Heat-resistant flexible-film optical waveguides from fluorinated polyimides

Heat-resistant flexible-film optical waveguides were fabricated from fluorinated polyimides. These waveguides operated in single mode and had low optical loss (0.3 dB/cm) at a wavelength of 1.3 mum for TE and TM polarizations. They also had good flexibility: The optical loss did not significantly change above a minimum radius of curvature of less than 20 mm. The birefringence of 9 x 10(-5) between the TE and TM polarizations is 2 orders of magnitude smaller than that for a waveguide upon a substrate. Moreover, these waveguides had high thermal stability and moisture resistance: The optical loss and single-mode behavior changed little after heating the waveguides at 420 degrees C for 1 h or after their exposure to 85% relative humidity at 85 degrees C for more than 350 h.     

Single-mode optical waveguides fabricated from fluorinated polyimides

Buried channel optical waveguides were fabricated from fluorinated polyimides. They operated in single mode and showed an optical loss of less than 0.3 and 0.7 dB/cm for TE and TM polarizations, respectively, at a wavelength of 1.3 mum. Moreover, these waveguides had high heat and moisture resistance; the optical loss did not significantly change after heating at 380 degrees C for 1 h or after exposure to 85% relative humidity at 85 degrees C for over 200 h.     

Indium tin oxide overlayered waveguides for sensor applications

The use of indium tin oxide (ITO) thin films as electrodes for integrated optical electrochemical sensor devices is discussed. The effect of various thicknesses of ITO overlayers exhibiting low resistivity and high transparency on potassium ion-exchanged waveguides fabricated in glass substrates is investigated over the wavelength range 500-900 nm. ITO overlayers are formed by reactive thermal evaporation in oxygen, followed by annealing in air to a maximum temperature of 320 degrees C. With air as the superstrate, losses in the waveguides were found to increase dramatically above 30-nm ITO thickness for TE polarization and above 50-nm thickness for TM. Losses were increased over the whole wavelength range for a superstrate index close to that of water. A one-dimensional, multilayer waveguide model is used in the interpretation of the experimental results.     

Single-crystal Y2O3-ZrO2 rectangular waveguides for ultrahigh-temperature sensing applications

High-quality Y2O3-ZrO2 single-crystal rectangular waveguides have been developed for ultrahigh-temperature sensing applications. Five waveguides, 0.55-1.12 mm wide and 52-65 mm long, were fabricated from a bulky cubic 21.2-mol. % Y2O3 stabilized ZrO2 single crystal that had been precisely cut and finely polished. At 900-nm wavelength, the average optical loss of these waveguides is approximately 0.016 dB/cm, which is much lower than that of Y2O3-ZrO2 single-crystal optical fibers grown by the laser-heated pedestal growth method. The tested waveguides survived a temperature higher than 2,300 degrees C, and their mechanical strength and chemical resistance were also acceptable. Experimental results show that these waveguides are promising for ultrahigh-temperature sensing applications.     

Fabrication of waveguides in sputtered films of GeAsSe glass via photodarkening with above bandgap light

We have fabricated and characterized waveguides in thin films of Ge₅As₃₄Se₆₁ glass sputtered onto silicon wafer substrates. The 5-μm width waveguides were fabricated by exposure to 514.5 nm light from an Ar³⁺ laser, with a lithographic exposure mask used to provide the lateral patterning for the waveguides. The measured losses of the waveguides ranged from 3.5 to 6.4 dB/cm. From SEM imaging, we concluded that scattering from microcracks at the glass–substrate interface was the dominant source of loss.     

Dip-coated thin-film polycarbonate optical waveguides

We report on our systematic and detailed study of fabrication and characterization of polycarbonate thin-film optical waveguides. Various waveguide properties, such as refractive index, optical loss, waveguide thickness, dispersion as a function of temperature of waveguides and substrates, and the effect of thickness gradient on optical guiding losses, have been reported. A comparative study of the properties of waveguides fabricated with dioxane and tetrahydrofuran as solvents is also presented.     

