External Conversion Research Articles

Enhanced performance of quantum cascade Raman laser

We present a self-consistent numerical approach for quantum cascade Raman laser (QC-RL) with a modified design to improve the device performance. Our modeling approach is based on monolithic integration of stimulated Raman scattering (SRS) and electrically pumped QC laser. The laser band structure utilizing techniques with both material-dependent effective mass and band nonparabolicity is calculated by solving the Schrodinger–Poisson equations self-consistently. A detailed analysis of output characteristics of the obtained structure is carried out within a simplified 4-level rate equations model taking into account the SRS process. The model accurately explains the operating characteristics found in QCLs, such as damping transient response and non-resonant behavior of modulation frequency response. Furthermore, modification of the structure is focused on improving the SRS in the QC-RL. This leads to an enhancement of the device performance such as threshold current, external quantum efficiency, conversion efficiency, turn-on delay and modulation response. The excellent agreement of the experimental data with the simulated light output-current characteristics confirms the validity of the model.

Improvement of absorption in organic solar cells using tri-layer anode

The use of transparent conductive oxides (TCOs) in organic solar cells applications requires deposition at low temperatures and with a limited heat treatment. This process results in high resistivity materials. To achieve practical resistivity levels, we need to replace the conventional electrode with an TCO/Metal/TCO tri-layer anode. This material yielded the desired electrical properties without a significant effect on its optical properties. The use of these tri-layer anodes in organic solar cells led to the modification in optical field distribution in the structure of the solar cells, and improves the external power conversion efficiency by maximising the absorption into the photoactive layer.

Fast wavelength conversion to generate 560 nm fluorescence for data transmission in polymer optical fibres

The use of a fluorescent fibre (15.5 mm in length) pumped in a forward direction with a 520 nm emitting LED for fast wavelength conversion to 560 nm is reported. In addition to the fact that this wavelength matches the attenuation minima of poly-methyl-methacrylate polymer optical fibres (POFs), the presented results show an external conversion efficiency of 4.7% and a bandwidth of 52.5 MHz. The device is a simple, inexpensive, compact, eye-safe and lightweight optical source at 560 nm. It can be suitable for single-channel ‘long-distance’ (>400 m) or WDM-based links using POFs, presenting a bandwidth–distance product of 50–100 MHz/100 m.

Supporting Disadvantaged Young People into Work: Insights from the Capability Approach

AbstractThe Capability Approach (CA) offers a perspective on the employment activation of young people that is concerned with their freedom to make choices that they value rather than focusing solely on outcomes, such as having to take any job. It incorporates empowerment and the individual and external conversion factors that influence the conversion of resources into functionings for young people, such as getting a job that they value. This article considers the implications of using the CA as a lens for analyzing youth activation polices. A more capability informed approach to employment activation would not measure success solely by the transition into work, but rather by whether it has improved the young person's capabilities, and might focus, for example, on more sustainable and valued careers and develop individuals' freedom of choice in the labour market. Using data from two UK case studies of third sector organizations that support young people into work, it explores these issues empirically, including the extent to which these employment activation programmes, in their current form, can enhance the capabilities of beneficiaries. Conclusions on the implications of a CA for employment activation are made.

Imagining STEM higher education futures: advancing human well-being

The paper explores a conceptual approach to the question of what it means to provide a university education that addresses equity, and encourages the formation of STEM graduates oriented to public-good values and with commitments to making professional contributions to society which will advance human well-being. It considers and rejects resource-based and utilitarian approaches to well-being and opts for Amartya Sen and Martha Nussbaum’s capabilities approach which involves developing graduates’ capabilities and functionings to be and do in ways which they have reason to value. The capabilities approach argues for educational and social development as the expansion of such freedoms. The approach is sensitive also to diversity through the focus on the choice of a plurality of functionings and a wide informational basis for policy, practice and social justice evaluations. It allows an analysis of how different individuals can convert available resources into functionings through attending to conversion factors: personal (internal) conversion factors, social conversion factors and environmental (external) conversion factors. All make a difference to how an individual converts her resources into functionings and makes advantage and disadvantage visible. There is a further education-specific issue, that of selecting core valuable capabilities because not any form of science and engineering education will do. Educators need to deliberate and agree which capabilities and corresponding functionings are valuable and so should be supported in policy and practice. In a capabilities-friendly education, education would aim to secure and distribute valuable education capabilities to diverse students (women and men, black and white, rural and urban and so on) paying attention to the social arrangements in education (pedagogies, institutional culture and education policy) and to conversion factors and barriers that might impede the development of opportunities and valued outcomes.

