Silicon Oxy-Nitride for the Low Refractive Index Layers in the Mirror Coatings of the Cryogenic Laser Interferometer Gravitational Waves Detector.

The low refractive index layers in the mirror coatings of the room-temperature laser interferometer gravitational waves detectors are silica deposited by the ion beam sputter method. However, the silica film suffers from the cryogenic mechanical loss peak, hindering its application for the next generation detector operated at cryogenics. New low refractive index materials need to be explored. We study amorphous silicon oxy-nitride (SiON) films deposited using the method of plasma-enhanced chemical vapor deposition. By changing the N_{2}O/SiH_{4} flow rate ratio, we can tune the refractive index of the SiON smoothly from nitridelike to silicalike of ∼1.48 at 1064 nm, 1550 nm, and 1950 nm. Thermal anneal reduced the refractive index down to ∼1.46 and effectively reduced the absorption and cryogenic mechanical loss; the reductions correlated with the N─H bond concentration decrease. Extinction coefficients of the SiONs at the three wavelengths are reduced down to 5×10^{-6}∼3×10^{-7} by annealing. Cryogenic mechanical losses at 10 K and 20 K (for ET and KAGRA) of the annealed SiONs are significantly lower than the annealed ion beam sputter silica. They are comparable at 120 K (for LIGO-Voyager). Absorption from the vibrational modes of the N─H terminal-hydride structures dominates over the absorption from other terminal hydrides, the Urbach tail, and the silicon dangling bond states in SiON at the three wavelengths.

Low cryogenic mechanical loss composite silica thin film for low thermal noise dielectric mirror coatings.

Thermal noise in dielectric mirror-coatings is the limiting factor for ultra-high-precision metrologies employing optical cavities and being operated at cryogenic temperatures. Silica film is an indispensable low-refractive-index material for mirror coatings but suffers from high cryogenic mechanical loss and hence contributes to high thermal noise. We partitioned a thick silica film into thin layers by introducing blocking layers of titania. The cryogenic mechanical loss of silica was significantly suppressed; the effect was more profound for thinner partitions. Elimination of the transitions of the long-range two-level systems with scales that exceed the thickness of the silica partition is hypothesized. Dielectric mirror coatings with blocking layers are proposed to reduce the cryogenic thermal noise of the coatings. The calculated reflectance spectrum is consistent with that of the conventional quarter-wave (QW) stack around 1550 nm, and the calculated absorptance increases 2.2 times over that of a conventional titania/silica QW stack where the titania is absorptive.

Thickness-dependent crystallization on thermal anneal for titania/silica nm-layer composites deposited by ion beam sputter method.

Crystallization following thermal annealing of thin film stacks consisting of alternating nm-thick titania/silica layers was investigated. Several prototypes were designed, featuring a different number of titania/silica layer pairs, and different thicknesses (in the range from 4 to 40 nm, for the titania layers), but the same nominal refractive index (2.09) and optical thickness (a quarter of wavelength at 1064 nm). The prototypes were deposited by ion beam sputtering on silicon substrates. All prototypes were found to be amorphous as-deposited. Thermal annealing in air at progressive temperatures was subsequently performed. It was found that the titania layers eventually crystallized forming the anatase phase, while the silica layers remained always amorphous. However, progressively thinner layers exhibited progressively higher threshold temperatures for crystallization onset. Accordingly it can be expected that composites with thinner layers will be able to sustain higher annealing temperatures without crystallizing, and likely yielding better optical and mechanical properties for advanced coatings application. These results open the way to the use of materials like titania and hafnia, that crystallize easily under thermal anneal, but ARE otherwise promising candidate materials for HR coatings necessary for cryogenic 3rd generation laser interferometric gravitational wave detectors.

A compact optical pickup head in blue wavelength with high horizontal stability for laser thermal lithography.

A compact optical pickup head in blue wavelength with a single-axial actuator i.e. focusing, for laser thermal lithography was designed, fabricated, and tested. The numerical aperture of the objective lens was 0.85. The linear range of the focus error signal was 3 µm. A planar spring structure for improving the horizontal stability was designed and incorporated into the actuator. We applied a modified push-pull method together with a static Blu-ray re-writable disc to test the horizontal stability of the pickup head. We found that the in-plane jitter of the pickup head in two orthogonal directions were 0.34 nm and 1.59 nm, respectively. We demonstrated an example of applying the pickup head to write an inorganic photo-resist GeSbSnO film, and well-defined pattern was obtained with ~220 nm spot size.

Fabrication of three-dimensional autocloned photonic crystal on sapphire substrate.

We applied the laser interference lithography method to form a patterned sapphire substrate (PSS). A three-dimensional photonic crystal was formed by autocloning the PSS with alternate Ta2O5/SiO2 coatings. A high total integrated reflectance (TIR) band was obtained around the 410 to 470 nm wavelength range that matched the emission spectrum of the gallium nitride (GaN) light-emitting diode (LED) for application in manipulating the light extraction of the sapphire-based GaN LED.

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.

Spectral shift by half free-spectral-range for microring resonator employing the phase jump phenomenon in coupled-waveguide and application on all-microring wavelength interleaver.

