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To overcome the defect of conventional nonlinear piezoelectric cantilever vibration energy harvesters, in this paper we conceive an elastic-support model to study the performance of energy converters under two types of variable-intensity excitation conditions: filtered Gaussian noises and pink noises. When excitation intensity is insufficient, thanks to the system's variable potential function, frequent bistable transition oscillations between two wells occur in elastic-support systems, while only weak oscillations in either well could be observed in rigid-support systems. In practical applications, the structural parameters of energy harvesters are not allowed to make real-time changes. If considered remaining the magnet interval and the spring's elastic stiffness unchanged while receiving stable maximum output voltage, elastic-support systems can be made full use toward variable-intensity filtered Gaussian noises. It has been proven that elastic-support systems are capable of adapting to random excitations with variable intensity, through which maximum power output and sufficient electromechanical energy conversion of the system can be accomplished. [ABSTRACT FROM AUTHOR]

In this paper, we present a highly accurate simplified lattice Boltzmann method (HSLBM) which can achieve the third-order of accuracy in space. By introducing virtual streaming nodes and decoupling the streaming distance from the mesh spacing, HSLBM effectively combines the local second-order simplified and highly stable lattice Boltzmann method (SHSLBM) and the overall high order scheme. The correlation between the streaming distance and the mesh spacing is accomplished by a high-order Lagrange interpolation algorithm. Through a series of tests, it is found that using 5 interpolation points and setting streaming distance as 1/5 of the mesh spacing can give optimal results. In general, HSLBM improves the accuracy of SHSLBM from the second order to the third order, while maintaining its merits like low memory cost, convenient implementation of physical boundary conditions, and good numerical stability. To match the accuracy of the flow solver, a linear extrapolation scheme with the third-order of accuracy is also proposed to determine the boundary values of the non-equilibrium distribution functions. Various benchmark tests are performed to demonstrate the robustness of HSLBM in simulating two- and three-dimensional incompressible viscous flows as well as its flexibility in problems with curved boundaries and on body-fitted meshes. [ABSTRACT FROM AUTHOR]

An immersed boundary-simplified lattice Boltzmann method is developed in this paper for simulations of two-dimensional incompressible viscous flows with immersed objects. Assisted by the fractional step technique, the problem is resolved in a predictor-corrector scheme. The predictor step solves the flow field without considering immersed objects, and the corrector step imposes the effect of immersed boundaries on the velocity field. Different from the previous immersed boundary-lattice Boltzmann method which adopts the standard lattice Boltzmann method (LBM) as the flow solver in the predictor step, a recently developed simplified lattice Boltzmann method (SLBM) is applied in the present method to evaluate intermediate flow variables. Compared to the standard LBM, SLBM requires lower virtual memories, facilitates the implementation of physical boundary conditions, and shows better numerical stability. The boundary condition-enforced immersed boundary method, which accurately ensures no-slip boundary conditions, is implemented as the boundary solver in the corrector step. Four typical numerical examples are presented to demonstrate the stability, the flexibility, and the accuracy of the present method. [ABSTRACT FROM AUTHOR]

The generation of broadband light within the telecommunication band has been instrumental to the design and characterization of advanced optical devices and systems. In this paper, stimulated degenerate four-wave mixing of an ultra-silicon rich nitride waveguide is investigated using a pulsed pump at 1.555 μm and incoherent broadband sources emitting in the 1.65 μm wavelength region as a signal. The waveguide possesses a large nonlinear parameter of 330 W−1/m as well as anomalous dispersion, required for phase matched parametric processes. The broadband idler ranging from 1.43 μm to 1.52 μm is generated using a coupled peak power of 4.6 W, spanning ∼100 nm at the −20 dB level, which is sufficient to cover parts of the E- and S-bands. The spectral span of the generated idler also agrees well with the calculation based on the phase-matching condition governing degenerate four-wave mixing. Cascaded incoherent four-wave mixing is also observed. Using a supercontinuum pump spanning from 1.1 to 1.7 μm with a coupled peak power of 26 W, an idler spanning from 1.2 to 1.4 μm is generated, equivalent to an idler on/off conversion efficiency of 27 dB. [ABSTRACT FROM AUTHOR]