Centrality Dependence of Low Momentum Direct Photon Production in = 200GeV Au+Au Collisions in PHENIX

Interest in the production of direct photons at low momentum in heavy ion collisions has increased recently with the rather puzzling recent PHENIX results on the observation of a large excess in the yield of direct photons in Au+Au collisions compared to the Ncou scaled p+p yield and the observation of a large direct photon v2 at pT below 3GeV/c. The two measurements are difficult to reconcile within the current understanding of the fireball evolution and the assumption that low momentum direct photons are dominantly emitted early in the collision. This has sparked much theoretical interest, with new calculations refining old works as well as calculations of newly proposed production mechanisms. Experimental constraints on newly proposed production mechanisms are crucial in understanding the evolution of the fireball. One would also desire experimental evidence whether the direct photons below pT of 3GeV/c are dominantly emitted early or late in the collision. In this proceedings, we show new PHENIX measurements of the detailed centrality dependence of direct photon production in the range of 0.4 < pT < 5GeV/c in Au+Au collisions at = 200GeV based on the high statistics of the combined 2007 and 2010 datasets. Photons are cleanly identified through their external conversion in the backplane of the Hadron Blind Detector to dielectron pairs. We observe the shape of the excess direct photon invariant yield in the range of 0.6 < pT < 2GeV/c is well described by an exponential fit with an inverse slope of 240MeV/c2 consistent with previous measurements. Additionally, the shape is seen to be the same for all centralities within uncertainties and the integrated yield follows a power law as a function of Npart. The new results will offer powerful constraints to current calculations which must predict the same centrality dependence.

High-power continuous-wave operation of a fiber optical parametric oscillator in L and U bands

Fiber optical parametric oscillators (OPOs), based on a highly-efficient four-wave mixing process in a χ(3) medium, are reviewed. Their capability to provide very large tuning ranges with high output power is discussed. A novel architecture for CW fiber OPOs is presented, which has allowed us to significantly extend the performance of these devices. To do so we used: (i) a highly-nonlinear fiber (HNLF) as the parametric gain medium; (ii) a narrowband tunable intracavity filter; (iii) a high output coupling fraction from the feedback loop (up to 3dB). With these features, we have been able to obtain excellent performance in terms of output power and tuning range, even with a reduced pump power. With only about 2W of pump power, we have obtained the following performances: (i) tuning range of 254nm; (ii) output power in excess of 1W at some wavelengths; (iii) external conversion efficiency in excess of 60% at some wavelengths; (iv) linewidth as low as 8GHz.This architecture of fiber OPO can be used for providing narrow linewidth tunable high-power CW radiation over hundreds of nanometers. Such sources could find applications in remote sensing, optical communication, nonlinear optics, etc.

Systematic studies of the centrality dependence of soft photon production in Au + Au collision with PHENIX

Since the earliest days of Heavy Ion Physics thermal soft photon radiation emitted during the reaction had been theorized as a smoking gun signal for formation of a quark-gluon plasma and as a tool to characterize its properties. In recent years the existence of excess photon radiation in heavy ion collisions over the expectation from initial hard interactions has been confirmed at both RHIC and LHC energies by PHENIX and ALICE respectively. There the radiation has been found to exhibit elliptic flow $v_2$ well above what can currently be reconciled with a picture of early emission from a plasma phase. During the 2007 and 2010 Au+Au runs PHENIX has measured a high purity sample of soft photons down to $p_T>0.4\,\text{GeV}/c$ using an external conversion method. We present recent systematic studies by PHENIX from that sample on the centrality dependence of the soft photon yield, and elliptic and triangular flow $v_2$ and $v_3$ in Au+Au collisions which fill in the experimental picture and enable discrimination of competing soft photon production scenarios.

