Visible to near-infrared octave spanning supercontinuum generation in tantalum pentoxide (Ta2O5) air-cladding waveguide.

In this work, for the first time, to the best of our knowledge, an anomalous dispersion CMOS-compatible Ta2O5 waveguide was realized, and broadband on-chip supercontinuum generation (SCG) was accordingly demonstrated. When pumped at a center wavelength of 1056 nm with pulses of 100 fs duration and peak power of 396 W, a supercontinuum ranging from 585 nm to 1697 nm was generated, comprising a bandwidth of more than 1.5 octaves and leading to an efficient SCG source. The excellent performance for Ta2O5 to generate SCG benefits mainly from its high nonlinear refractive index, which enhances the efficiency of the nonlinear conversion process.

Fabrication and analysis of tantalum pentoxide optical waveguide resonator of high thermal stability.

We fabricated waveguide resonators with high thermal stability using tantalum pentoxide thin film covered with PECVD silicon dioxide cladding. Without complex athermal design, low temperature dependence of 7.4 pm/°C and 8.15 pm/°C were measured in waveguide Bragg gratings (WBG) and Fabry-Perot resonator sandwiched by a pair of identical WBG mirrors, respectively. Suggested by semi-analytical perturbation calculations, the athermal properties of tantalum pentoxide waveguide grating are attributed not only to the low thermo-optical coefficient in tantalum pentoxide thin film but also to the strong chromatic dispersion of the guided modes. Guidelines are proposed to design waveguide-based frequency devices of low thermo-optical effect without complex athermal design.

Efficient wavelength conversion with low operation power in a Ta2O5-based micro-ring resonator.

The Ta2O5-based micro-ring resonator with an unloaded quality factor of 182,000 has been demonstrated to realize efficient nonlinear wavelength generation. The propagation loss of the resonator is 0.5 cm-1, and the buildup factor of the ring resonator is estimated to be ∼50. With a high buildup factor of the ring structure, the four-wave-mixing (FWM) conversion efficiency of -30 dB is achieved in the resonator with a pump power of 6 mW. Based on power-dependent FWM results, the nonlinear refractive index of Ta2O5 is estimated to be 1.4×10-14 cm2/W at a wavelength of ∼1550 nm. The demonstration of an enhanced FWM process in the Ta2O5-based micro-ring cavity implies the possibility of realizing FWM-based optical parametric oscillation in a Ta2O5-based micro-ring resonator.

Surface Engineering of Polycrystalline Silicon for Long-Term Mechanical Stress Endurance Enhancement in Flexible Low-Temperature Poly-Si Thin-Film Transistors.

The surface morphology in polycrystalline silicon (poly-Si) film is an issue regardless of whether conventional excimer laser annealing (ELA) or the newer metal-induced lateral crystallization (MILC) process is used. This paper investigates the stress distribution while undergoing long-term mechanical stress and the influence of stress on electrical characteristics. Our simulated results show that the nonuniform stress in the gate insulator is more pronounced near the polysilicon/gate insulator edge and at the two sides of the polysilicon protrusion. This stress results in defects in the gate insulator and leads to a nonuniform degradation phenomenon, which affects both the performance and the reliability in thin-film transistors (TFTs). The degree of degradation is similar regardless of bending axis (channel-length axis, channel-width axis) or bending type (compression, tension), which means that the degradation is dominated by the protrusion effects. Furthermore, by utilizing long-term electrical bias stresses after undergoing long-tern bending stress, it is apparent that the carrier injection is severe in the subchannel region, which confirms that the influence of protrusions is crucial. To eliminate the influence of surface morphology in poly-Si, three kinds of laser energy density were used during crystallization to control the protrusion height. The device with the lowest protrusions demonstrates the smallest degradation after undergoing long-term bending.

Self-phase modulation in highly confined submicron Ta2O5 channel waveguides.

Optical spectra broadening as a result self-phase modulation in a channel waveguide fabricated on a high quality tantalum pentoxide (Ta2O5) film by using RF sputtering is measured. The full-width at half maximum of the optical spectra for transverse electric (TE)/transverse magnetic (TM) polarizations of 42.5/31.7 nm is obtained using pulses of 10 nm at a wavelength of 800 nm with a peak-coupled power of 43.77 W. The nonlinear Kerr coefficients of 2.14 × 10-14 cm2/W and 1.92 × 10-14 cm2/W for TE and TM polarizations, respectively, are then extracted from the experiments using a theoretical model based on the method of moments. The obtained results on the nonlinearity further suggest that Ta2O5 is a promising material to develop nonlinear waveguide devices for integrated photonics.

