Stripe Lengths Research Articles

Combined Funnel, Concentrator, and Particle Valve Functional Element for Magnetophoretic Bead Transport Based on Engineered Magnetic Domain Patterns.

Controlled actuation of superparamagnetic beads (SPBs) within a microfluidic environment using tailored dynamic magnetic field landscapes (MFLs) is a potent approach for the realization of point-of-care diagnostics within Lab-on-a-chip (LOC) systems. Making use of an engineered magnetic domain pattern as the MFL source, a functional LOC-element with combined magnetophoretic "funnel", concentrator, and "valve" functions for micron-sized SPBs is presented. A parallel-stripe domain pattern design with periodically decreasing/increasing stripe lengths is fabricated in a topographically flat continuous exchange biased (EB) thin film system by ion bombardment induced magnetic patterning (IBMP). It is demonstrated that, upon application of external magnetic field pulses, a fully reversible concentration of SPBs at the domain pattern's focal point occurs. In addition, it is shown that this functionality may be used as an SPB "funnel", allowing only a maximum number of particles to pass through the focal point. Adjusting the pulse time length, the focal point can be clogged up for incoming SPBs, resembling an on/off switchable particle "valve". The observations are supported by quantitative theoretical force considerations.

Numerical simulation study of self-driven microdroplet on locally restrictive discontinuous wetting gradient surface using front tracking method

The motion of droplet on surface with discontinuous wetting gradient is of great importance for understanding lab-on-a-chip systems and other microfluidic devices. Different wetting gradients are known to be the main influencing factor in the droplet self-driven process, but the effect of different wall structures on the droplet migration process also deserves further investigation. In this paper, we analyze the self-driven process of liquid droplets on a local wetting gradient surface under microgravity conditions using front tracking method. The effects of different driving stripe lengths [Formula: see text], different restrictive stripe lengths [Formula: see text], and different surface wetting gradients Δcos⁡ θ on the droplet migration process and droplet morphology are analyzed. A theoretical formula that can predict the lateral spreading length of droplets is also proposed. The results show that different driving stripe length [Formula: see text] lengths and the wetting gradient Δcos θ have significant effects on the migration velocity of droplets, while different restrictive stripe length [Formula: see text] lengths have very significant effects on the final morphological characteristics of droplets. When restrictive stripe length [Formula: see text], the hindering effect generated by the restrictive region ΙΙΙ has more and more significant effects on the morphological structure of droplets in the migration process. When the correction factor ε = 0.735 in the prediction equation, the predicted value calculated by the theoretical equation has a good degree of similarity with the numerical simulation results.

Optical net gain measurement on Al0.07Ga0.93N/GaN multi-quantum wells.

We report net gain measurements at room temperature in Al0.07Ga0.93N/GaN 10-period multi-quantum well layers emitting at 367 nm, using the variable stripe length method. The separate confinement heterostructure was designed targeting electron-beam pumped lasing at 10 kV. The highest net gain value was 131 cm-1, obtained at the maximum pumping power density of the experimental setup (743 kW/cm2). The net gain threshold was attained at 218 kW/cm2 using 193 nm optical pumping. From these experiments, we predict an electron-beam-pumped lasing threshold of 370 kW/cm2 at room temperature, which is compatible with the use of compact cathodes (e.g. carbon nanotubes). In some areas of the sample, we observed an anomalous amplification of the photoluminescence intensity that occurs for long stripe lengths (superior to 400 µm) and high pumping power (superior to 550 kW/cm2), leading to an overestimation of the net gain value. We attribute such a phenomenon to the optical feedback provided by the reflection from cracks, which were created during the epitaxial growth due to the strong lattice mismatch between different layers.

Indication of cooperative light amplification in 5,5″-bis(4-biphenylyl)−2,2′:5,2″-terthiophene single crystals at room temperature

Among the large number of π-conjugated compounds, thiophene/phenylene co-oligomers have been one of the promising candidates for both optically pumped light amplification media and light emission media in electroluminescence devices owing to their high thermal stability, high photoluminescence (PL) quantum yield and high carrier mobility. Of these, the single-crystal state of 5,5″-bis(4-biphenylyl)−2,2′:5′,2″-terthiophene (BP3T), which has been frequently used as an active medium in organic light-emitting transistor devices, indicates an unusual light amplification process in its PL spectra. Herein, we explain two insights about the indication of a cooperative light amplification process in BP3T single crystals under optical pumping. The first one is excitation beam area dependence for the nonlinear amplification threshold in PL spectra at room temperature, which has previously been carried out at 12 K. The second one is time-delayed amplified emission in various excitation stripe lengths.