Using coupled-mode theory, we have shown that there is a pi phase jump between the input and the through/drop fields of a codirectional coupler when the gap width between the coupled-waveguides reaches certain values such that the length of the coupler equals to the odd integer (for through field) or even integer (for drop field) times of the Transfer Distance. We introduced an efficient numerical method based on combining the scattering matrix method and FDTD method for analyzing a microring that has material loss. By applying this method, we found that the phase jump phenomenon also occurs in a half-ring coupler when the gap width between the coupled half-ring waveguides reaches a critical value. We showed that, for a given operating bandwidth, it is important that the gap width between the rings has to be larger than a certain value in order to avoid the phase jump, or smaller in order to take advantage of the phase jump. Based on the phase jump phenomenon, we found that the through and the drop spectra of the single-arm and the double-arm microring can be manipulated to shift about one half free spectral range by selecting appropriate gap widths. A novel all-microring wavelength interleaver, based on the phase jump phenomenon, is proposed and numerically demonstrated.

Heat resistive dielectric multi-layer micro-mirror array in epitaxial lateral overgrowth gallium nitride.

Ta2O5 / SiO2 dielectric multi-layer micro-mirror array (MMA) with 3mm mirror size and 6mm array period was fabricated on c-plane sapphire substrate. The MMA was subjected to 1200 degrees C high temperature annealing and remained intact with high reflectance in contrast to the continuous multi-layer for which the layers have undergone severe damage by 1200 degrees C annealing. Epitaxial lateral overgrowth (ELO) of gallium nitride (GaN) was applied to the MMA that was deposited on both sapphire and sapphire with 2:56 mm GaN template. The MMA was fully embedded in the ELO GaN and remained intact. The result implies that our MMA is compatible to the high temperature growth environment of GaN and the MMA could be incorporated into the structure of the micro-LED array as a one to one micro backlight reflector, or as the patterned structure on the large area LED for controlling the output light.

Surface profile control of the autocloned photonic crystal by ion-beam-sputter deposition with radio-frequency-bias etching.

Growth of the autocloned Ta(2)O(5)/SiO(2) multilayer photonic crystal with a lateral sawtooth period was simulated. Ion-beam sputter (IBS) was applied to deposit the films and radio-frequency-bias (RF-bias) etching was applied simultaneously with the IBS on the Ta(2)O(5) film. Both simulation and experiment showed that the quality of the autocloning can be controlled by the RF-bias power; there is an intermediate power range within which the drop of peak-to-valley height variation of the sawtooth profile can be reduced significantly such that a high degree of autocloning can be achieved. Analysis showed that simultaneous deposition and etching at the proper RF-bias power on the Ta(2)O(5) film has the capability to compensate the flattening effect of the SiO(2) deposition such that the sawtooth surface profile can be maintained.

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.

Humidity effect on the decay of second-order nonlinearity in thermally poled fused silica.

Type I (GE124) and Type II (KV) fused silica were thermally poled in a vacuum and in air under identical poling conditions. Second-order nonlinear (SON) strength and nonlinear depth were found all to be the same. Samples were then stored in high and low humidity to study their SON stability. The SON of poled GE124 was stable over time despite different poling atmospheres and humidity in storage. The SON of both the air-poled and vacuum-poled KV samples decayed over time in both low and high humidity, with the exception that the air-poled KV sample stored in low humidity remained stable. High humidity accelerated the decay process of the KV samples. A porous surface model was used to interpret the decay mechanism. The decay curves implied multiple carriers or a multiple-porosity model for the decay mechanism.

Graphic method for numerical analysis of a periodically stratified thin-film omnidirectional reflector.

We introduce a graphic method for analyzing a periodically stratified multilayer omnidirectional reflector. When the dispersive refractive indices of the materials are known, the contours of constant omnibandwidth and constant center wavelength with respect to layer thickness can be numerically and graphically presented for analysis. Examples of this procedure for TiO2/SiO2 and Si/SiO2 periodically layered systems for the visible and near-infrared regions are given. A comparison of omnidirectional reflectors of quarter-wave and non-quarter-wave layer thicknesses is made by the graphic method.

Quasi-phase-matched second-harmonic generation in Ge-ion implanted fused silica channel waveguide.

Channel waveguides were formed on fused silica substrate by Ge-ion implantation with lithographically defined channels. Thermal poling was performed to form second order optical nonlinearity (SON) in the waveguides. Periodical photo masks were designed and fabricated on a mask glass. Periodical erasure of the SON in the channel waveguides by 266 nm UV light with the photo mask on the fused silica substrate produced periodical SON distribution in the waveguides. First order quasi-phase-matching second-harmonic generation from 1064 nm to 532 nm was demonstrated in the channel waveguides.

Simple design method for third-order dispersion compensation with a thin-film dispersion compensator.

A novel and simple numerical method for finding the best thin-film structures for third-order dispersion compensation has been achieved. A target third-order dispersion value is specified first; then the multilayer thin-film structure is optimized to have a second-order dispersion spectrum, which has the least deviation from linearity and has a slope that equals the specified third-order dispersion value. Numerical examples are presented for reflection-type and transmission-type thin-film compensators. Both types can achieve required phase compensation, but the reflection type has a flatter amplitude response than the transmission type.

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.