Four-wave-mixing in the loss low submicrometer Ta2O5 channel waveguide.

A degenerate four-wave-mixing (FWM) operation in the Ta2O5 submicrometer channel waveguide has been successfully demonstrated. The propagation loss of 1.5 dB/cm and total insertion loss of 5.1 dB are realized in a 12.6 mm long waveguide with inverse taper structure. The wavelength and quadratic pumping power-dependent measurements on optical transmission confirm FWM performance and characterize the nonlinearity of waveguide. The conversion efficiency of -50 dB at coupled pump power of 40 mW is observed, suggesting that the nonlinear refractive index of Ta2O5 waveguide at 1550 nm is estimated to be 1×10(-14) cm2/W. Our primary results indicate that the Ta2O5 submicrometer channel waveguide has great potential in developing nonlinear waveguide applications.

Low-loss and high-Q Ta(2)O(5) based micro-ring resonator with inverse taper structure.

A low-loss and high-Q Ta(2)O(5) based micro-ring resonator is presented. The micro-ring resonator and channel waveguide with core area of the 700 by 400 nm(2) were fabricated on amorphous Ta(2)O(5) thin films prepared by reactive sputtering at 300°C and post annealing at 650°C for 3 hours. The Ta(2)O(5) micro-ring resonator with a diameter of 200 µm was coupled to the channel waveguide with a coupled Q up to 38,000 at a 0.9 µm coupling gap. By fitting the transmission spectrum of the resonator, the extracted loss coefficient inside the ring cavity and transmission coefficient of TE mode were 8.1dB/cm and 0.9923, leading to the estimated unloaded Q of higher than 44,000. In addition, based on the cut-back method, the propagation loss and the coupling loss of Ta(2)O(5) channel waveguide with an inverse taper were 1.5dB/cm and 3.2 dB, respectively. The proposed Ta(2)O(5) technology offers an unique alternative for fabricating high performance guided wave devices, and may well lead to novel applications in photonic integrated circuits.

Preparation of insulating SiO2 nanostructured thin films by the sol-gel process.

Organic dispersion agents can effectively decrease the surface roughness of films. Here, films containing organic dispersion agents are used to produce metal-insulator-metal structures. It was found that addition of Triton caused films to become denser, and thicker, and the leak current of devices to decrease by 10 times compared with that without Triton because of its uniform dispersion in the films. The resulting films were used as the insulator layer of thin film transistors containing a semiconductor layer of evaporated pentacene. The interface between the insulator and semiconductor layers was found to affect the arrangement of pentacene, and O2 plasma was used to improve the interface activity to increase the order of the pentacene molecules and enhance the carrier mobility of the devices.

Design of multi-porous layer for dye-sensitized solar cells by doping with TiO2 nanoparticles.

We propose a multi-layer dye-sensitized solar cell (DSSC). Conventional DSSC components use a singular TiO2 particle size and a mono-layer active layer, but we demonstrate a multi-layer and multi-scale TiO2 particle based DSSC. Doping with large TiO2 particles can produce light scattering inside the DSSC component. Light scattering effects reduce TiO2 absorption at wavelengths of 200-300 nm. The unabsorbed light zig-zags between the Pt back electrode layer and the substrate, and enhances the Ru-dye absorption. To enhance the scattering, we doped the active layer with 20 wt% of large diameter TiO2. The multi-layer DSSC increases efficiency by about 15% compared with standard DSSCs.

Hierarchical structured TiO2 photoanodes for dye-sensitized solar cells.

A novel approach has been developed to fabricate hills-like hierarchical structured TiO2 photoanodes for dye-sensitized solar cells (DSSCs). The appropriately aggregated TiO2 clusters in the photoanode layer could cause stronger light scattering and higher dye loading that increases the efficiency of photovoltaic device. For detailed light-harvesting study, different molecular weights of polyvinyl alcohol (PVA) were used as binders for TiO2 nanoparticles (P-25 Degussa) aggregation. A series of TiO2 films with dissimilar morphology, the reflection of TiO2 films, absorbance of attached dye, amount of dye loading, and performance of fabricated DSSC devices, were measured and investigated. An optimized device had energy conversion efficiency of 4.47% having a higher dye loading and good light harvesting, achieving a 23% increase of short-circuit current J(sc) in DSSCs.