Variable Stripe Length method: influence of stripe length choice on measured optical gain.

The Variable Stripe Length (VSL) method is a very popular tool to measure the optical gain in thin film active devices. However, over the last decade experimental and theoretical evidence has been reported that cast doubt upon its reliability and that seriously discourages its application. Continuing in the path of highlighting the uncertainties associated with this method, this Letter soundly demonstrates that the particular choice of stripe lengths in the VSL measurements profoundly influences the optical gains retrieved by this method. Thus, a single set of VSL data may render gain values that differ by tens of cm-1. The observed gain variability is ascribed to a combination of unavoidable experimental noise and incorrect assumptions in the analytical treatment (small-signal approximation).

Sex differentiation based on the gular stripe in the apparently monomorphic Cape gannet

The Cape gannet, Morus capensis, has long been considered a monomorphic species despite attempts at phenotypically separating the sexes. In a further effort we here recorded morphometric measurements from genetically sexed Cape gannets. Discriminant function analysis was used to classify sexes according to these morphometric measurements. Culmen and gular stripe lengths were selected as the best discriminators, correctly classifying 65% of birds. Male Cape gannets had significantly longer culmens and gular stripes than females. We thereby provide the first evidence suggesting that the Cape gannet demonstrates some level of dimorphism. Nonetheless, morphometric criteria used to separate sexes holds limited value due to overlap in measurements between sexes.

Sex Differentiation Based on the Gular Stripe in the Apparently Monomorphic Cape Gannet

The Cape gannet, Morus capensis, has long been considered a monomorphic species despite attempts at phenotypically separating the sexes. In a further effort we here recorded morphometric measurements from genetically sexed Cape gannets. Discriminant function analysis was used to classify sexes according to these morphometric measurements. Culmen and gular stripe lengths were selected as the best discriminators, correctly classifying 65% of birds. Male Cape gannets had significantly longer culmens and gular stripes than females. We thereby provide the first evidence suggesting that the Cape gannet demonstrates some level of dimorphism. Nonetheless, morphometric criteria used to separate sexes holds limited value due to overlap in measurements between sexes.

LEEM image phase contrast of MnAs stripes

Low energy electron microscopy (LEEM) imaging of strained MnAs layers epitaxially grown on GaAs(001) reveals striped contrast features that become more pronounced and vary systematically in width with increasing defocus, but that are completely absent in focus. Weaker subsidiary fringe-like features are observed along the stripe lengths, while asymmetric contrast reversal occurs between under-focus and over-focus conditions. A Fourier optics calculation is performed that demonstrates that these unusual observations can be attributed to a phase contrast mechanism between the hexagonal α phase and orthorhombic β phase regions of the MnAs film, which self-organize into a periodic stripe array with ridge-groove morphology. The unequal widths of the α and β phase regions are determined accurately from the through focus series, while the height variation in this system can also be determined in principle from the energy dependence of contrast.

Polarization asymmetry and optical modal gain saturation via carrier–photon interaction in ZnO

The polarization dependence of modal gain was examined in ZnO using a variable stripe length method, where the transverse electric (TE) mode gain was dominant over the transverse magnetic (TM) mode gain due to the polarization asymmetry of the wurtzite structure. Modal gain saturation was also investigated using a modal gain contour map for the wavelength and stripe length. The TE modal gain and degree of polarization increased with increasing stripe length up to a threshold length (∼100 μm). At longer stripe lengths, rapid reduction in the carrier density along the stripe resulted in gain saturation and a spectral redshift.

Modal optical gain and cavity mode analysis of unstructured and optically structured ZnO nanocrystalline thin films

Deposition of high-crystal-quality ZnO nanocrystalline thin films (NCTFs) on a lattice-mismatched sapphire substrate was achieved using a pulsed laser deposition technique, and surface channel waveguides were fabricated by using a laser irradiation process. The optical structure defined on the film was characterized using a scanning electron microscope while the optical gain of unstructured and optically structured NCTFs was measured using the variable stripe length method (VSLM). We present a new analysis of a VSLM to study gain saturation for various stripe lengths, and its validity was confirmed by the consistent result obtained by using a widely used method for short stripe lengths. In the case of unstructured films, the inelastic exciton–exciton scattering dominates the stimulated emission (P-band) below Mott density while not only N-band due to the electron–hole plasma but also the exciton–exciton scattering contribute to the lasing beyond Mott density. These results suggest that the exciton–exciton scattering occurs in a localized space of a coherence scale, so the coexistence of the N-band and the P-band stimulated emission beyond the Mott density originates from the non-uniform spatial distribution. An induced cavity defined on a ZnO NCTF sample, showing constant periodic mode features, is confirmed which is attributed to local densification produced due to a refractive index change in a structured arc region.

