High Refractive Index Films Research Articles

Mechanical and thermoelastic characteristics of optical thin films deposited by dual ion beam sputtering

Mechanical and thermoelastic properties of optical films are very important to ensure the performance of optical interference filters and optical coating systems. We systematically study the growth and the mechanical and thermoelastic characteristics of niobium oxide (Nb(2)O(5)), tantalum oxide (Ta(2)O(5)), and silicon dioxide (SiO(2)) thin films prepared by dual ion beam sputtering. First, we investigate the stress (sigma), hardness (H), reduced Young's modulus (E(r)), and scratch resistance. Second, we focus on the methodology and assessment of the coefficient of thermal expansion (CTE) and Poisson's ratio (nu) using the two-substrate method. For the high refractive index films, namely, Nb(2)O(5) (n at 550 nm=2.30) and Ta(2)O(5) (n at 550 nm=2.13), we obtained H approximately 6 GPa, E(r) approximately 125 GPa, CTE=4.9x10(-6) degrees C(-1), nu=0.22, and H approximately 7 GPa, E(r) approximately 133 GPa, CTE=4.4x10(-6) degrees C(-1), and nu=0.27, respectively. In comparison, for SiO(2) (n at 550 nm=1.48), these values are H approximately 9.5 GPa, E(r) approximately 87 GPa, CTE=2.1x10(-6) degrees C(-1), and nu=0.11. Correlations between the growth conditions (secondary beam ion energy and ion current), the microstructure, and the film properties are discussed.

High refractive index and good mechanical property UV-cured hybrid films containing zirconia nanoparticles

UV-curable zirconia (ZrO 2) nanoparticle coatings, prepared by dispersing highly-crystalline zirconia nanoparticles (4 nm) in tetrahydrofuran with the aid of 3-glycidoxypropyltrimethoxysilane (GPTMS) and following addition of a cationic photoinitiator, were cast on silicon wafers (or glass substrates) by dip-coating or spin-coating and photopolymerized to get transparent ZrO 2 nanoparticle films. Ellipsometrical characterization indicates that the refractive index of the film changes from 1.63 to 1.77 at wavelength of 632 nm when the molar ratio of GPTMS-to-ZrO 2 decreases from 0.30 to 0.15. Nano-indentation tests show that the films exhibit robust mechanical performance though they are not heat-treated.

Studies on the laser-induced damage resistance of sol-gel derived ZrO2-TiO2 composite high refractive index films

ZrO2-TiO2 composite films with mol fraction of Ti in the total amount of Zr and Ti ranging from 0 to 100, have been prepared by the sol-gel spin-coating method. The microstructure, optical properties, and laser-induced damage resistance (LIDR) of these films were investigated, and emphasis was given to the compositional dependence of the laser-induced damage threshold (LIDT). The experimental results indicate that at a small expense of refractive index, the sol-gel process can considerably improve the LIDR of the films. Moreover, with increasing mol fraction of Ti from 0 to 100, a general decrease of the LIDT in a wide range of 57.1—20.9J/cm2(at 1053nm，10ns pulse duration and in “R/1” testing mode) was observed. Combining the structural and optical analysis results, a possible correlation between the LIDR and optical band gap of the prepared films is presented on the basis of the multiphoton absorption model.

Characterization of high refractive index semiconductor films by surface plasmon resonance

Si-based surface plasmon resonance (SPR) in the Kretschmann-Raether geometry is considered as a platform for the optical measurement of high refractive index films. The implementation of the SPR effect becomes possible due to the relatively high index of refraction of Si compared to most materials. As examples we study the SPR responses for some important semiconductor-based films, including laser-ablated porous silicon and thin germanium films. Using SPR data, we determine the refractive indices of these films for different parameters (thickness and porosity) and ambiences. We also discuss novel SPR biosensor architectures with the use of these solid films.

High Refractive Index Films Prepared from Titanium Chloride and Methyl Methacrylate via a Non-Aqueous Sol–Gel Route

Poly(methylmethacrylate)/silica/titania films were prepared via a nonaqueous sol–gel route at ambient temperature, followed by spin-coating and multistep baking. The acrylic monomers used were methyl methacrylate (MMA) and 3-(trimethoxysilyl)propyl methacrylate (MSMA). Silicic acid and titanium(IV) chloride were used as the precursors of the inorganic component. FTIR results indicated the successful bonding between TiO2 and SiO2. TEM images suggested the silica/titania particles were well dispersed in the Poly(methyl methacrylate) (PMMA) matrix with the particles size smaller than 40 nm in our study. The refractive index and extinction coefficient were also studied. The refractive index of the hybrid increased with increasing the titania content, and the hybrid films showed high optical transparency in visible region.