Effect of interface incorporation of cadmium nanocrystallites on the photovoltaic performance of solar cells based on CdS/Si multi-interface nanoheterojunction

A non-planar and multi-interface CdS/Si nanoheterojunction was prepared by growing CdS nanocrystallites (nc-CdS) onto silicon nanoporous pillar array (Si-NPA) using a chemical bath deposition (CBD) method. Utilizing the surface reducibility of Si-NPA, small quantities of Cd nanocrystallites (nc-Cd) were incorporated naturally into the CdS–Si interface during the CBD process. Solar cells with a device construction of ITO/CdS/Si-NPA/Al were prepared based on CdS/Si-NPA and their photovoltaic performances, including open circuit voltage, short circuit current density, external quantum efficiency and energy conversion efficiency, were measured under simulated AM 1.5G illumination. Compared with CdS/Si-NPA cells without nc-Cd incorporation, an energy conversion efficiency promotion by two orders of magnitude was achieved, which was ascribed to the reduction of the series resistance resulted from the nc-Cd incorporation at the interface. Our results show that the incorporation of nc-Cd at CdS–Si interface might be an effective path for obtaining high-efficiency solar cells based on CdS/Si multi-interface nanoheterojunctions.

Functional patterns for thin-film-type amorphous silicon solar cells

Three types of patterns acting as light-scattering centers were constructed on bulk glass surfaces and applied to solar cell fabrication. In order to fabricate a scattering center in the transparent conducted oxide (TCO) layer, the SiO2-based solution hydrogen silsesquioxane (HSQ; Dow Corning FOx-16) was used for direct nano-patterning. Direct nano-patterning is a simple and fast process that exploits the solvent permeability of poly(dimethylsiloxane) (PDMS). The nano- and micro-structured SiO2 layer fabricated directs diffused light into the active layer of the solar cell, enabling effective use of this layer. This increases the external quantum efficiency and conversion efficiency of the cell. Additionally, a thinner Si junction was fabricated to ensure the effect of each pattern.

Weak-signal conversion from 1550 to 532 nm with 84% efficiency.

We report on the experimental frequency conversion of a dim, coherent continuous-wave light field from 1550 to 532nm with an external photon-number conversion efficiency of (84.4±1.5)%. In contrast to previous works, our conversion efficiency value incorporates all losses before the photoelectric detection, including those introduced by frequency filters. We used sum-frequency generation, which was realized in a standing-wave cavity built around a periodically poled type I potassium titanyl phosphate (PPKTP) crystal, pumped by an intense field at 810nm. Our result is in full agreement with a numerical model. For optimized cavity coupler reflectivities, it predicts a conversion efficiency of up to 93% using the same PPKTP crystal.

Reconstruction of π0 and η mesons via conversion method in Au+Au at 1.23AGeV with HADES

Lepton pairs emerging from decays of virtual photons are one of the most promising probes of dense hadronic matter. The HADES experiment at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt studies dielectron radiation as well as strangeness production in various reactions, i.e. proton, deuteron and heavy-ion. The understanding of the corresponding experimental results calls for supporting studies from various model calculations. For a more model independent understanding of the dilepton cocktail the production cross sections of particles created during the freeze-out is crucial. In this contribution we will present results from a two photon analysis of Au+Au in 1.23 GeV/u collisions providing information on π0 and η mesons. Therefore the capability of HADES to detect e+e− pairs from external conversion of real photons will be explained and first results will be shown.

Electron and Hole Transport Layers: Their Use in Inverted Bulk Heterojunction Polymer Solar Cells

Bulk heterojunction polymer solar cells (BHJ PSCs) are very promising organic-based devices for low-cost solar energy conversion, compatible with roll-to-roll or general printing methods for mass production. Nevertheless, to date, many issues should still be addressed, one of these being the poor stability in ambient conditions. One elegant way to overcome such an issue is the so-called “inverted” BHJ PSC, a device geometry in which the charge collection is reverted in comparison with the standard geometry device, i.e., the electrons are collected by the bottom electrode and the holes by the top electrode (in contact with air). This reverted geometry allows one to use a high work function top metal electrode, like silver or gold (thus avoiding its fast oxidation and degradation), and eliminates the need of a polymeric hole transport layer, typically of an acidic nature, on top of the transparent metal oxide bottom electrode. Moreover, this geometry is fully compatible with standard roll-to-roll manufacturing in air and is less demanding for a good post-production encapsulation process. To date, the external power conversion efficiencies of the inverted devices are generally comparable to their standard analogues, once both the electron transport layer and the hole transport layer are fully optimized for the particular device. Here, the most recent results on this particular optimization process will be reviewed, and a general outlook regarding the inverted BHJ PSC will be depicted.

