Identical Absorber Research Articles

Self-starting coherent mode locking in a two-section laser with identical gain and absorber media

Coherent mode-locking (CML) represents a method for ultrashort pulse generation in lasers based on the self-induced transparency (SIT) phenomena, which allows us to overcome the principal limitations on the pulse duration imposed by the active medium's gain bandwidth in standard passively mode-locked lasers. So far, this regime was only studied in lasers with transition dipole moments in the absorber medium being twice larger than in the gain medium. This fact ensures that the $2\ensuremath{\pi}$-SIT-soliton in the absorber also forms the stable $\ensuremath{\pi}$-soliton in the gain but can be barely realized in experiments. In this paper we demonstrate theoretically that the self-starting stable coherent mode-locking regime can also arise if the same medium is used both in the gain and absorber laser sections, i.e., the transition dipole moments are equal. These results are both obtained analytically using the area theorem and confirmed by the numerical solution of the Maxwell-Bloch equations. Our findings can enable new opportunities in the area of ultrafast lasers.

Synthesis, Structure and Electromagnetic Properties of Nanocomposites with Three-component FeCoNi Nanoparticles

Infrared heating was used to synthesize FeCoNi/С nanocomposites, where nanoparticles of FeCoNi ternary alloy are stabilized and uniformly distributed in the carbon matrix volume. The authors studied the impact of synthesis temperature and percentage ratio of metals upon the structure, composition and electromagnetic properties. X-ray phase analysis and Mossbauer spectroscopy showed that ternary alloy nanoparticles with different compositions and crystalline lattice types can be formed with the rise in synthesis temperature and iron concentration. Resonator method was used to examine frequency dependencies of relative complex dielectric and magnetic permeabilities of nanocomposites in the range of 3–12 GHz. Calculation of reflection coefficient based on experimental permeability data showed that by varying synthesis temperature and percentage ratio of metals one can control the frequency range of effective absorption of electromagnetic waves. It was established that increase in relative iron content from 33 to 50 rel.% leads to the shift of minimal electromagnetic wave reflection coefficient band from f ~ 12+ GHz to frequency f ~ 6 GHz at identical absorber thickness.

Modeling of absorber pilot plant performance for CO2 capture with aqueous piperazine

A CO2 capture pilot plant with the advanced flash stripper was operated with aqueous piperazine (PZ) at The University of Texas Separations Research Program (UT-SRP) in March 2015. A wide range of absorber operating conditions (solvent concentration, lean loading, flue gas CO2, gas rate, liquid-to-gas ratio) and intercooling configurations was tested in the pilot plant campaign.The pilot plant measurements closed the material balance (±2%) around the absorber. 5m PZ was tested for the first time and outperformed 8m PZ at comparable absorber conditions due to enhanced mass transfer rates in the absorber. Operating with 5m PZ increased the number of transfer units (NTU) by ∼20% for each of 3 tests even though 8m PZ was operated with greater solvent capacity and greater driving force. Operation with full spray intercooling doubled the NTU compared to operation without spray or intercooling at identical absorber conditions.The pilot plant data were used for further validation of the rigorous rate-based absorber model in Aspen Plus®. The pilot plant absorber model has been developed over several pilot plant campaigns, and a consistent method for absorber model validation and correction utilizing a rigorous data reconciliation method with the Independence PZ model was used to predict pilot plant performance for the March 2015 campaign. Pilot plant data suggested that interfacial area correction and CO2 correction provided similar predictions of pilot plant absorber performance and could be used interchangeably to correct the model. The inhibitor correction to density-predicted loadings was consistent with model-predicted correction to titration data and might provide an explanation for pilot-model offsets. Pilot plant data also suggested a possible time dependence, and absorber NTU values were slightly over-predicted (mean ratio of model-predicted NTU to pilot plant-measured NTU=1.08). A possible explanation is that several high CO2 removal cases in the March 2015 campaign were outside the range where the previous model corrections were developed, where the controlling mass transfer resistance in the absorber differed. This observation provides an opportunity to investigate future model corrections at different column mass transfer limiting mechanisms. The column temperature profiles of different intercooling configurations were matched within 2% mean absolute percentage error.