Design of thin-film photonic crystal waveguides

We present numerical designs for single-mode leak-free photonic crystal waveguides exhibiting strongly anisotropic spatial and temporal dispersion. These structures may be produced quite simply by drilling regular arrays of holes into thin films of high refractive index, and permit the realization of highly compact optical elements and wavelength division multiplexing devices.

Comparison of Different High and Low Index Materials in the Manufacture of High Laser Damage Threshold Mirrors at the 351 nm Wavelength

Sol-gel material based mirrors have been produced by forming alternate layers of high refractive index and low refractive index thin films. These mirrors have proven to have a high laser induced damage threshold [LIDT]. Using nitric acid stabilized zirconia derived from zirconium-n-propoxide and base catalyzed silica, a 16 layer mirror with a reflectivity of better than 94% at 351 nm and 45° angle of incidence was fabricated. This had an LIDT of 7.7 J/cm2 at 351 nm with a 0.7 ns pulse width. Crazing prevented further layers being deposited. Both spin and dip coating were attempted with dip coating yielding the best results.

Application of plasma enhanced chemical vapor deposition silicon nitride as a double layer antireflection coating and passivation layer for polysilicon solar cells

Reflection losses in passivated emitter solar cell polycrystalline silicon solar cells have been reduced by the application of a double layer antireflection coating of plasma enhanced chemical vapor deposition silicon nitride (PECVD SixNyHz). The layer was deposited in a single wafer parallel plate reactor powered by a 13.56 MHz rf power supply using SiH4 and NH3 as the reactive gases. The layers deposited had refractive indexes of 2.50 and 1.95 at HeNe wavelength and thicknesses of 42.5 and 64.5 nanometers (nm), respectively. The overall reflectance measured in the wavelength range between 350 and 1150 nm was 8.5%. The extinction coefficient of the high refractive index film showed a significant increase in absorption for short wavelengths. However, the improvement in current collection was higher than expected from the overall reflectance and absorption of the film. Short circuit current was increased by 49% and open circuit voltage increased by 3.3% when compared with uncoated cells. These results imply a passivation effect which increases the open circuit voltage beyond the value expected solely from the current collection addition.

Influence of radiation interference on the shape of Raman spectra for amorphous hydrogen-free diamondlike carbon films

The influence of interference on the shape of Raman spectra from thin films is considered. High refractive index, low absorption and sufficient film thickness combined with high reflection from a substrate surface are the conditions to enhance the interference distortion of the Raman spectra. In the case of an amorphous hydrogen-free diamondlike carbon film deposited onto a highly reflecting aluminum layer, the interference dramatically distorts the ratio of Raman intensities at 500 and 1550 cm−1, by about a factor of four when the spectra are registered by using a conventional backscattering geometry. The effect is smaller but clearly noticeable for ordinary silicon-substrate samples as well. The magnitude of the interference modification is strongly influenced by geometrical factors such as the direction and polarization of the detected Raman scattered light.

Photonic band structure of guided Bloch modes in high index films fully etched through with periodic microstructure

Abstract By adapting the well-known ‘zigzag’ ray model for use with a periodic waveguide (i.e. replacing the plane wave rays with Bloch wave rays), we show that thin films of high refractive index, supported by a low index substrate and fully etched through with a periodic pattern, can support guided modes. From the dispersion relation of these guided Bloch modes, it is shown that the in-plane modal group velocity can be zero, suggesting applications in enhanced dipole-field interactions and control of spontaneous emission in waveguide lasers.

Photonic band structure of guided bloch modes in high index films fully etched through with periodic microstructure

By adapting the well-known 'zigzag' ray model for use with a periodic waveguide (i.e. replacing the plane wave rays with Bloch wave rays), we show that thin films of high refractive index, supported by a low index substrate and fully etched through with a periodic pattern, can support guided modes. From the dispersion relation of these guided Bloch modes, it is shown that the in-plane modal group velocity can be zero, suggesting applications in enhanced dipole-field interactions and control of spontaneous emission in waveguide lasers.

Quantitative characterization of optical anisotropy in high refractive index films

AbstractWaveguide coupling measurements of polymers have largely concentrated on the application of mode analysis to the study of thin supported films (such as spin coatings). The use of prism coupling to study thick, freestanding polymer films, however, has not been reported. In this paper, the ability of prism coupling to characterize the three‐dimensional optical properties of thick, freestanding polymer films and sheets is demonstrated. A modified prism coupling procedure is described that allows the determination of all three principal refractive indices in thick, three‐dimensionally anisotropic freestanding films. A Metricon prism coupler is used in a manner similar to an Abbé refractometer for the measurement of isotactic polypropylene, poly(ethylene terephthalate), PMDA‐ODA polyimide, and poly(phenylene sulfide). Three series of PMDA‐ODA films are also investigated in this study. The first series has been drawn to different extensions from three‐dimensionally random films. The second series has random orientation in the plane of the film but different degrees of planarity with respect to the through direction. The third series are commercial films of varying thickness. These three series of films are compared as to the optical an‐isotropy that is developed from the three different fabrication processes. © 1993 John Wiley & Sons, Inc.