Integrated-optical wavelength sensor with self-compensation of thermally induced phase shifts by use of a LiNbO3 unbalanced Mach-Zehnder interferometer

We demonstrate an integrated-optical unbalanced Mach-Zehnder interferometer in lithium niobate for detecting wavelength shifts of light sources, such as laser diodes and superluminescentdiodes at lambda = 844 nm. The output signal can be used to stabilize the light source. Because of the temperature dependence of the effective refractive index and the thermal expansion of the substrate, the device acts also as a temperature sensor. The temperature sensitivity of the interferometer was compensated for by the combination of proton exchanged- and annealed proton exchanged-channel waveguides by approximately two orders of magnitude. The thermo-optic coefficients of the extraordinary effective refractive index in integrated optical channel waveguides in LiNbO8 have been measured with high accuracy over a temperature range from 10 degrees C to 40 degrees C.     

Fabrication of channel waveguides from sol-gel-processed polyvinylpyrrolidone/ SiO(2) composite materials

Sol-gel-processed composite materials of polyvinylpyrrolidone (PVP) and SiO(2) were studied for optical waveguide applications. PVP is a polymer that can be crosslinked, so it is expected to have high thermal stability after crosslinking. However, thermal crosslinking and thermal decomposition of pure PVP take place around the same temperature, 200 °C, therefore pure PVP had a high optical propagation loss as a result of the absorption of the decomposed molecules after crosslinking. The incorporation of sol-gel-processed SiO(2) prevented the thermal decomposition of PVP and provided remarkably low optical propagation losses. The PVP/SiO(2)composite material also produced thick (>2-μm) crack-free films when the PVP concentration was 50% or higher. An optical propagation loss of 0.2 dB/cm was achieved at 633 nm in the 50% PVP/SiO(2) composite planar waveguide. Several aspects of the thermal stability of the waveguides were evaluated. The slab waveguide was then used for fabrication of channel waveguides with a selective laser-densification technique. This technique used metal lines fabricated with photolithography on the slab waveguide as a light absorbent, and these metal lines were heated by an Ar laser. The resultant channel waveguide had an optical propagation loss of 0.9 dB/ cm at 633 nm. This technique provides lower absorption loss and scattering loss compared with the direct laser-densification technique, which uses UV lasers, and produces narrow waveguides that are difficult to fabricate with a CO(2) laser.     

Platinum and palladium high-temperature transducers on langasite

There is a pressing need for the fabrication of surface acoustic wave (SAW) devices capable of operating in harsh environments, at elevated temperature and pressure, or under high-power conditions. These SAW devices operate as frequency-control elements, signal-processing filters, and pressure, temperature, and gas sensors. Applications include gas and oil wells, high-power duplexers in communication systems, and automobile and aerospace combustion engines. Under these high-temperature and power-operating conditions, which can reach several hundred degrees Centigrade, the typically fabricated aluminum (A1) thin film interdigital transducer (IDT) fails due to electro and stress migration. This work reports on high temperature SAW transducers that have been designed, fabricated, and tested on langasite (LGS) piezoelectric substrates. Platinum (Pt) and palladium (Pd) (melting points at 1769 degrees C and 1554.9 degrees C, respectively) have been used as thin metallic films for the SAW IDTs fabricated. Zirconium (Zr) was originally used as an adhesion layer on the fabricated SAW transducers to avoid migration into the Pt or Pd metallic films. The piezoelectric LGS crystal, used as the substrate upon which the SAW devices were fabricated, does not exhibit any phase transition up to its melting point at 1470 degrees C. A radio frequency (RF) test and characterization system capable of withstanding 1000 degrees C has been designed and constructed. The LGS SAW devices with Pt and Pd electrodes and the test system have been exposed to temperatures in the range of 250 degrees C to 750 degrees C over periods up to 6 weeks, with the SAW devices showing a reduced degradation better than 7 dB in the magnitude of transmission coefficient, /S21/, with respect to room temperature. These results qualify the Pt and Pd LGS SAW IDTs fabricated for the above listed modern applications in harsh environments.     