Performance evaluation of plastic solar air heater with different cross sectional configuration

Solar air heaters, SAHs, represent the primary component of solar energy usage systems. The SAHs are able to absorb the solar energy and convert it into thermal energy at the absorbing surface, and then transfer the thermal energy to an air flowing through the SAH. In this work, an experimental investigation was performed on three SAHs with different cross sectional shape to find the best configuration. The shapes are circular, semi-circular and half-circle plus isosceles triangle. All SAHs have an identical absorber with a half-circle shape. The mass flow rate of air was changed from 0.05 to 0.25kg/s. Results indicated that the highest efficiencies were accomplished for the circular configuration and reached to about 80% at a mass flow rate of 0.18kg/s. On the other hand, for the same conditions, the thermal efficiency reached about 64% and 48% for half-circle plus isosceles triangle and semi-circular shapes, respectively.

Temporal coupled-mode theory model for resonant near-field thermophotovoltaics

A temporal Coupled-Mode Theory model is developed to predict performance of resonant near-field ThermoPhotoVoltaic systems, which typically requires numerically intensive calculations. It is formulated for both orthogonal and non-orthogonal (coupled) modes and includes load-voltage dependencies and non-idealities, such as background absorption and radiation losses. Its good accuracy is confirmed by comparing with exact transfer-matrix calculations for two simple planar systems: a plasmonic emitter across a bulk semiconductor absorber and a metal-backed thin-film semiconductor emitter across an identical absorber.

4-Terminal Tandem Photovoltaic Cell Using Two Layers of PTB7:PC71BM for Optimal Light Absorption.

A 4-terminal architecture is proposed in which two thin active layers (<100 nm) of PTB7:PC71BM are deposited on a two-sided ITO covered glass substrate. By modeling the electric field distribution inside the multilayer structure and applying an inverse solving problem procedure, we designed an optimal device architecture tailored to extract the highest photocurrent possible. By adopting such a 4-terminal configuration, we numerically demonstrated that even when the two subcells use identical absorber materials, the performance of the 4-terminal device may overcome the performance of the best equivalent single-junction device. In an experimental implementation of such a 4-terminal device, we demonstrate the viability of the approach and find a very good match with the trend of the numerical predictions.

Plasmonic and diffractive nanostructures for light trapping—an experimental comparison

Metal nanoparticles and diffractive nanostructures are widely studied for enhancing light trapping efficiency in thin-film solar cells. Both have achieved high performance enhancements, but there are very few direct comparisons between the two. Also, it is difficult to accurately determine the parasitic absorption of metal nanoparticles. Here, we assess the light trapping efficiencies of both approaches in an identical absorber configuration. We use a 240 nm thick amorphous silicon slab as the absorber layer and either a quasi-random supercell diffractive nanostructure or a layer of self-assembled metal nanoparticles for light trapping. Both the plasmonic and diffractive structures strongly enhance the absorption in the red/near-infrared regime. At longer wavelengths, however, parasitic absorption becomes evident in the metal nanoparticles, which reduces the overall performance of the plasmonic approach. We have formulated a simple analytical model to assess the parasitic absorption and effective reflectivity of a plasmonic reflector and to demonstrate good agreement with the experimental data.

Erratum to: Non-synchronous and Localized Responses of Systems of Identical Centrifugal Pendulum Vibration Absorbers

The dynamic response of a system composed of a rotor subject to engine-order torsional excitation and fitted with multiple identical centrifugal pendulum vibration absorbers is considered. The absorbers’ desired synchronous response can lose stability in two ways: a jump behavior, which maintains the symmetry associated with absorber synchronicity, and loss of synchronicity via a symmetry-breaking response bifurcation. In this paper, we review the loss of stability of the synchronous response and consider the post-critical system response for the case of general path absorbers. We investigate the resulting post-bifurcation responses, which consist of two groups of absorbers each of which respond in a mutually synchronous manner within the group. By considering permutations of these responses, it is found that there are many such responses with some being dynamically stable. This analytical study is based on the method of averaging applied to a dimensionless form of the equations of motion. It is found that the initial symmetry-breaking bifurcations are generally subcritical, and the subsequent bifurcations can result in situations where several steady-state responses are possible over a wide range of operating conditions. Some of these responses are localized in nature leading to a significant reduction in feasible operating range of the system. It is shown that non-synchronous responses can exist at very small torque levels. Absorber overtuning widens the range of stable synchronous response, but it also widens the range where stable non-synchronous responses coexist with the stable synchronous response. These results provide important considerations for the tuning of identical absorber systems.

