Long-wavelength Range Research Articles

Near-Field and Far-Field Directional Conversion of Spoof Surface Plasmon Polaritons.

A compact metallic meta-structure is proposed to realize directional conversion between spoof surface plasmon polaritons (SSPPs) and propagating waves at millimeter wave and THz frequencies. The structure is constructed by embedding two slits or multi-slits array into a subwavelength metallic reflection grating. When the back-side of the structure is illuminated by an oblique beam with a fixed incident angle, the propagating wave will be unidirectionally converted into SSPPs with a considerable efficiency. Both the simulations and experiments demonstrate that the excitation ratio of the SSPPs between the two possible propagating directions (left and right) reaches up to about 340. Furthermore, assisted by the structure, near-field SSPPs can be also converted into far-field narrow beams with particular directions. Through frequency sweeping, wide-angle beam scanning is verified by theory and experiments. The work paves a new way for SSPPs launching and also provides fresh ideas for super-resolution imaging in the longer wavelength range.

Optical transmission of strained GaN/sapphire structures

In this work we correlated transmission spectra of GaN layers grown on sapphire substrates by hydride vapour phase epitaxy with biaxial stress measured in the layers. It was observed that the sign of stress in the GaN layer is changed by Si doping and growth conditions. Transmission curves are shifted relative to each other depending on the stress in the layer. The cut-off wavelength of the transmission curves has a tendency to shift near parallel to a shorter wavelength range when the GaN layer is under the compression biaxial stress. When the GaN layer is under the tensile biaxial stress the cut off wavelength has a tendency to shift near parallel to a longer wavelength range).

Length divergence of the lattice thermal conductivity in suspended graphene nanoribbons

Thermal properties of graphene have attracted much attention, culminating in a recent measurement of its length dependence in ribbons up to 9 $\ensuremath{\mu}\mathrm{m}$ long. In this paper, we use the improved Callaway model to solve the phonon Boltzmann transport equation while capturing both the resistive (umklapp, isotope, and edge roughness) and nonresistive (normal) contributions. We show that for lengths smaller than 100 $\ensuremath{\mu}\mathrm{m}$, scaling the ribbon length while keeping the width constant leads to a logarithmic divergence of thermal conductivity. The length dependence is driven primarily by a ballistic-to-diffusive transition in the in-plane (LA and TA) branches, while in the hydrodynamic regime when $10\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}lLl100$ $\ensuremath{\mu}\mathrm{m}$, the contribution from the in-plane branches saturates and the out-of-plane (ZA) branch shows a clear logarithmic trend, driven by the nonresistive normal contribution. We find that thermal conductivity converges beyond $Lg100$ $\ensuremath{\mu}\mathrm{m}$ due to the coupling between in-plane and flexural modes. This coupling leads to renormalization of ZA phonon dispersion in the long-wavelength range, preventing further divergence of thermal conductivity. We also uncover a strong dependence on sample width, which we attribute to the interplay between nonresistive normal and diffusive edge scattering in the Poisseuille flow regime. We conclude that normal processes play a crucial role in the length and width dependence of thermal transport in graphene in the hydrodynamic regime and dictate the relative in-plane (LA+TA) to out-of-plane (ZA) contribution to transport.

Polarization memory effect in the photoluminescence of nc-Si-SiOx light-emitting structures.

The polarization memory (PM) effect in the photoluminescence (PL) of the porous nc-Si−SiOx light-emitting structures, containing nanoparticles of silicon (nc-Si) in the oxide matrix and passivated in a solution of hydrofluoric acid (HF), has been investigated. The studied nc-Si−SiOx structures were produced by evaporation of Si monoxide (SiO) powder in vacuum and oblique deposition on Si wafer, and then the deposited silicon oxide (SiOx) films were annealed in the vacuum at 975 °C to grow nc-Si. It was found that the PM effect in the PL is observed only after passivation of nanostructures: during etching in HF solution, the initial symmetric nc-Si becomes asymmetric elongated. It was also found that in investigated nanostructures, there is a defined orientational dependence of the PL polarization degree (ρ) in the sample plane which correlates with the orientation of SiOx nanocolumns, forming the structure of the porous layer. The increase of the ρ values in the long-wavelength spectral range with time of HF treatment can be associated with increasing of the anisotropy of large Si nanoparticles. The PM effect for this spectral interval can be described by the dielectric model. In the short-wavelength spectral range, the dependence of the ρ values agrees qualitatively with the quantum confinement effect.

Determination and evaluation of the optical properties of dielectric liquids for concentrating photovoltaic immersion cooling applications

The optical properties of the immersion dielectric liquids are highly important for concentrating photovoltaic (CPV) systems with direct liquid-immersion cooling applications. A method for experimentally determining the optical transmittance of immersion dielectric liquids was proposed, based on the spectrophotometer measurement and photometry correction. Through the method, the optical transmittance of five candidate immersion dielectric liquids, both before and after long term stability test, was determined. Further, normalized photocurrent density (Jnp) was introduced to evaluate the effect of transmittance loss on CPV solar cells photocurrent. Results show that all the selected liquids exhibit much higher transmittance than that of EVA and silicone encapsulants in the spectrum range of 300–1100nm. De-ionized water is shown to be the strongest absorber in the longer wavelength range and thus it is not suitable for immersing multi-junction CPV solar cells. With the other four initial organic liquids presence, the bottom subcell Jnp of the specified three-junction solar cells decreased. In fact, this is a beneficial property, especially for dimethyl silicon oil and ethyl acetate. In addition, results from the long term stability test show that the most stable liquid for CPV immersion cooling applications was observed to be dimethyl silicon oil. This is because no degradation in its optical transmittance was detected after immersion tabbed CPV solar cells for 180days.

