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An equivalent circuit model has been developed to model a one-dimensional waveguide with many side-branch Helmholtz resonators. This waveguide constitutes a phononic crystal that has been shown to have decreased phase speed below the resonance frequency of an individual resonator. This decreased phase speed can be exploited to achieve super-resolution using broadband time reversal focusing techniques. It is shown that the equivalent circuit model is capable of quantifying this change in phase speed of the crystal and also the small-scale wave-resonator interactions within the crystal. The equivalent circuit model enables the parameterization of the physical variables and the optimization of the focusing bandwidth by balancing the combination of increasing resolution and decreasing amplitude near the resonance frequency. It is shown that the quality factor—in this case, the quality factor determined by the geometric shape of each resonator—controls the range of frequencies that are strongly affected by the Helmholtz resonators. [ABSTRACT FROM AUTHOR]

Understanding the elastic properties of materials is critical for their safe incorporation and predictable performance. Current methods of bulk elastic characterization often have notable limitations for in situ structural applications, with usage restricted to simple geometries and material distributions. To address these existing issues, this study sought to expand the capabilities of resonant ultrasound spectroscopy (RUS), an established nondestructive evaluation method, to include the characterization of isotropic multi-material samples. In this work, finite-element-based RUS analysis consisted of numerical simulations and experimental testing of composite samples comprised of material pairs with varying elasticity and density contrasts. Utilizing genetic algorithm inversion and mode matching, our results demonstrate that elastic properties of multi-material samples can be reliably identified within several percent of known or nominal values using a minimum number of identified resonance modes, given sample mass is held consistent. The accurate recovery of material properties for composite samples of varying material similarity and geometry expands the pool of viable samples for RUS and advances the method towards in situ inspection and evaluation. [ABSTRACT FROM AUTHOR]

We investigate, via temperature and excitation density dependent quasi-resonant confocal microphotoluminescence, the optical properties and internal quantum efficiency (IQE) of InGaN/GaN single quantum wells (QWs) on Ga-polar GaN microrods selectively grown by continuous flow metal organic vapor phase epitaxy on patterned SiO2/n-GaN/sapphire template. Seven samples were grown with different growth parameters for the InGaN/GaN QW. The homogeneity of their optical properties is analyzed by mappings along the m-plane facet of the microrods in order to get insight on the growth mechanisms of the shell. Excitation density dependent measurements show that the IQE is affected by the high doping level of the core, which is required to grow such high aspect-ratio structures. Local IQEs between 15±1% near the tip and 44±5% near the base of microrods are estimated from measurements at room and low temperature. By comparison with results reported on planar c-plane QWs, we conclude that the radiative recombination rate is the main limitation for the emission efficiency. [ABSTRACT FROM AUTHOR]

In acoustics, it is known that, for a given response signal at an arbitrary location, reciprocity and time reversal (TR) can be used to focus high levels of acoustic energy at that position. In solid media, elastic waves generally induce different disturbances in three directions. In this paper, both experimental and numerical wave propagation results for solid materials demonstrate the ability to use a scalar source, a three component detector and the reciprocal TR process to selectively focus each of the different vector components, either individually or collectively. The principle is explained from an analytical point of view. The numerical and experimental study demonstrates excellent temporal and spatial focalization. Applications of the selective vector component focusing can be found in damage imaging techniques using both linear or nonlinear ultrasonic waves. [ABSTRACT FROM AUTHOR]

This paper describes a time reversal (TR) method of spatially illuminating a source signal which has been masked by another source signal. This masking occurs as a result of inherent limitations in the traditional TR process. The selective source reduction (SSR) method employs a subtraction technique where one TR focus is selectively reduced to illuminate the masked focus. Experimental results and considerations are presented to demonstrate the SSR method for two elastic wave pulses emitted simultaneously from two spatially separated surficial sources and to examine the limitations of the method. A blind test was conducted to demonstrate that no a priori information about the source(s) is required. Spatial and/or temporal characteristics of multiple close-proximity sources can be resolved with the use of the illumination method. The measurements show that the SSR method’s limitations are chiefly due to imperfect temporal reconstruction of the source function in the time reversed focal signal, which consequently limits signal reduction. [ABSTRACT FROM AUTHOR]