Temperature-programmed GC using silicon microfabricated columns with integrated heaters and temperature sensors

Columns were fabricated in silicon substrates by deep reactive-ion etching. The channels were sealed with a glass wafer anodically bonded to the silicon surface. Heaters and temperature sensors were fabricated on the back side of each column chip. A microcontroller-based temperature controller was used with a PC for temperature programming. Temperature programming, with channel lengths of 3.0 and 0.25 m, is described. The 3.0-m-long channel was fabricated on a 3.2 cmx3.2 cm chip. Four columns were fabricated on a standard 4-in. silicon wafer. The 0.25-m-long channel was fabricated on a 1.1 cmx1.1 cm chip, and approximately 40 columns could be fabricated on a 4-in. wafer. All columns were coated with a nonpolar poly(dimethylsiloxanes) stationary phase. A static coating procedure was employed. The 3.0-m-long column generated about 12000 theoretical plates, and the 0.25-m-long channel generated about 1000 plates at optimal carrier gas velocity. Linear temperature ramps as high as 1000 degrees C/min when temperature programmed from 30 to 200 degrees C were obtained with the shorter column. With the 0.25-m-long column, normal alkanes from n-C5 through n-C15 were eluted in less than 12 s using a temperature ramp rate of 1000 degrees C/min. Temperature uniformity over the column chip surface was measured with infrared imaging. A variation of about 2 degrees C was obtained for the 3.0-m-long channel. Retention time reproducibility with temperature programming typically ranged from +/-0.15% to +/-1.5%. Design of the columns and the temperature controller are discussed. Performance data are presented for the different columns lengths.     

Low-Loss Optical Waveguides for the Near Ultra-Violet and Visible Spectral Regions with Al(2)O(3) Thin Films from Atomic Layer Deposition

In this work, we report low-loss single-mode integrated optical waveguides in the near ultra-violet and visible spectral regions with aluminum oxide (Al₂O₃) films using an atomic layer deposition (ALD) process. Alumina films were deposited on glass and fused silica substrates by the ALD process at substrate/chamber temperatures of 200 °C and 300 °C. Transmission spectra and waveguide measurements were performed in our alumina films with thicknesses in the range of 210-380 nm for the optical characterization. Those measurements allowed us to determine the optical constants (n_w and k_w), propagation loss, and thickness of the alumina films. The experimental results from the applied techniques show good agreement and demonstrate a low-loss optical waveguide. Our alumina thin-film waveguides are well transparent in the whole visible spectral region and also in an important region of the UV; the measured propagation loss is below 4 dB/cm down to a wavelength as short as 250 nm. The low propagation loss of these alumina guiding films, in particular in the near ultra-violet region which lacks materials with high optical performance, is extremely useful for several integrated optic applications.     

Evaluation of hafnium nitride thin films sputtered from a hafnium nitride target

Hafnium nitride (HfN) thin films were prepared on Si (100) substrates by radio frequency magnetron sputtering with a compound target. Nitrogen composition, work function and electrical resistivity were investigated to evaluate thin film properties. Nitrogen composition and work function had little dependence on argon gas pressure and radio frequency power. Electrical resistivity showed strong correlation with the substrate temperature. When thin films were fabricated at room temperature, the electrical resistivity was 100 μΩ cm, and it became lower with an increase in the substrate temperature. When the films were fabricated at 600 °C, the resistivity became less than 50 μΩ cm.     

Thermal, structural, optical, and dielectric properties of (100 - x)Li2B4O7 - x(BaO-Bi2O3-Nb2O5) glasses and glass-nanocrystal composites

