S. Chao

Mixed films of TiO 2 –SiO 2 deposited by double electron-beam coevaporation

We used double electron-beam coevaporation to fabricate TiO(2)-SiO(2) mixed films. The deposition process included oxygen partial pressure, substrate temperature, and deposition rate, all of which were real-time computer controlled. The optical properties of the mixed films varied from pure SiO(2) to pure TiO(2) as the composition of the films varied accordingly. X-ray diffraction showed that the mixed films all have amorphous structure with a SiO(2) content of as low as 11%. Atomic force microscopy showed that the mixed film has a smoother surface than pure TiO(2) film because of its amorphous structure.Linear and Bruggemans effective medium approximation models fit the experimental data better than other models.

Low-loss dielectric mirror with ion-beam-sputtered TiO 2 –SiO 2 mixed films

Ion-beam-sputtered TiO(2)-SiO(2) mixed films with 17% SiO(2) concentration were used as high-refractive-index layers in a multilayered-stack dielectric mirror. Experimental results indicated that total loss of the as-deposited mirror was 34% lower than that of the as-deposited conventional mirrors with pure TiO(2) films used as high-refractive-index layers. In addition, annealing reduced total loss of the mirrors. Although decreasing with an increasing annealing temperature, total loss of the conventional mirrors dramatically increased above ~200 degrees C annealing temperature, owing to increased scattering from an amorphous-to-crystalline phase transition in the TiO(2) films. In addition, total loss of the mirrors with the mixed films continuously decreased with an increasing annealing temperature up to 400 degrees C without the phase transition. Total loss was reduced 88% by means of decreasing absorption in the mixed films. Moreover, the annealed mirror with mixed films was better than both the as-deposited mirror and the conventional mirror with pure films in terms of laser-damage resistance.

CHARACTERISTICS OF ION-BEAM-SPUTTERED HIGH-REFRACTIVE-INDEX TIO2-SIO2 MIXED FILMS

The ion-beam-sputtering method was used to deposit TiO2-SiO2 mixed films on silica substrates. The SiO2 concentration ranged from 0 to 17%, and the refractive index ranged from ∼2.7 to ∼2.3 in visible wavelength. All the structures of the as-deposited films were amorphous. The refractive index and the extinction coefficient decreased with increased SiO2 concentration. High temperature annealing reduced the optical absorption for all films. There was a phase transition from amorphous to polycrystalline anatase at high temperature. Surface roughness, and thus optical scattering, increased drastically with the appearance of the phase transition. The phase transition temperature was higher for films with higher SiO2 concentration. The ion-beam-sputtered TiO2-SiO2 mixed film could sustain higher temperature annealing, resulting in lower extinction coefficient, than that of the ion-beam-sputtered pure TiO2 film. The phase diagram of the mixed film system was given.

ANNEALING EFFECT ON ION-BEAM-SPUTTERED TITANIUM DIOXIDE FILM

We found that the extinction coefficient of ion-beam-sputtered titanium dioxide films first decreased with increasing annealing temperature then increased drastically when annealing temperature was increased above ~200 degrees C for 24 h of annealing time. The decreasing extinction coefficient with annealing temperature was attributed to a reduction in absorption owing to oxidation of the film by annealing. The film structure remained amorphous to 200 degrees C annealing temperature. The drastic increase of extinction coefficients above ~200 degrees C was associated with the appearance of an anatase polycrystalline structure and was attributed to scattering by the polycrystalline structure. With shorter annealing time the transition temperature from amorphous to polycrystalline anatase was higher. Guidance for reducing the optical loss of laser mirrors is proposed.