Rare-Earth-Free Direct-Emitting Light-Emitting Diodes for Solid-State Lighting

The advent of a wide-bandgap GaN p-n junction has enabled highly efficient blue light-emitting diodes (LEDs) in GaInN/GaN heteroepitaxy. The system also enables a much wider range of emission wavelengths. We summarize progress in epitaxial materials development of green, yellow, and orange direct-emitting LEDs that bypass the steps of external phosphor conversion to achieve higher stability, efficiency, and higher overall color rendering quality for wider adoption of one of the widest power savings resources so far identified.

Designing III-V multijunction solar cells on silicon

Single junction Si solar cells dominate photovoltaics but are close to their efficiency limits. This paper presents ideal limiting efficiencies for tandem and triple junction multijunction solar cells subject only to the constraint of the Si bandgap and therefore recommending optimum cell structures departing from the single junction ideal. The use of III-V materials is considered, using a novel growth method capable of yielding low defect density III-V layers on Si. In order to evaluate the real potential of these proposed multijunction designs, a quantitative model is presented, the strength of which is the joint modelling of external quantum efficiency and current-voltage characteristics using the same parameters. The method yields a single parameter fit in terms of the Shockley-Read-Hall lifetime. This model is validated by fitting experimental data of external quantum efficiency, dark current, and conversion efficiency of world record tandem and triple junction cells under terrestrial solar spectra without concentration. We apply this quantitative model to the design of tandem and triple junction solar cells, yielding cell designs capable of reaching efficiencies without concentration of 32% for the best tandem cell and 36% for the best triple junction cell. This demonstrates that efficiencies within a few percent of world records are realistically achievable without the use of concentrating optics, with growth methods being developed for multijunction cells combining III-V and Si materials.

The reversal of the laser-beam-induced-current contrast with varying illumination density in a Cu2ZnSnSe4 thin-film solar cell

We apply an array of correlated spatially-resolved techniques, including μ-Raman/photoluminescence/reflectance/laser-beam-induced-current in conjunction with scanning electron microscopy and atomic force microscopy, to study the impact of the microscopic-scale thickness inhomogeneity of CdS layer in a Cu2ZnSnSe4 thin-film solar cell. Thicker CdS regions are found to cause more light reflecting loss thus yield lower external quantum efficiencies and energy conversion efficiencies than the general area. However, these regions show much less efficiency degradation at high illumination intensity, leading to an inversion of laser-beam-induced-current contrast in the area mapping. While improving the CdS layer uniformity can boost the device performance, the finding further points out the possibility of operating thin-film photovoltaic devices based on the similar materials (such as CuInGaSe2, CdTe, Cu2ZnSn(S,Se)4) under a substantially higher illumination density for concentrated photovoltaic and photo-detection.

Photovoltaic effect in multiphase Bi-Mn-O thin films.

We report an external solar power conversion efficiency of ~0.1% in Bi-Mn-O thin films grown onto (111) oriented Niobium doped SrTiO3 (STO) single crystal substrate by pulse laser deposition (PLD). The films contain BiMnO3 (BMO) and Mn3O4 (MO) phases, which both grow epitaxially. The growth conditions were tailored to obtain films with different Bi/Mn ratios. The films were subsequently illuminated under a sun simulator (AM 1.5 G). We find that the Bi/Mn ratio in the film affects the magnitude of the photo induced voltage and photocurrent and therefore the photovoltaic conversion efficiency. Specifically, a higher Bi/Mn ratio (towards unity) in the film increases the power conversion efficiency. This effect is described in terms of a more favorable energy band alignment of the film/substrate hetero-structure junction, which controls photo carrier separation.

Pore size dependence of self-assembled type photonic crystal on dye-sensitized solar cells efficiency utilising Chlorine e6

There are few reports on photoelectric conversion efficiency using naturally-occurring dyes for dye-sensitized solar cells (DSSC). This is because the matching with an excited electronic level of naturally-occurring dye to the conduction band of semiconductor is problematic; the excited electrons are easily relaxed to the steady state with fluorescence or heat emission. We examined the fluorescence inhibition effect of a self-assembled photonic crystal using Chlorine e6 dye. Chlorine e6 is derived from chlorophyll and has a long excited electron lifetime. We prepared TiO2 inverse opals with various particle sizes by liquid phase deposition and described their effect on DSSCs with regard to structural, optical and electrochemical properties. In addition, we explored the implications of fluorescence lifetime measurements relative to the photonic band diagram and the amount of adsorbed dye. Although the main factor affecting the external photoelectric conversion efficiency was the diffusion resistance of the electrolyte and the contact resistance between TiO2 interfaces, the possibility that the dye fluorescence lifetime, i.e. the photonic band structure, can affect the internal quantum efficiency per one dye molecule was also investigated.