Polarisation control for surface emitting lasers

A polarisation control method for surface emitting lasers is proposed. By preparing high refractive index films on one side of dielectric multilayer reflector walls, a differential loss for orthogonal polarisations can be provided. A GaAlAs/GaAs device was fabricated based on this design and the polarisation control has been demonstrated up to 1.4 times the threshold.

11. Some salient observations in processing CS 3Sb photocathodes on thin-film coated glass substrates

The semitransparent alkali antimonide photocathodes such as CS 3 Sb and Na 2 KSb(Cs) are processed in uhv region on glass substrates, wherein they are not prone to substrate contaminants. In interference photpemission after Kossel (1969) , the photocathodes are processed on thin dielectric films of high refractive index like TiO 2 , SiO etc. While processing the transmissive interference photocathode of CS 3 Sb thin films on SiO coated substrates by the normal method (activation of evaporated Sb film by Cs vapours at 150°C), the cathode luminous sensitivity turns out to be very low, about 5 μA/1m. In the case of cathodes on TiO 2 substrates, luminous sensitivity of 25 μA/1m is registered. Several investigations have been conducted to analyse the cause of the deterioration phenomena mainly due to the chemical compatibility of the photocathode with the substrate and the surface effects. This has been taken care of by changing the normal processing technique to a coevaporation method, by which consistent photoelectric yields of 45 μA/1m are obtained for photocathodes on SiO as well as TiO 2 thin-film coated glass substrates. The optical and photoelectrical properties of interference photocathodes are presented. A photoelectric gain of 1.45 for TiO 2 -CS 3 Sb and 1.25 for SiO-CS 3 Sb combinations in the tuned region are respectively achieved. This development of interference photocathode technology offers an additional advantage of spectral maximum selectivity.

Strip-loaded waveguide formed in a graded-index LiNbO_3 planar waveguide

An As(40)Se(50-x)S(x)Ge(10) film strip loaded waveguide, formed in the graded-index LiNbO(3) planar waveguide, has been demonstrated. Analytical results show that the optical field confinement in the waveguide loaded by the high refractive index film becomes large near the film cutoff thickness for the fundamental mode. Photostructural effect of the chalcogenide glass overcomes difficulty in precisely controlling film thickness. A 3-D waveguide has been achieved by loading As(40) Se(10)S(40)Ge(10) film 10 microm wide on a Ti diffused LiNbO(3) planar waveguide. Optical confinement in the waveguide has been improved intensively with the aid of the photostructural effect of the film.

Optical field distribution in a waveguide loaded with high refractive-index film; modulation efficiency improvement in a planar-type modulator

The change of optical field distribution by loading a high refractive-index film on a slab waveguide has been analyzed. It is found that the optical field is remarkably pulled to the film-waveguide boundary when the thickness of the loading film is close to the cutoff thickness. The optical energy content in the loading film has been calculated, and we have found that the energy existing in the film is easily made to be little (e.g., less than 10%) by the appropriate choice of the refractive index of the loading film. The effect is well utilized for the improvement of modulation efficiency in the waveguide modulator with planar-type electrodes.

Automatic monitoring of deposition conditions during rf sputtering of dielectric materials

Modern optics often require dielectric thin films of precisely controlled optical indices. One method of producing such films is by rf sputtering at extremely low deposition rates (∼0·1 μ h −1). Thermal and plasma instabilities occur during the deposition process (often > 10 h duration) and the sputtering system therefore requires continuous monitoring to ensure appropriate corrective action. The objective of this paper is to examine the technology involved in developing supervision circuitry to monitor the parameters of a conventional commercially available rf sputtering system employed in the fabrication of high optical quality dielectric films for integrated-optics applications. The techniques used have overcome the difficulties of operating analogue circuitry in a high rf noise environment. A novel reflection-interferometer, enabling continuous monitoring of sputtered film thickness in a close spaced target-substrate system, was developed. The design has been shown to possess enhanced alignment tolerance, particularly for high refractive index films.

Infrared Filters of Evaporated Gallium Arsenide

Films of gallium arsenide have been produced by vacuum evaporation on to amorphous substrates and their optical properties have been measured. A simple, effective way of evaporating gallium arsenide is described. The design of absorption filters using evaporated semiconductor films of high refractive index is considered and the manufacture and properties of such a filter are described.