid thin films by an ex-situ method that involved a spin coating and multistep baking process. The field emission scanning electron microscope image showed a flat and uniform morphology of the hybrid thin film. TiO₂ domains were in the nanometer range, thus avoiding light scattering. The refractive index at 633 nm of the hybrid thin film reached 2.05, which suggested potential applications of the film to anti-reflective coatings and optical waveguides.     

Thickness and refractive index of DPPC and DPPE monolayers by multiple-beam interferometry

The thickness and refractive index of 1,2-dipalmitoyl-sn-glycero-3-phosphatidyl choline (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE) monolayers Langmuir--Blodgett (LB) deposited on mica were measured in dry air and bulk water using multiple-beam interferometry (MBI). Measurements of thickness using atomic force microscopy (AFM) of identical monolayers, and X-ray reflectivity (XRR) of the monolayers on quartz were taken for comparison. The measurement of the properties of solid-supported monolayers in dry air allows lipid optical properties to be determined free from solvent effects. The thickness and refractive index measured by MBI were 25.5?±?0.6 Å and 1.485?±?0.007 for DPPE monolayers, and 23.9?±?0.5 Å and 1.478?±?0.006 for DPPC monolayers in dry air. These thicknesses are consistent with the other techniques used in this work as well as other measurements in the literature. The refractive indices of solid-supported lipid monolayers have not been previously measured. The values are higher than previous measurements on black lipid films done by reflectometry, which is attributed to increased lipid packing density and the absence of hydrocarbon solvents. Applying water to the monolayers had no measurable effect on their properties, indicating that any change in hydration was below detection.

A Study on Liquid Hybrid Material for Waveguides—Synthesis and Property of PSQ‐Ls for Waveguides

Silicate‐based inorganic‐organic hybrid polymer systems have many unique properties including thermal stability and photo‐stability, chemical resistance with the combination of tunable optical properties. Two kinds of new UV‐patternable hybrid materials PSQ‐Ls were synthesized by a sol‐gel process at room temperature, which can be used for low cost fabrication of optical waveguides. Thick films (up to 8.31 µm) can be coated by a single spin‐coating process without any cracking and the average surface roughness (Ra), detected by atomic force microscopy (AFM), is below 0.5 nm. The optical properties (refractive index, birefringence, and optical loss at 1310 nm and 1550 nm, respectively) of the PSQ‐Ls films are investigated by a prism coupler. The refractive index of PSQ‐Ls can be exactly tuned from 1.4483 to 1.5212 by blending PSQ‐LH (n_TE=1.5212 @ 1310 nm) and PSQ‐LL (n_TE=1.4483 @ 1310 nm). The maximum refractive index contrast is about 4.8%. After post‐baking, birefringences of the films are below 0.0005 and optical losses are about 0.2 dB · cm^?1 at 1310 nm, 0.7 dB · cm^?1 at 1550 nm, respectively. Furthermore, the PSQ‐Ls films also show outstanding thermal stability in air atmospheres.     

Investigation of optical properties of amorphous Ge15Se85-xCux thin films using spectroscopic ellipsometry

Different compositions of amorphous Ge₁₅Se_85-xCu_x thin films were deposited onto glass substrates by the thermal evaporation technique. Their amorphous structural characteristics were studied by X-ray diffraction (XRD). The optical constants (n, k) of amorphous Ge₁₅Se_85-xCu_x thin films were obtained by fitting the ellipsometric parameters (ψ and Δ) data for the first time using three layers model system in the wavelength range 300–1100 nm. It was found that the refractive index, n, increases with the increase of Cu content. The possible optical transition in these films is found to be indirect transitions. The optical energy gap decreases linearly from 1.83 to 1.44 eV with increasing the Cu. The experimental transmittances spectrum can be simulated using the thickness and optical constants modeled by spectroscopic ellipsometry model.     