Introduced in this paper is a new method of determining and investigating focal positions of time reversed elastic wave fields. This method exploits the temporally symmetric nature of time reversed acoustics focused signals as they are akin to the autocorrelation function of the forward propagation received signals. Contrasting this symmetry with the degree of asymmetry at regions away from the focal location provides details about the original source that cannot be retrieved when using other standard imaging conditions. [ABSTRACT FROM AUTHOR]

We investigated the degradation of cleaved facets of (Al,In)GaN laser diodes in different atmospheres. We found that operation in water-free atmospheres with sufficient oxygen shows a slow degradation. Operation in atmospheres with water vapor causes a fast degradation and an oxidation on the facet. This deposition is a permanent damage to the laser diode. If the laser diode is operated in pure nitrogen, we find a thick deposition on the facet, which shows high absorption. This deposition can be removed by either high optical output powers or by operation in atmospheres with sufficient oxygen. We also explain the influence of these coatings to the degradation behavior and see these coatings as the reason for unstable kinks in the L–I characteristics during operation. [ABSTRACT FROM AUTHOR]

In this paper, the multimodal nonlinear elastic behavior of concrete, which is representative of a consolidated granular material, is modeled numerically. Starting from a local three-dimensional softening law, the initial stiffness properties are re-estimated according to the local strain field. The experiments deal with samples of thermally damaged concrete blocks successively excited around their first three modes of vibration. The geometry of these samples cannot be described by a one-dimensional approximation in these experiments where compressional and shear motions are strongly coupled. Despite this added complexity, the nonlinear behavior for the three modes of vibration of the samples is well captured by the simulations using a single scalar nonlinear parameter appropriately integrated into the elasticity equations. It is shown that without sufficient attention paid to the latter, the conclusions would have brought erroneous statements such as nonlinearity dispersion or strain type dependence. [ABSTRACT FROM AUTHOR]

We report a method to determine the radiative efficiency (ngrad) of GaN-based light-emitting diodes using excitation density and bias dependent room temperature photoluminescence (PL) measurements selectively exciting the active region. Considering carrier escape by tunnelling out of the active region, we extrapolate the generation rate of charge carriers from photocurrent measurements under reverse bias. A model describing the recombination of carriers including phase-space filling is then fitted to excitation density dependent PL data obtained under forward bias to extract ngrad. Results show that grad vs. carrier density is asymmetric around its maximum due to phasespace filling. [ABSTRACT FROM AUTHOR]

In this letter, the equivalence between local and global measures of nonclassical nonlinear elasticity is established in a slender resonant bar. Nonlinear effects are first measured globally using nonlinear resonance ultrasound spectroscopy (NRUS), which monitors the relative shift of the resonance frequency as a function of the maximum dynamic strain in the sample. Subsequently, nonlinear effects are measured locally at various positions along the sample using dynamic acousto elasticity testing (DAET). After correcting analytically the DAET data for three-dimensional strain effects and integrating numerically these corrected data along the length of the sample, the NRUS global measures are retrieved almost exactly. [ABSTRACT FROM AUTHOR]

This letter presents a three-dimensional model to describe the complex behavior of nonlinear mesoscopic elastic materials such as rocks and concrete. Assuming isotropy and geometric contraction of principal stress axes under dynamic loading, the expression of elastic wave velocity is derived, based on the second-order elastic constants (λ, µ), third-order elastic constants (l, m, n), and a parameter α of nonclassical nonlinear elasticity resulting from conditioning. We demonstrate that both softening and recovering of the elastic properties under dynamic loading is an isotropic effect related to the strain tensor. The measurement of the conditioning is achieved using three polarized waves. The model allows the evaluation of the third-order elastic constants uncoupled from conditioning and viscoelastic effects. The values obtained are similar to those reported in the literature using quasi-static loading. [ABSTRACT FROM AUTHOR]

