Gap Layer Research Articles

A Low-Threshold Miniaturized Plasmonic Nanowire Laser with High-Reflectivity Metal Mirrors.

A reflectivity-enhanced hybrid plasmonic GaAs/AlGaAs core-shell nanowire laser is proposed and studied by 3D finite-difference time-domain simulations. The results demonstrate that by introducing thin metal mirrors at both ends, the end facet reflectivity of nanowire is increased by 30–140%, resulting in a much stronger optical feedback. Due to the enhanced interaction between the surface charge oscillation and light, the electric field intensity inside the dielectric gap layer increases, resulting in a much lower threshold gain. For a small diameter in the range of 100–150 nm, the threshold gain is significantly reduced to 60–80% that of nanowire without mirrors. Moreover, as the mode energy is mainly concentrated in the gap between the nanowire and metal substrate, the output power maintains >60% that of nanowire without mirrors in the diameter range of 100–150 nm. The low-threshold miniaturized plasmonic nanowire laser with simple processing technology is promising for low-consumption ultra-compact optoelectronic integrated circuits and on-chip communications.

Goos–Hänchen effect on a graphene-based hyperbolic metamaterial slab

In this paper, the tunable Goos–Hanchen (GH) effect on a graphene-based hyperbolic metamaterial slab has been investigated using the transfer matrix and finite element methods. It was shown that the excitation of the surface polaritons in the THz frequency region results in enhanced GH shift at the interface of the slab which can be controlled and tuned by changing the orientation of the optical axis of the slab, the chemical potential of graphene monolayers and the thickness of the air gap layer. These may have interesting applications in the optical devices.

Multi-slice representational learning of convolutional neural network for Alzheimer\u2019s disease classification using positron emission tomography

BackgroundAlzheimer’s Disease (AD) is a degenerative brain disorder that often occurs in people over 65 years old. As advanced AD is difficult to manage, accurate diagnosis of the disorder is critical. Previous studies have revealed effective deep learning methods of classification. However, deep learning methods require a large number of image datasets. Moreover, medical images are affected by various environmental factors. In the current study, we propose a deep learning-based method for diagnosis of Alzheimer’s disease (AD) that is less sensitive to different datasets for external validation, based upon F-18 fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT).ResultsThe accuracy, sensitivity, and specificity of our proposed network were 86.09%, 80.00%, and 92.96% (respectively) using our dataset, and 91.02%, 87.93%, and 93.57% (respectively) using the Alzheimer’s Disease Neuroimaging Initiative (ADNI) dataset. We observed that our model classified AD and normal cognitive (NC) cases based on the posterior cingulate cortex (PCC), where pathological changes occur in AD. The performance of the GAP layer was considered statistically significant compared to the fully connected layer in both datasets for accuracy, sensitivity, and specificity (p < 0.01). In addition, performance comparison between the ADNI dataset and our dataset showed no statistically significant differences in accuracy, sensitivity, and specificity (p > 0.05).ConclusionsThe proposed model demonstrated the effectiveness of AD classification using the GAP layer. Our model learned the AD features from PCC in both the ADNI and Severance datasets, which can be seen in the heatmap. Furthermore, we showed that there were no significant differences in performance using statistical analysis.

Electrical modulation of the LWIR absorption and refractive index in InAsSb-based strained layer superlattice heterostructures

InAsSb-based strained layer superlattices (SLS) have strong fundamental absorption, which can be easily modified in a controlled manner by injecting excess carriers. This makes them attractive for intensity modulation of infrared lasers as well as beam steering and spatial beam shaping with a nanosecond-scale time response. This paper reports the modulation of the fundamental absorption and the refractive index by carrier injection in a 3.45-nm-period InAsSb0.65/InAsSb0.35 SLS with a low temperature energy gap of 85 meV grown by molecular beam epitaxy on a GaSb substrate with a GaInSb metamorphic buffer. The SLS absorber was sandwiched by n- and p-type wider energy gap layers for electrical injection and confinement of excess carriers. The population of conduction band states was obtained by measuring the intensity modulation of a 10.6 μm CO2 laser for temperatures ranging from T = 77 to 200 K. An increase of the electron quasi-Fermi level with electrical injection up to 20 meV was observed. The experimental data imply a decrease in the Auger coefficient with temperature, from 3 × 1024 cm6/s at 77 K to 1 × 1024 cm6/s at T = 200 K attributed to recombination involving two electrons and a heavy hole. The refractive index changes obtained by electrical injection of excess carriers can reach 0.05 at T = 77 K and 0.035 at T = 200 K, which are at least three orders of magnitude greater than those obtained with electro-optical materials.