Photoreactive Langmuir–Blodgett–Kuhn multilayer assemblies from functionalized liquid-crystalline side chain polymers,I. Homopolymers containing azobenzene chromophores

Stable monomolecular layers of a liquid-crystalline homopolymer bearing azobenzene chromophores in the mesogenic side groups could be prepared at the water–air interface by slow compression at room temperature. From the area per monomer unit of A₀≈0.25 nm² a structural model with tightly packed azobenzene groups oriented nearly parallel to the surface normal was deduced. After transfer to a solid support in a Y-type mode the obtained Langmuir–Blodgett–Kuhn (LBK) multilayer assemblies of varying thicknesses (from a double layer to a thick film of d = 238 nm, capable of guiding optical modes) were prepared and analyzed by X-ray reflection and optical evanescent wave techniques. A monolayer thickness of d₀ = 2.52 nm, as well as the anisotropic index of refraction of these layers, could thus be evaluated. Photo-isomerization by UV-irradiation (at λ = (360±30)nm) was examined by UV-VIS absorption spectroscopy. X-ray reflectivity data of samples illuminated for 30 min showed a complete loss of the layered structure of the LBK samples. As a result strongly light-scattering films were obtained. Surface plasmon microscopy was used to demonstrate the usefulness of this photo-induced order–disorder transition for optical information storage.     

Investigations into the response of an optical gas sensor based on polymeric LB films

The response of an NO₂ sensing system based on LB films of a polysiloxane with azobenzene chromophoric side-chains has been investigated. Changes in absorbance on exposure to 100 ppm NO₂ have been recorded using UV-visible absorption spectroscopy from which changes in extinction coefficient (Δ k ≈ 0.033 at 500 nm) have been determined. Shallow angle X-ray reflectivity (SAXR) studies indicate a change in layer thickness from 2.10nm in air to 2.31 nm in 10000 ppm NO₂ together with loss of Bragg detail. Changes in real refractive index (Δn ≈ 0.107 over most of the visible region) for films in air and 100 ppm NO₂ have been deduced from reflectance spectra.     

Optical reflectivity and absorption measurements of sodium C222 sodide

The reflectivity at normal incidence of single crystals of Na⁺C222 · Na⁻ was measured from 350 to 2500 nm in a microreflectivity apparatus. The reflectivity spectrum shows a single peak at 630 ± 10 nm (1.97 eV) with a peak reflectivity of up to 60%. No rise in the reflectivity was found toward longer wavelengths. This absence of a plasma edge confirms that the concentration of conduction electrons is smaller than 1.8 × 10²⁰/cm³ as expected from the low electrical conductivity of this sodide. The absorption spectrum of thin solid films of Na⁺C222 · Na⁻, formed by vapor deposition, was measuredin situ in a vacuum evaporator. The absorption peak was at 650 ± 10 nm (1.91 eV), with a full-width at half-maximum of 0.37 eV. The reflectivity and absorption data were used together to estimate the indices of refraction and absorption and the components of the complex dielectric constant as a function of wavelength.     

Theoretical studies of the band structure and optoelectronic properties of ZnOxS1−x

In the present article, we have revisited the electronic band gap nature of ZnO_xS_1?x (0 ≤ x ≤ 1) with the recently developed modified Becke and Johnson exchange potential and the calculated band gaps are found consistent with the experimental results. We expect that the band gap bowing parameter obtained in the present work will be close to the experimental one. As the optical properties of ZnO_xS_1?x (0 ≤ x ≤ 1) are very important, therefore different optical parameters like dielectric functions, refractive index and reflectivity are also calculated. The results are illustrated in terms of band structures, band gap energy as a function of oxygen composition, total and partial density of states. © 2012 Wiley Periodicals, Inc.     