We demonstrate the possibility of selectively imaging the features of a barely visible impact damage in a laminated composite plate by using an air-coupled time reversal mirror. The mirror consists of a number of piezoelectric transducers affixed to wedges of power law profiles, which act as unconventional matching layers. The transducers are enclosed in a hollow reverberant cavity with an opening to allow progressive emission of the ultrasonic wave field towards the composite plate. The principle of time reversal is used to focus elastic waves at each point of a scanning grid spanning the surface of the plate, thus allowing localized inspection at each of these points. The proposed device and signal processing removes the need to be in direct contact with the plate and reveals the same features as vibrothermography and more features than a C-scan. More importantly, this device can decouple the features of the defect according to their orientation, by selectively focusing vector components of motion into the object, through air. For instance, a delamination can be imaged in one experiment using out-of-plane focusing, whereas a crack can be imaged in a separate experiment using in-plane focusing. This capability, inherited from the principle of time reversal, cannot be found in conventional air-coupled transducers. [ABSTRACT FROM AUTHOR]

In the framework of k · p -theory, semipolar ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN quantum wells (QWs) have equivalent band structures and are expected to have identical optical polarization properties. However, ( 20 2 ¯ 1 ) QWs consistently exhibit a lower degree of linear polarization (DLP) than ( 20 2 ¯ 1 ¯ ) QWs. To understand this peculiarity, we investigate the optical properties of ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) InGaN/GaN single QW light-emitting diodes (LEDs) via resonant polarization-resolved photoluminescence microscopy. LEDs were grown on bulk substrates by metal organic vapor phase epitaxy with different indium concentrations resulting in emission wavelengths between 442 nm and 491 nm. We discuss the origin of their DLP via k · p band structure calculations. An analytical expression to estimate the DLP in the Boltzmann-regime is proposed. Measurements of the DLP at 10 K and 300 K are compared to m-plane LEDs and highlight several discrepancies with calculations. We observe a strong correlation between DLPs and spectral widths, which indicates that inhomogeneous broadening affects the optical polarization properties. Considering indium content fluctuations, QW thickness fluctuations, and the localization length of charge carriers, we argue that different broadenings apply to each subband and introduce a formalism using effective masses to account for inhomogeneous broadening in the calculation of the DLP. We conclude that the different DLP of ( 20 2 ¯ 1 ) and ( 20 2 ¯ 1 ¯ ) QWs might be related to different effective broadenings of their valence subbands induced by the rougher upper QW interface in ( 20 2 ¯ 1 ) , by the larger sensitivity of holes to this upper interface due to the polarization field in ( 20 2 ¯ 1 ) , and/or by the different degrees of localization of holes. [ABSTRACT FROM AUTHOR]

Developed in the late 1980s, Nonlinear Resonant Ultrasound Spectroscopy (NRUS) has been widely employed in the field of material characterization. Most of the studies assume the measured amplitude to be proportional to the strain amplitude which drives nonlinear phenomena. In 1D resonant bar experiments, the configuration for which NRUS was initially developed, this assumption holds. However, it is not true for samples of general shape which exhibit several resonance mode shapes. This paper proposes a methodology based on linear resonant ultrasound spectroscopy, numerical simulations and nonlinear resonant ultrasound spectroscopy to provide quantitative values of nonlinear elastic moduli taking into account the 3D nature of the samples. In the context of license renewal in the field of nuclear energy, this study aims at providing some quantitative information related to the degree of micro-cracking of concrete and cement based materials in the presence of thermal damage. The resonance based method is validated as regard with concrete microstructure evolution during thermal exposure. [ABSTRACT FROM AUTHOR]

This paper describes the principle behind a high amplitude non-contact acoustic source based on the principle of time reversal (TR), a process to focus energy at a point in space. By doing the TR in an air filled, hollow cavity and using a laser vibrometer in the calibration of the system, a non-contact source may be created. This source is proven to be more energetic than an off the shelf focused ultrasound transducer. A scaled up version of the proposed source has the potential to allow nondestructive evaluation processes that require high amplitude, such as nonlinear elastic wave spectroscopy (NEWS) techniques. [ABSTRACT FROM AUTHOR]