Characterization of tandem organic solar cells comprising subcells of identical absorber material

AbstractRecently organic tandem solar cells with record efficiency had been shown comprising identical absorber materials in both subcells. Such structures pose new challenges for characterization. The standard test methods for measuring spectral response of tandem solar cells can not be applied. The standard procedures demand for different bias illumination during measuring spectral response allowing to select the subcell being current limiting. With subcells comprising identical absorber materials, thus having identical absorption spectra, such a selection is not trivial. In this paper, we show that with the help of detailed optical simulations of such tandem organic solar cells, their characterization is possible, and we apply the proposed method to a sample structure. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Optimizing limited solar roof access by exergy analysis of solar thermal, photovoltaic, and hybrid photovoltaic thermal systems

An exergy analysis was performed to compare a conventional (1) two panel photovoltaic solar thermal hybrid (PVT x2) system, (2) side by side photovoltaic and thermal (PV+T) system, (3) two module photovoltaic (PV) system and (4) a two panel solar thermal (T x2) system with identical absorber areas to determine the superior technical solar energy systems for applications with a limited roof area. Three locations, Detroit, Denver and Phoenix, were simulated due to their differences in average monthly temperature and solar flux. The exergy analysis results show that PVT systems outperform the PV+T systems by 69% for all the locations, produce between 6.5% and 8.4% more exergy when matched against the purely PV systems and created 4 times as much exergy as the pure solar thermal system. The results clearly show that PVT systems, which are able to utilize all of the thermal and electrical energy generated, are superior in exergy performance to either PV+T or PV only systems. These results are discussed and future work is outlined to further geographically optimize PVT systems.

Variation of the defect density in a-Si:H and μc-Si:H based solar cells with 2 MeV electron bombardment

Improvement of the performance of solar cells based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon requires understanding of the role of the deep defects – dangling bonds – in the bulk of the intrinsic a-Si:H or μc-Si:H absorber layers. A straightforward way to understand how these defects may affect the performance of the cells is to investigate changes in the device performance upon variation in the defect density.In the present work solar cells with a-Si:H and μc-Si:H absorber layers were exposed to 2MeV electron bombardment. The performance of the cells after various bombardment doses and annealing steps was evaluated in view of the changes in the defect density of intrinsic layers, measured with ESR on nominally identical absorber layers irradiated in parallel with the cells.The defect density was varied over a range of 2 orders of magnitude. In the solar cells a strong degradation of performance is observed upon irradiation with the biggest effect on the short circuit current density JSC for both types of absorber layers. In most cases both VOC and JSC recover after the final annealing step (at 160°C) for both types of cells.

Simulation of a metamaterial beam for mechanical wave absorption

In this paper, we present a metamaterial beam consisting of a uniform isotropic beam with many small mass-spring systems serving as mechanical wave absorbers. Based on the analysis of two different mass-spring systems, negative effective mass and negative effective stiffness are explained theoretically. The governing equations of a unit cell of a metamaterial beam are derived using the Hamilton’s principle. The mechanical wave absorbabilities of the following two different finite metamaterial beams are analyzed by numerical simulation:one is the bean in which mass-spring systems with linearly varying elastic coefficients are uniformly distributed, and the other is the bean in which four identical absorber subgroups composed of spring-damper subsystems with linearly varying natural frequenciesare uniformly distributed. The results reveal that the mechanical wave transmitted in the metamaterial beam is absorbed by resonating with spring-mass absorbers, which verifies the effectivity of the proposed metamaterial beam on mechanical wave absorption.

Temporal position encoding photoacoustics: A technique for surface absorber mapping

A nondestructive photoacoustic method for the evaluation of surface absorber distributions is presented. Ultrasound generated by absorption of a pulsed laser source is detected using optical beam deflection. Temporal encoding of the spatial absorber distribution is achieved using a nonparallel orientation of the probe beam relative to the sample surface. Sound waves launched from individual surface absorbers travel different distances before interacting with the probe beam, thus giving rise to acoustic signatures at different times. With this approach, we resolved eleven identical absorber features in a 3–cm-wide sample. Decreasing the probe beam diameter enhances the temporal and/or spatial resolution. A limiting spatial resolution of 175μm is predicted. The sound wave intensity decays in a roughly inverse square-root manner with increasing distance of travel.