Transparent glasses in the system (100 - x)Li2B4O7 - x(BaO-Bi2O3-Nb2O5) (x = 10, 20, and 30) were fabricated via the conventional melt-quenching technique. The amorphous and glassy characteristics of the as-quenched samples were established by the differential thermal analyses (DTA) and X-ray powder diffraction (XRD) studies. Glass-nanocrystal composites (GNCs) i.e., the glasses embedded with BaBi2Nb2O9 (BBN) nanocrystals (10-50 nm) were produced by heat-treating the as-quenched glasses at temperatures higher than 500 degrees C. Perovskite BBN phase formation through an intermediate fluorite-like phase in the glass matrix was confirmed via XRD and transmission electron microscopic (TEM) studies. The optical transmission properties of these GNCs were found to have a strong compositional (BBN content) dependence. The refractive index (n = 1.90) and optical polarizability (alphao = 15.3 x 10(-24) cm3) of the GNC (x = 30) were larger than those of as-quenched glasses. The temperature dependent dielectric constant (epsilonr) and loss factor (D) for the glasses and GNCs were determined in the 100-40 MHz frequency range. The epsilonr was found to increase with increase in heat-treatment temperatures, while the loss of the glass-nanocomposites was less than that of as-quenched glasses. The sample heat-treated at 620 degrees C/1 h (x = 30) exhibited relaxor behavior associated with a dielectric anomaly in the 150-250 degrees C temperature range. The frequency dependence of the dielectric maximum temperature was found to obey the Vogel-Fulcher relation (Ea = 0.32 eV and Tf = 201 K).     

Investigation of thermal annealing of optical properties and residual stress of ion-beam-assisted TiO2 thin films with different substrate temperatures

Titanium oxide films were prepared by ion-beam-assisted deposition on glass substrates at various substrate temperatures. The effect of the temperature of thermal annealing from 100 degrees C to 300 degrees C on the optical properties and residual stress was investigated. The influence on the stoichiometry and residual stress of titanium oxides deposited at different substrate temperature was discussed. The residual-stress was minimum and the extinction coefficient was maximum at an annealing temperature of 200 degrees C with a substrate temperature of 150 degrees C. However, when the substrate temperature was increased to 200 degrees C and 250 degrees C, the residual stress was minimum and the extinction coefficient was maximum at an annealing temperature of 250 degrees C. The spectra of x-ray photoelectron spectroscopy reveal that the films lost oxygen and slowly generated lower suboxides at the annealing temperature at which the residual stress was minimum and the extinction coefficient was maximum. As the annealing temperature increased above the temperature at minimum stress, the lower suboxides began to capture oxygen and form stable oxides. TiO2 films deposited at substrate temperatures of 200 degrees C and 250 degrees C were more stable than films deposited at 150 degrees C.     

Temperature dependant characteristics of zinc oxide nanorods based heterojunction diode

Temperature-dependant characteristics of heterojunction diode made by n-ZnO nanorods grown on p-silicon substrates has been characterized and demonstrated in this paper. ZnO nanorods were grown onto the silicon substrate via simple thermal evaporation process by using metallic zinc powder in the presence of oxygen at approximately 550 degrees C without the use of any metal catalysts or additives. The as-grown ZnO nanorods were characterized in terms of their structural and optical properties. The detailed structural studies by XRD, TEM, HRTEM and SAED revealed that the grown nanorods are well-crystalline with the wurtzite hexagonal phase and preferentially grown along the [0001] direction. The as-grown n-ZnO nanorods grown on p-Si substrate were used to fabricate p-n heterojunction diode. The fabricated p-n junction diode attained almost similar turn-on voltage of approximately 0.6 V. The values of turn-on voltage and least current are same with the variations of temperature (i.e., 27 degrees C, 70 degrees C and 130 degrees C).     

Microstructure of magnesium fluoride films deposited by boat evaporation at 193 nm

Single layer magnesium fluoride (MgF2) was deposited on fused-silica substrates by a molybdenum boat evaporation process at 193 nm. The formation of various microstructures in relation to the different substrate temperatures and deposition rates were investigated. The relation between these microstructures (including cross-sectional morphology, surface roughness, and crystalline structures), the optical properties (including refractive index and optical loss) and stress, were all investigated. It was found that the laser-induced damage threshold (LIDT) would be affected by the microstructure, optical loss, and stress of the thin film. To obtain a larger LIDT value and better optical characteristics, MgF2 films should be deposited at a high substrate temperature (300 degrees C) and at a low deposition rate (0.05 nm s(-1)).     