A High Optical-Gain Organic Crystal Comprising Thiophene/Phenylene Co-Oligomer Nanomolecules

We have estimated the optical gain for a crystal of a thiophene/phenylene co-oligomer, 1,4-bis(5-phenylthiophen-2-yl)benzene. We prepared the crystal by a vapor phase growth method and measured emission spectra by exciting it with a pulse laser and changing the pumped stripe lengths. With increasing the stripe lengths, the emission intensity was increased and the emission spectra were gain narrowed around 516 nm with its full width at half maximum down to approximately 6 nm. The optical gain spectrum was determined from the emission spectra that varied as a function of the pumped stripe lengths. The gain spectrum was peaked at 516 nm with the maximum net optical gain coefficient being 75 cm(-1). This peak location is in excellent agreement with the peak position of the gain narrowed emission spectra. The results indicate that the determination of the gain spectrum is important for laser applications of the material. The oligomer crystal in the present studies is a good candidate for them.

Optical Modal Gain Saturation of Exciton-Exciton Scattering and Electron-Hole Plasma in ZnO

The optical modal gain spectrum for a ZnO thin film was measured by using a variable stripe length method. Using the dierential equation for amplified spontaneous emission in a stripe geometry, we obtained stripe-length-dependent gain spectra, which enabled us to investigate the gain saturation in long stripe lengths. When the stripe was shorter than the saturation length, our analysis was in a good agreement with the gain obtained by commonly-used analysis [1]. For intermediate density ( nMott). Both gains saturate with increasing stripe length, but the electron-hole plasma becomes relatively dominant for long stripe lengths ( 1000 µm) while the exciton-exciton scattering is relatively suppressed. This result suggests that exciton-exciton scattering gain saturation is more vulnerable to increase in the stripe length increase since the excitonic stimulated emission occurs on a coherence scale of the localized area.

Correlation of self-organized surface nanostructures and anisotropic electron transport in nonpolar ZnO (10−10) homoepitaxy

Self-organized surface nanostructures were formed during laser molecular beam epitaxy of nonpolar ZnO (10−10). The growth mechanism of the nanostructures was related to a Schwoebel barrier effect generated during ZnO homoepitaxy. Small islands elongated along the [001] direction were generated on two-dimensional (2D) growing surfaces above the critical thickness. With increasing film thickness, the anisotropic islands developed homogeneous nanostripe arrays with stripe lengths above 5μm and connected by some branches. Highly anisotropic surface morphology markedly influenced electron transport of ZnO films and Mg0.12Zn0.88O∕ZnO multiquantum wells (MQWs) with conductivity parallel to the nanostripe arrays by more than one order of magnitude larger than that observed perpendicular to the nanostripe arrays. We propose that the origin of anisotropic conductivity of ZnO films resulted mainly from anisotropically distributed scattering centers of electrons related to grain boundaries between nanostripe arrays. Furthermore, the surface nanostructures consisting of nanostripe arrays gave rise to the anisotropic surface roughness at the heterointerface between Mg0.12Zn0.88O and ZnO layers and contributed to the anisotropy of 2D electron transport of the MQWs at low temperatures.

Chick feather pattern recognition

A crescent model is proposed for chick wing image processing and feather pattern recognition, thereby implementing chick sex separation by machine vision technology. The crescent shape delineates the region of interest in a wing image by an arc of large radius and an arc of small radius at two off-centred circles. Wing feathers are divergently distributed in the crescent region, manifesting as an oriented stripe pattern. Male chick feathers gradually change in length from short to long and then to short in accordance with the crescent envelope. Female chick feathers alternate the stripe lengths, following a long–short–long stripe pattern. Based on this knowledge, a chick feather pattern can be numerically characterised by a stripe length sequence and a stripe endpoint sequence. For pattern classification, the first-order differences of these two sequences are used. The mean value of the stripe endpoint difference sequence is the most efficient feature in male–female chick classification. Experimental results justified the model and feature selection strategy, and showed the feasibility of automatic chick sex separation.