Loss assessment in random crystal polarity gallium phosphide microdisks grown on silicon.

III-V semiconductors grown on silicon recently appeared as a promising platform to decrease the cost of photonic components and circuits. For nonlinear optics, specific features of the III-V crystal arising from the growth on the nonpolar Si substrate and called antiphase domains (APDs) offer a unique way to engineer the second-order properties of the semiconductor compound. Here we demonstrate the fabrication of microdisk resonators at the interface between a gallium-phosphide layer and its silicon substrate. The analysis of the whispering gallery mode quality factors in the devices allows the quantitative assessment of losses induced by a controlled distribution of APDs in the GaP layer and demonstrates the relevance of such a platform for the development of polarity-engineered III-V nonlinear photonic devices on silicon.

Enhancement of the air gap membrane distillation system performance by using the water gap module

Abstract The negative effect of an air gap layer presented between the membrane and cooling plate on air gap membrane distillation (AGMD) performance was diminished largely by inserting a water gap membrane distillation (WGMD) module in series. The new design of air-gap–water-gap membrane distillation (AG-WG)MD was evaluated experimentally by comparing with an AGMD system under different operating conditions. In theory, mass and heat transfer in the new (AG-WG)MD and imitative AGMD systems were analyzed. Experimental outcomes showed that a new (AG-WG)MD design profoundly enhanced flux (Pd) and gained output ratio (GOR), and greatly decreased energy consumption (STEC) and heat input (EH.I). At a concentration of 5,000 mg/L, coolant temperature of 20 °C, and flow rate of 18 L/h, Pd was promoted by 76.26%, 40.84%, 35.45%, 30.91%, and GOR by 46.38%, 33.46%, 31.27%, 26.65%, in addition to STEC being reduced about 55.63%, 46.81%, 43.66%, 38.30%, and EH.I around 31.31%, 25.84%, 23.53%, 20.55%, from the AGMD to (AG-WG)MD system at feed temperatures of 50 °C, 60 °C, 70 °C, and 80 °C, respectively. The outcomes proved that the AGMD performance could be significantly promoted by integrating with WGMD in a combined MD system. This combination increased the temperature difference across the membrane and decreased thermal-concentration boundary layers for the AGMD system.

Quasi-Heteroface Perovskite Solar Cells.

Perovskite solar cells (PSCs) have attracted unprecedented attention due to their rapidly rising photoelectric conversion efficiency (PCE). In order to further improve the PCE of PSCs, new possible optimization path needs to be found. Here, quasi-heteroface PSCs (QHF-PSCs) is designed by a double-layer perovskite film. Such brand new PSCs have good carrier separation capabilities, effectively suppress the nonradiative recombination of the PSCs, and thus greatly improve the open-circuit voltage and PCE. The root cause of the performance improvement is the benefit from the additional built-in electric field, which is confirmed by measuring the external quantum efficiency under applied electric field and Kelvin probe force microscope. Meanwhile, an intermediate band gap perovskite layer can be obtained simply by combining a wide band gap perovskite layer with a narrow band gap perovskite layer. Tunability of the band gap is obtained by varying the film thicknesses of the narrow and wide band gap layers. This phenomenon is quite different from traditional inorganic solar cells, whose band gap is determined only by the narrowest band gap layer. It is believed that these QHF-PSCs will be an effective strategy to further enhance PCE in PSCs and provide basis to further understand and develop the perovskite materials platform.

Defect properties of solar cells with layers of GaP based dilute nitrides grown by molecular beam epitaxy

Dilute nitrides lattice-matched to GaP were studied to explore the possibilities to improve their properties by additional indium or arsenic content in the GaPN alloy for further utilization in solar cells. Admittance spectroscopy shows that intrinsic layers of GaPNAs and InP/GaPN grown by molecular-beam epitaxy have unintentional background silicon donor doping. Deep-level transient spectroscopy allowed us to reveal several defect levels. In GaPNAs, two defect levels were detected at Ec − 0.58 eV and Ev + 0.44 eV, with respective concentrations of 4 × 1015 cm−3 and 2 × 1015 cm−3. After thermal annealing, these could be reduced by a factor of two and by more than one order of magnitude, respectively, leading to an increase of external quantum efficiency and open-circuit voltage of solar cells. The InP/GaPN layer exhibits a defect level at Ec − 0.44 eV (with a concentration of 2 × 1014 cm−3), which is of similar nature as the one at Ec − 0.58 eV in GaPNAs. Furthermore, unlike in GaPNAs, defect levels close to midgap were also detected in the InP/GaPN layer. These non-radiative recombination centers lead to poorer photoelectric properties of solar cells based on InP/GaPN as compared to those based on GaPNAs. Therefore, the introduction of arsenic in the compound and post-growth thermal annealing allowed us to reduce the defect concentrations in dilute nitrides and improve photoelectrical properties for photovoltaic applications.