Seeing with New Eyes

Creative Nursing 2013 has been about innovating: learning to think in new ways and anticipating instead of simply reacting to change. Futures Thinking framed our first issue; futurist Joel Barker showed how to conduct strategic exploration to address the future systematically. Other articles described a new paradigm of palliation in the care of individuals with dementia, practical and ethical challenges inherent in conducting research on patients, and the need for proactive awareness of the potential for error that dwells in the everyday chaos and complexity of our practice environments. "The Net Generation" addressed a segment of the nursing profession who have "never known a time when the Internet has not been a major presence in our lives" and the implications for care planning, the nursing process, and nonverbal communication.Issue #2, Cognitive Creativity, celebrated the life of the mind that characterizes our humanity. Creativity scholars Alfonso Montuori and Gabrielle Donnelly cited everyday creativity-a phenomenon that permeates every dimension of life- with particular emphasis on group and collaborative creativity. They shared with other scholars a recognition of the paradoxical nature of creativity. Our Outcomes feature reported on research into thinking styles and creativity preferences among nursing students and practicing nurses in Spain. Issue #2 contained a strong emphasis on creative ways to improve nursing education: incorporation of transdisciplinarity into curricula; a portrait of holistic nurse educators; helping master 's degree nursing students write clearly and relate to the literature in a personal way; and strategies for teaching nursing students intentional, accountable communication to increase safety.In Issue #3, Manifestations of Creativity, nursing scholar Daniel Pesut presented models of creative thinking, principles of directed creativity for quality improvement efforts, best practices for leading innovation in organizations, and self-assessment tools. From the Netherlands came a description of one of the most innovative programs we have ever featured in Creative Nursing: Buurtzorg-a nurse-led, nurse-run organization of self-managed teams that provide home care to patients in their neighborhoods. The Buurtzorg model is based on principles of trust, professionalism, creativity, simplicity, and collaboration. Other articles highlighted intrapreneurship, the role of social media in health care, integrating clinical reasoning into nursing curricula, an innovative implementation of a falls prevention program, the birth of a free clinic, and a moving account of a nurse's reconnection with a former patient many years after their nurse-patient relationship in an ICU.This current issue honors the goal of innovating: transformation. All transformation involves change, but not all change is transformative. Some change is retrograde and not in the positive sense of returning to a golden mean. There are names for the kind of change that does no good: backsliding, reverting to type, sinking to a new low, and bad money driving out good. No, true transformative change comes from innovation-seeing with new eyes.In this issue, nursing professors Patricia Freed and Dorcas McLaughlin from the School of Nursing at St. Louis University advocate for stimulating and supporting students' creative processes and impulses through story catching, embodied learning, and stimulating imaginations. The authors echo Hildegard Peplau in stating, "Failure to fully develop functions that have been associated with the right, creative brain could be an expensive error for a discipline which values the centrality of the nurse-patient relationship."Mental health professional Jeffrey Jamerson cites the narrative theory paradigm that our perception of the outside world is constructed within, through language and narrative-the internal and external conversations people have with themselves and others. Jamerson has developed a curriculum to help at-risk youth examine and then rewrite those conversations through expressive remix therapy. …

Semitransparent organic photovoltaics using a near-infrared absorbing cyanine dye

A selective near-infrared absorbing heptamethine cyanine dye (Cy7-P) electron donor was used for the fabrication of bilayer solar cells with the acceptor C60. Using a reflective metal top electrode, solar cell optimization resulted in an external power conversion efficiency (η) of 1.5% for layer thicknesses of ~20nm for Cy7-P and of ~40nm for C60, with using either PEDOT:PSS or MoO3 as anode and Alq3 as cathode buffer layers. Highly transparent devices were then fabricated by using silver/Alq3 cathodes. Average visible transmittance (450–670nm) values of, for example, 67.2% (η=0.7%), 62.1% (η=0.9%) or 50% (η=1%) were obtained for different thickness combinations of silver and Alq3. Optimized solar cells had a maximum transparency of 79.8% at 568nm. The operational stability under 1sun illumination was T80=30h, after which 80% of the initial cell performance was reached.