A 2 μm electro-optically cavity-dumped semiconductor disk laser (SDL) with a pulse full width at half maximum of 3 ns, a pulse peak power of 30 W, and repetition rates adjustable between 87 kHz and 1 MHz is reported. For ns-pulse cavity dumping the SDL was set up with a 35-cm long cavity into which an intra-cavity Brewster-angled polarizer prism and a Pockels cell for rotation of the linear polarization were inserted. By means of internal total reflection in the birefringent polarizer, pulses are coupled out of the cavity sideways. This variant of ns-pulse 2-μm SDL is well suited for applications such as high-precision light detection and ranging or ns-pulse laser materials processing after further power amplification. [ABSTRACT FROM AUTHOR]

This study broadens vibration-like techniques developed for osseointegration monitoring to the nonlinear field. The time reversed elastic nonlinearity diagnostic is applied to two mock models. The first one consists of tightening a dental implant at different torques in a mock cortical bone; the second one allows one to follow glue curing at the interface between a dental implant and a mock jaw. Energy is focused near the implant interface using the time reversal technique. Two nonlinear procedures termed pulse inversion and the scaling subtraction method, already used successfully in other fields such as contrast agents and material characterization, are employed. These two procedures are compared for both models. The results suggest that nonlinear elasticity can provide new information regarding the interface, complementary to the linear wave velocity and attenuation. The curing experiment exhibits an overall low nonlinear level due to the fact that the glue significantly damps elastic nonlinearity at the interface. In contrast, the torque experiment shows strong nonlinearities at the focus time. Consequently, a parallel analysis of these models, both only partially reflecting a real case, enables one to envisage future in vivo experiments. [ABSTRACT FROM AUTHOR]

Nonlinear elastic wave spectroscopy (NEWS) has been shown to exhibit a high degree of sensitivity to both distributed and isolated nonlinear scatterers in solids. In the case of an isolated nonlinear scatterer such as a crack, by combining the elastic energy localization of the time reversal mirror with NEWS, it is shown here that one can isolate surfacial nonlinear scatterers in solids. The experiments presented here are conducted in a doped glass block applying two different fixed frequency time-reversed signals at each focal point and scanning over a localized nonlinear scatterer (a complex crack). The results show a distinct increase in nonlinear response, via intermodulation distortion, over the damaged area. The techniques described herein provide the means to discriminate between linear and nonlinear scatterers, and thus to ultimately image and characterize damaged regions. [ABSTRACT FROM AUTHOR]

In this Letter, the time reversal mirror is used to focus elastic energy at a prescribed location and to analyze the amplitude dependence of the focus signal, thus providing the nonlinearity of the medium. By varying the frequency content of the focused waveforms, the technique can be used to probe the surface, by penetrating to a depth defined by the wavelength of the focused waves. The validity of this concept is shown in the presence of gradual and distributed damage in concrete by comparing actual results with a reference nonlinear measurement and X ray tomography images. [ABSTRACT FROM AUTHOR]