Substrate-dependent optical absorption characteristics of titanium dioxide thin films

We used the electron-beam evaporation method in various oxygen partial pressure environments to deposit TiO(2) thin films on various glass substrates at 300 degrees C. We found the threshold oxygen partial pressures above which the film is transparent are different for films on various substrates. Below the threshold oxygen partial pressure, the refractive index and the extinction coefficient of the films varied from substrate to substrate. The films on substrates with higher threshold oxygen partial pressure were associated with a higher extinction coefficient and a higher growth rate. These phenomena are correlated with the appearance of rutile phase in the anatase phase, which is also correlated with variations in the Al(2)O(3) and Na(2)O content in the substrates. The Al(2)O(3) content in the substrate tends to enhance the formation of rutile phase in the film and to give a higher extinction coefficient for the film, while the Na(2)O content in the substrate tends to retard the rutile formation in the film and to give a lower extinction coefficient for the film.

Light extraction enhancement for InGaN/GaN LED by three dimensional auto-cloned photonics crystal

Three dimensional (3-D) auto-cloned photonics crystal (APhC) of Ta2O5/SiO2 multi-layers was fabricated on the backside of the sapphire wafer that had InGaN/GaN multi-quantum well LED on the front side. 94% light extraction enhancement in comparison to the LED without APhC was obtained. Electrical properties of the LED did not altered by the APhC and its fabrication process. Experimental evidences showed that light extraction enhancement mechanism is two-folded: for rays that are emitted from the source and incident at lower angle of incidence to the APhC, the APhC acts as a high reflector; for rays incident at higher angle of incidence to the APhC, first order diffracted light from the APhC appears, the diffracted light is concentrated around the surface normal and is therefore capable of escaping.

TiO 2 –SiO 2 mixed films prepared by the fast alternating sputter method

We introduced the fast alternating sputter method and its application on deposition of TiO(2)-SiO(2) mixed films. By using fast alternating sputter, the TiO(2) and SiO(2) were completely mixed in the film, and no thinpair structure could be found by x-ray diffraction. The structure of the mixed films was amorphous in a wide composition range. The optical properties of the mixed films in the visible and near infrared changed from SiO(2)-dominant to TiO(2)-dominant as TiO(2) content in the film increased.

Time dependence of ferroelectric coercive field after domain inversion for lithium‐tantalate crystal

We found the ferroelectric coercive field of LiTaO3, both in forward and reverse direction, vary with time after the domain is inverted. The coercive field drops when the domain is inverted, then gradually recovers. This phenomenon is light sensitive. The existence of a net time‐varying internal electric field after domain inversion is hypothesized. The internal field is composed of the depolarization field, which is due to the spontaneous electric dipole moments, and an opposite direction time‐varying space‐charge field which is due to the redistribution of free‐carriers transport under the influence of the depolarization field. Electro‐optical effect caused by the internal electric field has been observed by means of an in situ optical monitoring technique for the domain inversion process. The in situ optical monitoring technique is based on using the LiTaO3 thin‐plate crystal as a low finesse Fabry–Perot interferometer.

MQWs InGaN/GaN LED with embedded micro-mirror array in the epitaxial-lateral-overgrowth gallium nitride for light extraction enhancement

Multi-quantum wells (MQWs) InGaN/GaN LEDs, 300 microm x 300 microm chip size, were fabricated with Ta(2)O(5) / SiO(2) dielectric multi-layer micro-mirror array (MMA) embedded in the epitaxiallateral- overgrowth (ELOG) gallium nitride (GaN) on the c-plane sapphire substrate. MQWs InGaN/GaN LEDs with ELOG embedded patterned SiO(2) array (P-SiO(2)) of the same dimension as the MMA were also fabricated for comparison. Dislocation density was reduced for the ELOG samples. 75.2% light extraction enhancement for P-SiO(2)-LED and 102.6% light extraction enhancement for MMA-LED were obtained over the standard LED. We showed that multiple-diffraction with high intensity from the MMA redirected the trap lights to escape from the LED causing the light extraction enhancement.

Quasi-phase-matched second-harmonic generation in ultraviolet-assisted periodically poled planar fused silica

First-order quasi-phase-matched second-harmonic generation of 1064 to 532 nm in a thermally poled planar fused-silica plate with periodic UV erasure of the second-order nonlinearity was successfully implemented. We obtained a 1:2.9 ratio of d31:d33 for UV-grade fused silica in support of the proposed mechanism for electric-field-induced second-order nonlinearity in this material.