An end fire printed monopole antenna based on electromagnetic band gap structure

In this paper, a moderate gain monopole antenna is proposed for GSM and Wi-Max applications. The antenna structure is constructed from a traditional printed monopole on Roger substrate of 3006 family. To enhance the antenna performance ultimately, an Electromagnetic Band Gap (EBG) layer is introduced to the antenna structure. A numerical analysis is applied based on a Finite Integral Technique (FIT) of CST Microwave Studio (CSTMWS) formulations to characterize the EBG unit cell in terms of S-parameters, dispersion and reflection diagram, as well as the constitutive electromagnetic properties. The antenna performance is tested numerically with and without the EBG structure in terms of S-parameters and radiation patterns. It is found that the proposed antenna provides an excellent matching, S11< −10 dB, at 1.85 and 3.3 GHz with 2.88 dBi and 5.8 dBi gain, respectively. Nevertheless, the Finite Element Method (FEM) based on Ansoft High Frequency Structure Simulator (HFSS) software package is invoked to compute the antenna performance. The antenna structure is fabricated and tested experimentally. Finally, an excellent agreement is found between the obtained numerical results from both software packages and measurements.

Direct Growth of Vertically Orientated Nanocavity Arrays for Plasmonic Color Generation

AbstractPlasmonic structural colors, arising from resonance interactions between photons and metallic nanostructures, have been developed rapidly for high‐end applications. However, common structural color materials and fabrication methods usually have open plasmonic nanostructures and limited scalability, respectively. Here, a new scheme based on Ag nanowire arrays/SiO2 composite metamaterial films with subwavelength enclosed nanostructures involved that combine a dielectric gap layer and a metal mirror is presented. The whole stacked structure can be simply prepared only via magnetron sputtering without any other procedures. Specifically, by changing deposition parameters, the geometry size and sub‐10 nm periodic parameters of the structure unit cell array can be finely tuned in a controllable and reproducible way. By experiments and simulations, it is demonstrated how interwire coupled plasmonic transverse modes in vertically orientated nanocavity arrays control multiple nanocavity standing‐wave resonances at visible wavelengths, generating three primary colors‐included bright and saturated colors across a wide gamut. Large‐area and uniform structural colors, whether on rigid or flexible substrates, show angle‐insensitive and air‐stable features. In a wider perspective, this work suggests that the material scheme and fabrication advances represent a robust platform for plasmonic color designing, theory exploring, and large‐scale manufacturing.

Low temperature epitaxial growth of GaP on Si by atomic-layer deposition with plasma activation

An approach for epitaxial growth of GaP layers on Si substrates at low temperature (380 °C) by plasma-enhanced atomic layer deposition (PEALD) is explored. A significant improvement of the crystalline properties of the GaP layers is obtained using additional in-situ Ar plasma treatment. The epitaxial growth for the first 20–30 nm of GaP on Si is demonstrated from transmission electron microscopy. Moreover, the use of in-situ Ar plasma treatment during the PEALD process allows one to increase the growth rate per cycle from 0.9 ± 0.1 Å/cycle to 1.9 ± 0.1 Å/cycle and reduce the RMS roughness from 3.76 nm to 1.88 nm. The effect of Ar plasma treatment on the electronic properties of the GaP/Si interface is studied by deep level transient spectroscopy (DLTS). A defect level at (0.33 ± 0.03) eV below the conduction band is observed in the subsurface layer of Si for the GaP/Si structure grown under Ar plasma treatment. However, the defect response observed by DLTS vanishes after rapid thermal annealing at 500 ºC in nitrogen ambient.

Heat blockage of air gap for inner overheating of high-voltage power cable and delay of early detection

Early detection for inner overheating of high-voltage cable is important for safe power supply. A new radial heat transfer model is developed for a typical 110 kV cable with an air gap layer. Numerical analyses are conducted for dynamic temperature field in cable induced by hot copper core with different thicknesses of air gap. The results show that the air gap has an important heat blockage for the outward heat transfer in cable. The air gap causes the temperature inside the overheated cable to rise faster and the temperature outside slower. The air gap not only reduces the response sensitivity of the surface temperature on inner overheating but also induces the inner layers to heat up and even break down. Finally, the non-dimensional temperatures on cable surface indicate that the detection of inner overheating cable would be delayed if the air gap is not considered in calculation models.