Picosecond laser ultrasonic measurements of interlayer elastic properties of 2H-MoSe2 and 2H-WSe2

We have used the ultrafast pump-probe technique known as picosecond ultrasonics to generate and detect coherent acoustic phonon pulses with frequencies reaching 40 GHz in exfoliated crystals of MoSe₂ and WSe₂ on Si and sapphire substrates. We report picosecond time-resolved reflectivity data from samples ranging from 180 nm to 920 nm in thickness and compare our results to a 1D simulation of strain-induced changes in the optical reflectivity. We find the longitudinal sound velocity along the c-axis (the interlayer direction) to be 2800 m/s ± 40 m/s for MoSe₂ and 2510 m/s ± 60 m/s for WSe₂. We also report the measured lifetime of longitudinal acoustic phonons approaching 40 GHz to be 0.85 ± 0.2 ns and compare this value with predictions of relaxation damping and 3-phonon models, as well as discuss its relationship to the predicted thermal conductivity of MoSe₂.     

Sol–gel-derived titania-hydroxypropylcellulose hybrid thin films of high refractive indices: solution components affecting the refractive index and uncracking critical thickness

Optically transparent, ca. 200–800 nm thick TiO₂-hydroxypropylcellulose (HPC) hybrid thin films were prepared from Ti(OC₃H₇ⁱ)₄–HPC–HCl–H₂O–C₃H₇ⁱOH solutions by the sol–gel method, where the as-deposited films were dried at 120 °C. The effects of the amount of HPC, H₂O and HCl in the starting solutions on the refractive index and uncracking critical thickness of the films were studied, where the effects on the critical thickness was discussed on the basis of in situ stress measurements during heating. The increase in HPC content increased the critical thickness and lowered the refractive index. The increase in HCl content resulted in a decrease in critical thickness and an increase in refractive index. Larger H₂O contents gave rise to a maximum in critical thickness while the refractive index was unaffected. Such variation in critical thickness with varying solution compositions was demonstrated to result from the differences in in-plane stress generated during heating. By optimizing the processing parameters an 810 nm thick TiO₂–HPC hybrid film of a refractive index of 1.84 was obtained.     

Kinetics of CN(v=0) and CN(v=1) with H2 HCl and HBr

CN radicals have been generated in their X ²Σ⁺ (v=0) and (v= 1 ) levels by pulsed laser photolysis of NCNO at 532 nm, and time-resolved laser-induced fluorescence has been used to measure the rates of their removal by H₂, HC1 and HBr. The rate constants for removal of CN(v= 1 ) by these three species are 1.2 ± 0.3, 1.1 ± 0.2 and 1.3 ± 0.1 times the rate constants for reaction of CN(v=0). The results can be interpreted in terms of vibrationally adiabatic theory and a CN vibrational frequency which is almost the same in the transition state as in the isolated radical.     

An Integrated Optical Biosensor (IOBS)

Abstract

We have demonstrated an integrated optical biosensor (IOBS) employed as differential refractometer with a resolution down to n_C.=5 × 10^?5. The minimum volume required for measurements is V=L_x?L_y?L_z=1?10^?4 microlitres where L_x?L_y=1mm?0.1mm is the illuminated grating area and L_z is the penetration depth of the evanescent field of the guided mode into the cover medium.

The sensor described was successfully used to monitor enzyme activities by measuring the change of refractive index of a substrate converted by an enzyme. Because the sensor responds not only to refractive index changes of a cover medium but also to thickness changes of absorbed layers it should be possible to measure biological interactions such as between antigen/antibody, inhibitor/enzyme or receptor/cell by coupling of either respective biomolecule onto the waveguiding film. Thus we have shown in preliminary experiments that on coupling covalently to the grating rabbit anti-human-IgG (both of similar molecular weight around 150 000) a pronounced attenuation of the intensity of the guided mode was observed due to the increased adlayer thickness (to be published).

It should be possible to miniaturize the unit using channel waveguides fabricated by a combination of photolithography and etching techniques with the aim to obtain implantable clinical integrated optical biosensors.

The sensitivity can be further improved electronically by measuring directly the intensity of the guided mode using a second grating as an output coupler. This will increase the signal to noise ratio substantially.