The study of CdSe thin film prepared by pulsed laser deposition for CdSe/CdTe solar cell

Over the past few years, the studies of CdS/CdTe solar cells have become the mainstream, which have achieved good performance of exceeding 20 %. Here, we prepare CdSe thin films by pulsed laser deposition (PLD) which is one of promising techniques to synthesize semiconductor thin films. In this paper, the optical and electrical properties as well as the crystal structure of CdSe thin films deposited under different temperatures are investigated. We find that the deposition rate adds firstly, and then decreases with the increase of substrate temperature. Transmittance spectrum suggests the absorption edges of CdSe thin films under the condition of high temperature move towards short wavelength. The band gaps of CdSe thin films are 1.948, 1.976, 2.013 and 1.978 eV at the deposition temperature of 150, 250, 350, 450 °C, respectively. X-ray diffraction analysis indicates the formation of cubic phase with a strong (111) preferential orientation. The surface morphology of CdSe thin film and the cross-sectional structure of CdSe/CdTe layer are also analyzed. Based on these studies, we further fabricate the CdSe (PLD)/CdTe solar cells that present excellent performance of response in the range of long wavelength over 900 nm and a good efficiency of near 10 % has been realized.

Optical and Phonon Characterization of Ternary CdSe x S1−x Alloy Quantum Dots

Ternary CdSe x S1−x alloy quantum dots (QDs) were synthesized using a wet chemical method. Their morphology, particle size, structural, optical, and vibrational properties were investigated using transmission electron microscopy, x-ray diffraction, UV–Vis, fluorescence and Raman spectroscopy, respectively. The optical and vibrational properties of the QDs can be controlled by adjusting the Se/S molar ratio. The absorption and emission peaks shift to a longer wavelength range when increasing the Se content. The presence of two CdSe-like and CdS-like longitudinal optical phonon modes was observed. The dependencies of the optical and phonon modes on the Se content are discussed in detail.

Analysis of external quantum efficiency and conversion efficiency of thin crystalline silicon solar cells with textured front surface

The present study aims to develop a simple analytical model that simulates the effect of the front surface texturization on thin crystalline silicon solar cells performance. It provides an analytical solution to the complete set of equations needed to determine the effect of the texturization on the performance of an $$\hbox {n}^{++}/ \hbox {n}^{+}/\hbox {p}$$n++/n+/p solar cell. The increase in external quantum efficiency (EQE), enhancement of conversion efficiency, contributions of the different cell regions to EQE, and effects of the physical parameters of each region were simulated and evaluated in comparison with $$\hbox {n}^{++}/ \hbox {n}^{+}/\hbox {p}$$n++/n+/p flat solar cells. Overall, the findings reveal that, for the same physical parameters of the different regions of the solar cells, the textured cells have better EQE and better conversion efficiency than the flat ones. The cell conversion efficiency can be improved by up to 0.83 %. This enhancement can be attributed to the decrease of the front surface reflectance for short-wavelength range and the increase of the light path for long-wavelength range.

Investigation of halloysite nanotubes with deposited silver nanoparticles by methods of optical spectroscopy

Halloysite nanotube composites covered by silver nanoparticles with the average diameters of 5 nm and 9 nm have been studied by methods of optical spectroscopy of reflectance/transmittance and Raman spectroscopy. It has been established that silver significantly increases the light absorption by nanocomposites in the range of 300 to 700 nm with a maximum near 400 nm, especially for the samples with the nanoparticle size of 9 nm, which is explained by plasmonic effects. The optical absorption increases also in the long-wavelength spectral range, which seems to be due to the localized electronic states in an alumosilicate halloysite matrix after deposition of nanoparticles. Raman spectra of nanocomposites reveal intense scattering peaks at the local phonons, whose intensities are maxima for the samples with the silver nanoparticle sizes of 9 nm, which can be caused by plasmonic enhancement of the light scattering efficiency. The results show the ability to use halloysite nanotube nanocomposites in photonics and biomedicine.

Synthesis and optical properties of glass with cadmium sulfide nanoparticles

A new approach to the synthesis of silicate glass with cadmium sulfide (CdS) nanoparticles has been developed. It has been demonstrated that replacement of cadmium sulfide with cadmium oxide and zinc sulfide used as sources of cadmium and sulfur allows a substantial reduction of sulfur volatilization during the melting. The produced samples have been investigated by the methods of spectrophotometry in the ultraviolet and visible ranges of the optical range. Thermal treatment of the samples at temperatures exceeding the glass formation point initiates the growth of CdS nanoparticles, which results in the increase of their average size from 3 to 5.2 nm and a significant shift of the fundamental absorption edge to the longer wavelength range from 380 to 480 nm.