The development of a process chain allowing for rapid prototyping of GaN-based light-emitting diodes (LEDs) is presented, which does not rely on photolithography. Structuring of the epitaxial layers is realized by direct-writing laser ablation, allowing a flexible chip layout that can be changed rapidly and at low cost. Besides contact metallization and trench formation, mesa definition is the most critical processing step. For mesa formation and to expose the n-GaN contact layer, the epitaxial grown p-GaN layer together with the active region has to be removed completely without forming cracks or crystal defects in the n-GaN layer or the mesa sidewalls, which would cause sidewall leakage currents. In developing an appropriate laser ablation process that meets these requirements, three different laser systems have been employed in a comparative study. These are a frequency-tripled picosecond (ps) Nd:YVO4 laser emitting at a wavelength of 355 nm and a pulse length of 10 ps and two 20 nanosecond (ns) pulse length laser systems, operating at a wavelength of 248 nm (Excimer laser) and 355 nm (frequency-tripled Nd:YVO4 laser), respectively. First, the laser sources are compared regarding the morphological properties of the resulting laser trenches. Due to band filling effects resulting in optical bleaching of the GaN material when irradiating with ps-laser pulses at 355 nm, the resulting ablation process suffers from cracking. Laser ablation using ns-pulses at both 355 nm and 248 nm leads to crack-free material removal up to a well-defined depth. To keep reverse-bias leakage currents at a level comparable to that of conventional dry-etched mesa-LEDs, subsequent wet etching is essential to remove residues in the mesa-trenches irrespective of the laser source used. Besides wet etching, an additional annealing step has to be applied to mesa-trenches fabricated using ns- and ps-laser pulses at a wavelength of 355 nm. Due to the larger penetration depth at 355 nm, defects causing an increased leakage current are generated in the quantum well region during laser irradiation, which manifest themselves by a spectrally broad defect luminescence. To separate and quantify the contributions from the mesa sidewall leakage and from the areal leakage through the p-n-junction, a series of LEDs have been fabricated and analyzed for which the mesa perimeter length was varied while keeping the mesa area constant. In this way, it is shown that laser ablation with ns-pulses at 248 nm results in the lowest sidewall leakage current as well as forward bias voltage-current and output power-current characteristics very similar to those of conventionally dry etched reference LEDs. [ABSTRACT FROM AUTHOR]

The optical gain of single quantum well laser structures on semipolar [formula]-GaN in dependence of the optical polarization and the resonator orientation has been studied by variable stripe length method. The c′-[formula] resonator shows maximum gain in TE mode, followed by the m-[formula]-resonator with extraordinary polarization. The anisotropic gain behaviour is explained by valence sub-band ordering and birefringence of the wurtzite crystal, resulting in a modification of the transition matrix element for stimulated emission. Measurements are accompanied by 6 × 6 k · p band structure calculations and gain analysis. [ABSTRACT FROM AUTHOR]

We report the influence of size controlled interfacial voids on the optical properties of non-epitaxial polycrystalline ZnO films grown by atomic layer deposition. Interfacial voids generated due to nanoscale Kirkendall effect [A. D. Smigelskas and E. O. Kirkendall, Trans. AIME 171, 130, (1947)], along the ZnO-Al2O3 interface, enhanced the ratio of near band edge to defect luminescence of ZnO films up to eight folds. This improvement is attributed to stress relaxation caused by formation of interfacial voids that allowed greater grain growth within the polycrystalline ZnO films upon annealing. Larger crystals have a lower surface-to-volume ratio profile which lessens the defect emission by decreasing surface effects on the optical properties of ZnO. [ABSTRACT FROM AUTHOR]

Si is the most common dopant in (Al)GaN based devices acting as a donor. It has been observed that Si induces tensile strain in (Al)GaN films, which leads to an increasing tendency for cracking of such films with the increase of Si content and/or the increase of Al content. Based on x-ray investigations, the Si-doped films have a larger in-plane lattice constant than their undoped buffer layers, indicating involvement of a mechanism other than the change of lattice constants expected from an alloying effect. In this work, we present a model about Si dislocation interaction while debating other proposed models in the literature. According to our model, Si atoms are attracted to the strain dipole of edge-type dislocations in (Al)GaN films. It is expected that Si is more incorporated on that side of the dislocation, which is under compression leading to the formation of off-balanced dipoles with reduced compressive component. In response to such off-balanced dipoles-appearing as tensile dominant strain dipoles-the dislocation lines climb in order to accommodate the excess tensile strain. However, this dislocation climb mechanism is hindered by forces exerted by vacancies created due to the climb process. Accordingly, we have observed a lower strain level in our Si doped layers when they contain fewer dislocations. These findings were further supported by x-ray diffraction, transmission electron microscopy, and micro-photoluminescence investigations. [ABSTRACT FROM AUTHOR]

With the recent application of time reversed acoustics and nonlinear elasticity to imaging mechanical damage, the development of time reversal based nondestructive evaluation techniques has begun. Here, diffusion bonded metal disks containing intentionally disbonded regions are analyzed using the time reversed elastic nonlinearity diagnostic. The nonlinear results are compared with linear ultrasonic imaging (C scan). Scanning electron microscopy is shown to illustrate the differences between the features seen by the linear and nonlinear methods. [ABSTRACT FROM AUTHOR]