Stability of deuterated amorphous silicon solar cells

In order to elucidate the microscopic mechanism for the earlier observed enhanced stability of deuterated amorphous silicon solar cells we conducted a side-by-side study of fully deuterated intrinsic layers on crystalline silicon substrates using the free-electron laser facility at Nieuwegein (FELIX) to resonantly excite the Si–D stretching vibration and measure the various relaxation channels available to these modes, and of p-i-n solar cells with identical intrinsic absorber layers on glass/TCO substrates to record the degradation and stabilization of solar cell parameters under prolonged light soaking treatments. From our comparative study it is shown that a-Si:D has a superior resistance against light-induced defect creation as compared to a-Si:H and that this can now be explained in the light of the `H collision model' since the initial step in the process, the release of H, is more likely than that of D. Thus, a natural explanation for the stability as observed in a-Si:D solar cells is provided.

Technical note Copper oxide coatings for use in a linear solar Fresnel reflecting concentrating collector

Electroplating selective copper oxide coatings were prepared on the surface of a rectangular absorber ( α = 0.92 and ε 100°C = 0.18) for operation in conjunction with a prototype linear solar Fresnel reflecting concentrating collector. Overall heat loss coefficients ( U L) of the selectively coated absorber at temperatures up to ∼300°C, optical efficiency ( η o) and stagnation temperature of the concentrator–absorber system were determined experimentally. The results have been compared with those obtained from an identical black painted absorber. Thermal efficiency ( η c) computed from the results of U L and η o measurements at a temperature of 250°C provides a value of 49% with the selectively coated absorber and 48% with the black painted absorber.

Performance characteristics of spray-pyrolysed selective cobalt-oxide coated tubular absorber operated with a linear solar concentrator

Spray-pyrolysed selective cobalt-oxide (CoOx) coatings were prepared on the surface of a bright nickel-plated copper tubular absorber (α = 0.89–0.91 and ϵ100°C = 0.18) for operation in conjunction with a prototype linear Fresnel reflector solar concentrator (LFRSC). Some preliminary tests were conducted to study the optical and thermal performance characteristics of the selective cobalt-oxide coated absorber in the concentrated solar flux. The tests conducted included determination of the overall heat loss coefficient UL of the absorber at temperatures from 50 to ∼ 120°C, and the optical efficiency ηo of the concentrator-absorber system, and measurement of the stagnation temperature of the absorber with the prototype solar concentrator. Based on the results of UL and ηo measurements, the thermal efficiency η of the concentrator-absorber system at a working temperature of 115°C has been determined for a typical beam radiation Ib of 600 W/m2. Further, comparison of the results of this study with those obtained using a dimensionally identical black-painted absorber indicates that the performance of the selective cobalt-oxide coated absorber is considerably superior to that of an ordinary black-painted absorber.

Thermodynamic feasibility of an absorber heat recovery cycle for solar airconditioning

A feasibility analysis of a water-lithium bromide absorber heat recovery (AHR) absorption cooling system, suitable for solar airconditioning is presented. In the proposed system the absorber heat (which is usually rejected to the surroundings in the conventional H 2OLiBr system) could be utilized to some extent for reducing the heat input requirement at the generator and hence improving the coefficient of performance. The analysis was carried out for the production of 278 K and 283 K evaporator temperatures with identical condenser and absorber temperatures of 303 K. The effects of generator temperature, condenser and absorber temperatures on the coefficient of performance and relative circulation are discussed. It is concluded that an absorber heat recovery cycle yields a higher COP as compared to the basic cycle at higher generator temperatures, unlike aqua-ammonia absorption systems.

Hyperfine interaction in polycrystalline paramagnetic ytterbium oxide studied by the Mössbauer effect

Abstract The isotopes of 170, 171, 172, 174 Yb were used to study the hyperfine interaction in polycrystalline paramagnetic Yb 2 O 3 between 2.5 °K and 77.3 °K. Asymmetric quadrupole — as well as magnetic dipole interactions were established for the two different lattice sites, which exist in the sesquioxides. We observed a temperature dependence of the magnetic hyperfine field whereas a temperature dependence of the electric field gradient could not be detected. The hyperfine interactions are discussed by using electronic wave functions which one can derive from first order perturbation theory. Experimental and theoretical results are compared. By using the identical absorber for all measurements the ratios of the quadrupole moments of the first excited states have been determined with high accuracy.