We present the design, fabrication, and characterization of photonic crystal waveguides (PhCWs) on an ultra-silicon-rich nitride (USRN) platform, with the goal of augmenting the optical nonlinearities. The design goals are to achieve an optimized group index curve on the PhCW band edge with a non-membrane PhCW with symmetric SiO2 undercladding and overcladding, so as to maintain back-end CMOS compatibility and better structural robustness. Linear optical characterization, as well as nonlinear optical characterization of PhCWs on ultra-silicon-rich nitride is performed at the telecommunication wavelengths. USRN’s negligible two-photon absorption and free carrier losses at the telecommunication wavelengths ensure that there is no scaling of two-photon related losses with the group index, thus maintaining a high nonlinear efficiency. Self-phase modulation experiments are performed using a 96.6 μm PhCW. A 1.5π phase shift is achieved with an input peak power of 2.5W implying an effective nonlinear parameter of 1.97 x 104 (W m)-1. This nonlinear parameter represents a 49 x enhancement in the nonlinear parameter from the slow light effect, in good agreement with expected scaling from the measured group index. [ABSTRACT FROM AUTHOR]

Multiphoton absorption coefficients and nonlinear refractive indices of germanium in the near and mid-infrared (2-5 µm) are reported. The nonlinear coefficients are measured by open and closed aperture Z-scan with 150 fs pulses at a repetition rate of 1 kHz. The nonlinear refractive index of Ge has a peak value of 9:1 x 10-5 cm²/GW at a wavelength of 3 µm. The effect of free electrons generated by multiphoton absorption is discussed by investigating the variation of multiphoton absorption coefficients at different input powers. Kramers-Kronig relations are also discussed with regard to the relationship between nonlinear refractive index and two photon absorption coefficient. [ABSTRACT FROM AUTHOR]

In this paper, a three-dimensional simplified and unconditionally stable lattice Boltzmann method (3D-USLBM) is proposed for simulating incompressible isothermal/thermal flows. This method is developed by reconstructing solutions to the macroscopic governing equations recovered from the lattice Boltzmann equation and resolved in a predictor-corrector scheme. The final formulations of 3D-USLBM only involve the equilibrium and the non-equilibrium distribution functions. Among them, the former is calculated from the macroscopic variables and the latter is evaluated from the difference between two equilibrium distribution functions at different locations and time levels. Thus, 3D-USLBM directly tracks the evolution of macroscopic variables, which yields lower cost in virtual memory and facilitates the implementation of physical boundary conditions. A von Neumann stability analysis was performed on the present method to theoretically prove its unconditional stability. By imposing a regular Lagrange interpolation algorithm, this method can be flexibly extended to a nonuniform Cartesian mesh or body-fitted mesh with curved boundaries. Four numerical tests, that is, plane Poiseuille flow, 3D lid-driven cavity flow and 3D natural convection in a cubic cavity, and concentric annulus, were conducted to verify the stability, accuracy, and flexibility of the presented method. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate an on-chip polycrystalline PbTe photoconductive detector integrated with a chalcogenide glass waveguide. The device is monolithic ally fabricated on silicon, operates at room-temperature, and exhibits a responsivity of 1.0 A/W at wavelengths between 2.1 and 2.5 µm. [ABSTRACT FROM AUTHOR]

We demonstrate an on-chip sensor for room-temperature detection of methane gas using a broadband spiral chalcogenide glass waveguide coupled with off-chip laser and detector. The waveguide is fabricated using UV lithography patterning and lift-off after thermal evaporation. We measure the intensity change due to the presence and concentration of methane gas in the mid-infrared (MIR) range. This work provides an approach for broadband planar MIR gas sensing. [ABSTRACT FROM AUTHOR]

The group velocity dispersion (GVD) of silicon nitride waveguides, prepared using plasma enhanced chemical vapor deposition, is studied and characterized experimentally in support of nonlinear optics applications. We show that the dispersion may be engineered by varying the geometry of the waveguide and demonstrate measured anomalous GVD values as high as -0.57 ps2/m and normal GVD values as high as 0.86 ps2/m. We also experimentally demonstrate the absence of any observed nonlinear loss at the telecommunications wavelength at peak intensities of up to 12 GW/cm2. Spectral broadening due to self phase modulation in silicon nitride waveguides with a nonlinear parameter of 1.4 W-1/m is also demonstrated. [ABSTRACT FROM AUTHOR]