Evaluations of the performance of multi-metallic layered composite cladding for the light water reactor accident tolerant fuel

Multi-metallic layered composite (MMLC) fuel cladding is designed to survive a longer period of time during accident scenarios for light water reactor (LWR) fuel. One proposed MMLC cladding concept joining Zircaloy to steel via interaction barrier layers will increase corrosion resistance, but result in a neutron absorption penalty. This cladding could also result in negative impacts due to mismatches between the mechanical behavior of its layers. In this study, the mechanical performance of the MMLC has been evaluated in two ways: small scale thermo-mechanical simulations and full-length fuel rod simulations. Both were carried out using the BISON fuel performance code. The small scale simulations predicted a reduction in residual stress and elastic deformation compared with standard Zircaloy cladding. The fuel rod simulations predicted decreased creep strain, larger pellet-clad gaps, and possible plastic deformation under long time use when compared to Zircaloy and stainless-steel cladding alone. The performance of various gap widths and layer thicknesses were compared to assist in the design of MMLC cladding; we recommend an MMLC with a Zircaloy/steel layer thickness ratio of at least 1.75 and an initial gap width of 40 μm.

Measurement of differential cross sections of deuteron elastic scattering on 31P for EBS purposes

The 31P(d,d0)31P elastic scattering differential cross-sections were measured, for the first time, in the energy range Ed,lab=900-2400 keV, using a variable energy step, for elastic backscattering spectrometry (EBS) purposes. The measurements were performed at the 5.5 MV TN11 HV Tandem Accelerator of the N.C.S.R. “Demokritos”, implementing a high precision goniometer. The experimental setup consisted of five silicon surface barrier (SSB) detectors, placed at the laboratory scattering angles between 130° and 170° (in steps of 10°). The target used was a thin GaP layer evaporated on a self-supporting carbon foil. To validate the obtained results, benchmarking measurements were performed, using a polished GaP crystal. The determined cross-section datasets are compared with the corresponding ones using Rutherford’s formula, and similarities and discrepancies will be discussed and analyzed.

Two-photon pumped spaser based on the CdS/ZnS core/shell quantum dot–mesoporous silica–metal structure

The spaser (a plasmonic nanolaser) has rapidly advanced as a subwavelength light source candidate. Herein, we introduce a spaser based on a quantum-dot, mesoporous-oxide, and metal structure from top to bottom consisting of CdS/ZnS core/shell quantum dots, a mesoporous silica film (MSF), and an Au film, respectively. Two-photon pumping using femtosecond laser pulses at 800 nm creates amplified spontaneous emission at approximately 451 nm. The advantages of MSF as a dielectric gap layer are examined through numerical simulations. Measuring the dependence of the luminescence intensity on the average pump power confirms the occurrence of two-photon up-conversion luminescence.

Study of pit formation in MBE grown GaP on misoriented Si

GaP-based alloys can be grown lattice-matched to Si, making them an attractive choice for use in Si-based multijunction solar cells. This work focuses on the growth of GaP on Si with the aim to improve the surface quality of GaP. The Si wafers used in this study were of precise (001), (001) 4° offcut toward [110], and (001) 6° offcut toward [110] orientations. GaP of high crystalline quality was grown, and high-resolution x-ray diffraction and atomic force microscopy (AFM) measurements revealed the presence of pits on the surface of GaP. Similar pits were also observed on surface of Si post growth when AFM imaging was done after chemically etching the GaP layers. The use of offcut wafers demonstrated a reduction in the pit density from over 100 μm−2 to values less than 1 μm−2 in both GaP and Si.

Single-Precursor Intermediate Shelling Enables Bright, Narrow Line Width InAs/InZnP-Based QD Emitters

Bright, narrow spectrum infrared emitters, particularly Cd- and Pb-free materials, are of interest for bioimaging, photodetection, and telecommunications. InAs-based quantum dots (QDs) are promising emitters in this spectral range; however, efforts to increase the photoluminescence quantum yield (PLQY) tend to broaden the PL line width as a consequence of interfacial defect formation when thick shells, lattice-mismatched with the core, are employed. Here we report that developing a single-precursor complex for InZnP growth enables uniform shell growth that maintains the excellent size dispersion (6%) of the cores. The introduction of this intermediate layer is key to facilitate the subsequent growth of different shells to improve radiative recombination without sacrificing size uniformity. The growth of InAs/InZnP/ZnSe leads to a PL full-width at half-maximum (fwhm) of 100 meV at 1.12 eV with a PLQY of 14%. We then further introduce an additional GaP layer to increase the radiative/nonradiative relative rate. InAs/InZnP/GaP/ZnSe QDs reach a PLQY of 23% while maintaining a narrow fwhm.