Investigation of gain and optically pumped lasing in GaInNAs/GaAs multi‐quantum well structures

We present experimental investigation of the optical gain in GaInNAs/GaAs multi-quantum well structures using the variable stripe length method. The amplified spontaneous edge emission was measured at different excitation intensities and stripe lengths. The obtained optical gain values at a temperature of 5 K are typically in the order of 100–150 cm−1, while at room temperature a maximum of 50 cm−1 could be observed. Optically pumped lasing has been detected from cleaved samples at room temperature for excitation densities above 300 kW/cm2.

The effect of current density, stripe length, stripe width, and temperature on resistance saturation during electromigration testing

Resistance saturation as a function of current density, stripe length, stripe width, and temperature is investigated for a two-level structure with Ti/AlCu/Ti/TiN stripes and interlevel W stud vias. A simple model based on first principles is presented, which relates the maximum fractional resistance change to the current density and stripe length. Experimental results for stripe lengths of 30, 50, 70, and 100 μm are in good agreement with the model predictions. Estimated void sizes based on the resistance saturation data are consistent with the actual void sizes determined from scanning electron microscopy analysis. A weak temperature dependence is found for 0.33 μm-wide samples in the range 170–250°C, while a strong width dependence is observed between 0.33 and 1.50 μm- wide samples. The width dependence is qualitatively explained in terms of a relaxed bulk modulus that depends on the aspect ratio of the interconnect lines.

Dynamics of amplified spontaneous emission in InAs/GaAs quantum dots

We investigate the dynamics of amplified spontaneous emission (ASE) in self-assembled InAs/GaAs quantum dots, using pulsed optical excitation of an edge-emitting sample at room temperature. A material gain of 1.5×104 cm−1 is determined for 800 nm excitation with 1.5 μJ/cm2 pulses. Using photoluminescence up-conversion, we show that increases in both electron–hole pair density and photon density in the excited stripe cause a significant decrease in the decay time from 2.2 ns, corresponding to spontaneous emission, to about 0.9 ns, corresponding to stimulated emission or ASE. A carrier capture time of 10 ps limits the onset of the ASE process for short stripe lengths.

Failure of the modal gain model in a GaN based laser diode

The optical gain in a Nichia ® group-III nitride based blue laser diode has been measured at room temperature using the variable stripe length method and nanosecond excitation. Except for stripe lengths less than 50 μm the device shows considerable deviation from the expected exponential intensity behavior. However, for short stripe lengths large gain values of up to 650 cm −1 are observed when exciting with 20 MW cm −2. Plasma recombination from a quantum confined level is suggested as the gain mechanism.

The effect of current density and stripe length on resistance saturation during electromigration testing

Resistance saturation as a function of current density and stripe length has been investigated for a two-level structure with Ti/TiN/AlCu/Ti/TiN stripes and interlevel W stud vias. A simple model relating the resistance change at saturation to the current density and stripe length is formulated for structures with short stripe lengths and blocking boundaries at both ends. Experimental results for stripe lengths of 25, 50, or 100 μm are in good agreement with the model predictions.

The electromigration short-length effect in Ti-AlCu-Ti metallization with tungsten studs

The electromigration short-length effect has been investigated by testing a two-level structure with Ti-AlCu-Ti stripes and interlevel tungsten (W) stud vias. This investigation represents a complete study of the short-length effect using a technologically realistic test structure. Lifetime measurements and resistance changes as a function of time were used to describe this phenomenon, where the latter approach provides new insights into the electromigration behavior of multilayered metallizations. A linear increase in resistance was followed by a resistance change with time that approached zero. For the same product of current density and stripe length, longer stripes increased in resistance to higher values than shorter stripes. The sigma of the lognormal distribution increased as the current density decreased and/or as the maximum allowed resistance change increased. The lifetime, or t50, at relatively small current densities did not obey Black’s empirical equation. Rather, the lifetime data obeyed a modified version of this equation that includes a critical current density jc as a new parameter. As an alternate approach to quantifying the short-length effect, we propose a novel and practical model for determining jc that focuses on the apparent saturation of the resistance increase with time of the W stud chains. Unlike the modified Black’s model, the resistance saturation approach allows one to estimate lower bound limits of jc. The threshold product, (jL)c, is determined from the modified Black’s model and from the resistance saturation model for stripe lengths of 50, 70, and 100 μm. Both models indicate that jc strongly depends on the fail criterion, or the magnitude of the resistance change, but is independent of temperature in the range 175–250 °C. No evidence was found of a (jL)c below which no electromigration-induced damage occurs.