In this work, the results of applying nonlinear dynamics to study progressive material fatigue in human bone are described. Material nonlinear dynamical response has been shown to be associated with mechanical damage. The progressive mechanical damage experiments were conducted in cortical bone extracted from a human femur. After each damage step, the material dynamical nonlinear response was measured by applying wave modulation and extracting a nonlinear parameter proportional to the sideband amplitude. The nonlinear parameter increases rapidly with damage step, indicating increased damage after the initial cycling procedure, while the quasistatic stiffness taken from the cycling experiments shows little change. [ABSTRACT FROM AUTHOR]

The authors measure the interplay of piezoelectric field and built-in potential on carrier recombination and quantum confined Stark effect in green light emitting InGaN quantum wells by modulating the external bias voltage of the pn junction. Time-resolved electroluminescence shows a temporal separation of carrier injection into the active region and radiative recombination within the InGaN quantum wells. During the time when the bias voltage is off, the piezoelectric field is partially compensated by the built-in potential of the pn junction, resulting in an increased radiative recombination rate. [ABSTRACT FROM AUTHOR]

Near- and far-field dynamics of edge-emitting (Al,In)GaN laser diodes are measured simultaneously with a 100 nm spatial and a 5 ns temporal resolution using a scanning near-field microscope. We reconstruct the phase distribution at the laser diode facet. Beam steering and near-field mode dynamics are interpreted in terms of thermal and carrier induced change of refractive index in the waveguide. [ABSTRACT FROM AUTHOR]

The exciton is observed in (In,Al)GaN laser diodes as resonance in the optical gain spectra and in the spectra of the carrier induced change of the refractive index. The observed instability of the exciton with respect to the free electron-hole plasma with increasing carrier densities is accompanied by a blueshift of the exciton resonance due to the quantum confined Stark shift. The experiments confirm central points of many-body simulations of InGaN/GaN quantum wells. The exciton becomes unstable near threshold and so lasing occurs from the electron–hole plasma. [ABSTRACT FROM AUTHOR]

So far, most of the studies on GaN doped with beryllium have mainly concentrated on possible p-type doping. Unfortunately, realization of p-type conductivity in such a way appeared to be very difficult. It seems, however, that bulk crystals doped with beryllium can be used as white light converters in the monolithic white light emitting diodes. To realize monolithic white light emitting diode, we used blue light emitting diodes and a single GaN:Be crystal as converter. High value of the Color Rendering Index gives hope for obtaining an effective light converter based on gallium nitride doped with beryllium. [ABSTRACT FROM AUTHOR]

A nonlinear scatterer is simulated in the body of a sample and demonstrates a technique to locate and define the elastic nature of the scatterer. Using the principle of time reversal, elastic wave energy is focused at the interface between blocks of optical grade glass and aluminum. Focusing of energy at the interface creates nonlinear wave scattering that can be detected on the sample perimeter with time-reversal mirror elements. The nonlinearly generated scattered signal is bandpass filtered about the nonlinearly generated components, time reversed and broadcast from the same mirror elements, and the signal is focused at the scattering location on the interface. [ABSTRACT FROM AUTHOR]

Resonant ultrasound spectroscopy (RUS) is a method whereby the elastic tensor of a sample is extracted from a set of measured resonance frequencies. RUS has been used successfully to determine the elastic properties of single crystals and homogeneous samples. In this paper, we study the application of RUS to macroscopic samples of mesoscopically inhomogeneous materials, specifically rock. Particular attention is paid to five issues: the scale of mesoscopic inhomogeneity, imprecision in the figure of the sample, the effects of low Q, optimizing the data sets to extract the elastic tensor reliably, and sensitivity to anisotropy. Using modeling and empirical testing, we find that many of the difficulties associated with using RUS on mesoscopically inhomogeneous materials can be mitigated through the judicious choice of sample size and sample aspect ratio.