Growth and properties of aligned ZnO nanowires and their applications to n-ZnO/p-Si heterojunction diodes

Aligned n-ZnO nanowires were synthesized via simple thermal evaporation process by using metallic zinc powder in the presence of oxygen on p-silicon (Si) substrate. The as-synthesized aligned ZnO nanowires were characterized in terms of their structural and optical properties by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction process (XRD) and room-temperature photoluminescence (PL) properties. The detailed structural and optical studies revealed that the as-grown nanowires are single crystalline with the wurtzite hexagonal phase and exhibit good optical properties. From application point of view, the as-grown aligned n-ZnO nanowires on p-Si substrates were used to fabricate heterojunction diodes. The fabricated heterojunction diodes exhibited good electrical (I-V) properties with the turn-on voltage of approximately 1.0 V. A temperature-dependant (from 25 degrees C approximately 130 degrees C), I-V characteristics for the fabricated device was also demonstrated in this paper. The presented results demonstrate that the simply grown aligned n-ZnO nanowires on p-Si substrate can be efficiently used for the fabrication of efficient heterojunction devices.     

Atmospheric pressure chemical vapor infiltration (CVI) for the preparation of biomorphic SiC ceramics derived from paper

Chemical Vapor Infiltration of biological structures such as paper is used here to produce biomorphic SiC ceramics with high temperature resistance. The biological substrate materials are infiltrated with tetramethylsilane (TMS) at atmospheric pressure and elevated temperatures of 790 degrees C. A simple tube furnace (hot-wall reactor) is used for the infiltration process. As result, porous SiC-ceramics are grown which are around 20% smaller and 70% lighter than the initial substrates. This can be explained by the pyrolytic reaction of the substrates while heating them up to 790 degrees C, which is necessary for the infiltration process. Nevertheless, besides the shrinking of the substrates the geometrical form remains nearly unchanged. The resulting materials were heated up to 1000 degrees C in oxygen atmosphere in order to analyze their resistance against oxidation. After this treatment, all of them were still mechanically stable and of unchanged shape while a further mass loss was observed. This confirms the high temperature stability of the prepared materials.     

Propagation loss and birefringence properties around 1.55 μm of polymeric optical waveguides fabricated with cross-linked silicone

We newly synthesized cross-linked silicone from methyl and deuterated phenyl (d-phenyl) groups in various amounts and used the resultant polymers to fabricate single-mode optical waveguides. We measured the losses and birefringence of the waveguides by the cutback method and Senalmont ellipsometry, respectively. The propagation loss decreased as we increased the d-phenyl content. The propagation loss was 0.23 dB/cm at 1.55 μm for a waveguide fabricated using cross-linked silicone with a 95-mol.% d-phenyl content. In contrast, the birefringence increased with increasing d-phenyl content. This indicates that the increase in phenyl content influences the orientation of optically anisotropic phenyl groups in the film, resulting in a birefringence increase. The birefringence value was 2.0×10⁻⁴ for a waveguide fabricated from cross-linked silicone with a 27-mol.% d-phenyl content.     

The effects of substrate and annealing on structural and electrochemical properties in LiCoO2 thin films prepared by DC magnetron sputtering

LiCoO2 thin films were fabricated by direct current magnetron sputtering method on STS304 and Ti substrates. The effects of substrate and annealing on their structural and electrochemical properties of LiCoO2 thin film cathode were studied. Crystal structures and surface morphologies of the deposited films were investigated by X-ray diffraction and field emission scanning electron microscopy. The as-deposited films on both substrates have amorphous structure. The (104) oriented perfect crystallization was obtained by annealing over 600 degrees C in STS304 substrate. The LiCoO2 thin film deposited on Ti substrate shows the (003) texture after annealing at 700 degrees C. The electrochemical properties were investigated by the cyclic voltammetry and charge-discharge measurement. The 600 degrees C-annealed LiCoO2 film deposited on STS304 substrate exhibits the inithial discharge capacity of 22 uAh/cm2 and the 96% capacity retention rate at 50th cycles. The electrochemical measurement on annealed films over 600 degrees C was impossible due to the formed TiO2 insulator layer using Ti substrate. As a result, it was found that the STS304 substrate seems to be more suitable material than the Ti substrate in fabricating LiCoO2 thin film cathode.