Influence of plasma on electrophysical properties of the GaP/n-Si isotype heterojunction grown by PE-ALD

The work is devoted to studying the influence of an additional step of argon plasma treatment on the quality of the GaP/n-Si isotype heterojunction grown by plasma-enhanced atomic-layer deposition (PE-ALD). Deep-level transient spectroscopy measurements demonstrated that argon plasma leads to the formation of defects with a high concentration in silicon wafers, unlike the PE-ALD mode without a step of hydrogen or argon plasma treatment. In addition, GaP layers obtained by PE-ALD have defects with an activation energy of 0.40 eV and 0.62 eV for processes with and without argon plasma treatment, respectively.

Comparison of Morphological Profiles and Performance Variables between Female Volleyball Players of the First and Second Division in Portugal.

Knowledge of players’ anthropometric profiles may contribute to a better understanding of the differences between their performance levels. For example, vertical jump height, which is a major indicator of a volleyball player’s performance, is influenced by favorable anthropometric variables. This study’s aim was to describe anthropometric profiles of elite female volleyball players competing in the 1st and 2nd divisions in Portugal and to link these profiles with performance skills such as jump height obtained during blocking and spiking actions. Fifty-nine volleyball players were analyzed according to their competitive level, forming three independent groups: GA (n = 20, A1 division, ranked first), GB (n = 21, A1 division, ranked last) and GC (n = 18, A2 division). Anthropometric data collected included body mass and height, arm span, seven skinfolds (triceps, biceps, subscapular, suprailiac, abdominal, thigh and calf), four body perimeters (relaxed brachial, contracted brachial, thigh and calf), and two body diameters (humeral and femoral). Performance data included the height obtained during block and spike actions. Significant differences were found between groups (GA/GB from A1 and GC from A2). Players in the GA had the highest body mass (68.05 ± 6.62 kg, p < 0.05), body height (176.35 ± 6.21 cm, p < 0.05), arm span (177.59 ± 6.09 cm, p < 0.05), lean mass (53.51 ± 4.94 kg, p < 0.05) and vertical jump heights (block: 0.36 ± 0.06 m and spike: 0.43 ± 0.05 m, p < 0.05). As expected, the fat mass percentage of GA players was the lowest (21.30 ± 2.61%). The results suggest that anthropometric profiles of volleyball players may vary according to the competitive level. The higher body mass, body height, arm span, and lean mass presented by GA players in comparison with GC players suggest these variables are important for top-level performance, since these athletes also exhibited higher jump heights. Variables such as height and arm span have a considerable genetic influence and could be important for early talent identification in volleyball. Other variables such as body mass, lean mass and vertical jump performance are more complex, since they also reflect the effects of environmental and training conditions.

Investigation of the Electroluminescence Mechanism of GaN-Based Blue and Green Light-Emitting Diodes with Junction Temperature Range of 120–373 K

Junction temperature (Tj) and current have important effects on light-emitting diode (LED) properties. Therefore, the electroluminescence (EL) spectra of blue and green LEDs were investigated in a Tj range of 120–373 K and in a current range of 80–240 mA based on accurate real-time measurements of Tj using an LED with a built-in sensor unit. Two maxima of the emission peak energy with changing Tj were observed for the green LED, while the blue LED showed one maximum. This was explained by the transition between the donor-bound excitons (DX) and free excitons A (FXA) in the green LED. At low temperatures, the emission peak energy, full width at half maximum (FWHM), and radiation power of the green LED increase rapidly with increasing current, while those of the blue LED increase slightly. This is because when the strong spatial potential fluctuation and low exciton mobility in the green LED is exhibited, with the current increasing, more bonded excitons are found in different potential valleys. With a shallower potential valley and higher exciton mobility, excitons are mostly bound around the potential minima. The higher threshold voltage of the LEDs at low temperatures may be due to the combined effects of the band gap, dynamic resistance, piezoelectric polarization, and electron-blocking layer (EBL).