Coupled chirped vertical gratings are proposed for on-chip group velocity dispersion engineering. The device circumvents the need for directional couplers or circulators to redirect compensated data. Finite difference time domain simulations and experimental verification of fabricated devices are performed. Dispersion values of up to //8.4×105 psnmkm are demonstrated. [ABSTRACT FROM AUTHOR]

In this letter, a low-cost alternative for forming high grade silicon germanium (SiGe) by a laser-induced crystallization of an amorphous Ge layer deposited directly on Si+ preamorphized implantation Si substrate is demonstrated. The results show that a fully strained epitaxial SiGe layer on the Si (100) substrate can be obtained at laser fluence above the epitaxial threshold. This is due to a liquid-phase epitaxial regrowth process of the laser annealing induced melted layer. Below the epitaxial threshold, polycrystalline SiGe is formed due to explosive recrystallization process. Simultaneous boron activation is achieved with the SiGe formation, a result due to the high temperature induced by the laser annealing. [ABSTRACT FROM AUTHOR]

In this letter, the effect of vacancies generated by preirradiated laser on dopant diffusion and activation in preamorphized silicon substrate has been studied. Laser-induced melting in silicon was used to generate excess vacancies near the maximum melt depth before silicon substrate amorphization and subsequent boron implantation. We demonstrate that by matching the preirradiated laser melt depth with the implant amorphize depth, it can effectively reduce the silicon self-interstitials released from the end-of-range defect band. The results show great suppression in boron transient enhanced diffusion and significant removal of end-of-range defects. This is attributed to the recombination of laser-generated excess vacancies with preamorphizing induced free silicon interstitials at the end-of-range region. [ABSTRACT FROM AUTHOR]

The effect of group velocity dispersion and free carrier lifetime on pulse quality and nonlinear phase acquisition is studied in a two-stage pulse compressor on a silicon chip. Appropriately, designing the dispersion in the nonlinear medium enables compressed pulses with more than 90% of the pulse energy confined in the main lobe to be generated. Free carrier lifetime is observed to impact nonlinear phase acquisition in the single pulse regime. The pulse compression factor decreases monotonically as the pulse repetition rate increases. Conditions for efficient compression at high repetition rates in the absence of free carrier quenching techniques are discussed. [ABSTRACT FROM AUTHOR]

The core of equivalent source method based near-field acoustic holography is to solve the equivalent source strength with the measured pressure and the transfer matrix, which is a typical ill-posed inverse problem. In this work, an explicit formula is proposed in which the regularization parameter is proportional to the product of the hologram distance and the maximum singular value of the transfer matrix. Utilizing the proposed approach, robust acoustic field reconstruction can be achieved even when the hologram distance is 5 times larger than the average microphone spacing.

A novel non-contact measurement technique had been developed for the mechanical properties of materials in Split Hopkinson Pressure Bars (SHPB). Instead of the traditional strain gages mounted on the surfaces of bars, two shutters were mounted on the end of bars to directly measure interfacial velocity using Fiber Micro-Displacement Interferometer System for Any Reflector. Using the new technique, the integrated stress-strain responses could be determined. The experimental technique was validated by SHPB test simulation. The technique had been used to investigate the dynamic response of a brittle explosive material. The results showed that the new experimental technique could be applied to the dynamic behavior in SHPB test.

Pb quantum well films with atomic-scale uniformity in thickness over macroscopic areas were prepared on Si(111)-7x7 surfaces. As a probe molecule, O(2) was used to explore the effect of electron confinement in the metal films on the surface reactivity. X-ray photoelectron spectroscopy results showed clear oscillations of oxygen adsorption and Pb oxidation with the thickness of the Pb films. The higher reactivity to O(2) on the films with 23 and 25 ML Pb has been attributed to their highest occupied quantum well states being close to the Fermi level (E(F)) and the high density of the electron states at E(F) (DOS-E(F)), as evidenced by the corresponding ultraviolet photoelectron spectroscopy. A dominant role of DOS-E(F) was suggested to explain the quantum modulation of surface reactivity in metal quantum well films.