High-Speed Antenna-Coupled Terahertz Thermocouple Detectors and Mixers

An antenna-coupled nanothermocouple (ACNTC) is an integrated structure consisting of a dipole nanoantenna and a nanothermocouple (NTC). ACNTCs are excellently suited as polarization-sensitive detectors and mixers for the long-wavelength far-infrared range around 30 THz. Radiation collected by the integrated nanoantenna and fed into the hot junction creates a temperature difference between the hot and cold junctions of the thermocouple, which results in open-circuit voltage due to the Seebeck effect. Due to the geometry-dependence of the Seebeck coefficient in nanowires, we realize single-metal ACNTCs. The fundamentals of single-metal NTCs are discussed. The thermal dynamics of NTCs is investigated showing that NTCs could exhibit mixer and detector intermediate frequency and low frequency cutoffs beyond 100 GHz. We provide experimental evidence of ACNTCs.

Theoretical Study of Local Surface Plasmon Resonances on a Dielectric-Ag Core-Shell Nanosphere Using the Discrete-Dipole Approximation Method**Supported by the Anhui Provincial Natural Science Foundation under Grant Nos 1308085QA19 and 1408085QA15, the Key Scientific Research Foundation of Anhui Provincial Education Department under Grant Nos KJ2013A180, KJ2015A223, KJ2015ZD28 and AQKJ2015B017, and the Young Foundation of Anqing Normal University under Grant Nos KJ201313

The local surface plasmon resonances (LSPRs) of dielectric-Ag core-shell nanospheres are studied by the discretedipole approximation method. The result shows that LSPRs are sensitive to the surrounding medium refractive index, which shows a clear red-shift with the increasing surrounding medium refractive index. A dielectric-Ag core-shell nanosphere exhibits a strong coupling between the core and shell plasmon resonance modes. LSPRs depend on the shell thickness and the composition of dielectric-core and metal-shell. LSPRs can be tuned over a longer wavelength range by changing the ratio of core to shell value. The lower energy mode ω− shows a red-shift with the increasing dielectric-core value and the inner core radius, while blue-shifted with the increasing outer shell thickness. The underlying mechanisms are analyzed with the plasmon hybridization theory and the phase retardation effect.

On the possibility of separation of aerosol and methane absorption in the long-wavelength spectral range for giant planets

To determine the aerosol absorption component of the effective optical depth τeffκ, a special method has been developed. It is based on the implausible effect of the decrease of the aerosol scattering component τeffa in dependence on the atmospheric pressure P, if τeffκ = 0. It is the elimination of this effect by selecting the values of τeffκ that allows the latter to be determined. This method was tested with the data of observations of the North Equatorial Belt (NEB) of Jupiter’s disk in the methane absorption bands at λ = 619 and 727 nm. This allowed us to determine the values of τeffκ and the imaginary part of the refractive index of aerosol particles (ni = 0.00063, 0.00065, 0.0007, and 0.00069 at λ = 605.5, 631.3, 714.7, and 741.4 nm, respectively) and to correctly specify the characteristic of the vertical structure of clouds of the planet.

Broadband absorption enhancement in plasmonic thin-film solar cells with grating surface

The plasmonic thin-film solar cells with grating surface is structured and simulated by Comsol Multiphysics software using finite element method. The absorption efficiency of solar cells has been systemically studied by considering structure characteristic parameters. The absorption of grating surface cell is much broader and stronger than that of smooth surface on a-Si at the wavelength from 400 to 700nm. The value of total absorption efficiency (TAE) increases from 47% to 69.3%. The embedded Ag nanoparticle array contributes to the improvement of the absorption of a-Si at longer wavelength range. The localized surface plasmon resonance is induced by Ag nanoparticles, and so that the TAE is increased to 75.1% when the radius of nanoparticle is 60nm at the bottom of a-Si with periodic width 200nm. The grating surface always plays a role to suppress light scattering from the active region, so more light can be absorbed again by a-Si in the infrared-region. Therefore, the results have significance in providing a theoretical foundation for the applications of thin-film solar cell.

Flexible luminescent waveguiding photovoltaics exhibiting strong scattering effects from the dye aggregation

Luminescent solar concentrators have received renewed interest because they can harvest solar radiation without the need for expensive tacking systems. In this work, highly efficient luminescent waveguiding photovoltaic devices exhibiting mechanical flexibility and solar concentration ability have been prepared by integrating Si solar cells with soft polydimethylsiloxane (PDMS) waveguides. We observed that segregated dyes in the PDMS waveguides induce strong scattering effects in the long-wavelength range. The scattered photons were transported effectively to the solar cells, leading to high power conversion efficiencies (PCEs). A single module exhibited a remarkable power conversion efficiency of 4.62±0.02%. After stacking two waveguides, we achieved PCEs as high as 5.23±0.01%, with a projected PCE approaching 12%.

On the linear stability of magnetized jets without current sheets – non-relativistic case

In this paper we consider stability of magnetized jets that carry no net electric current and do not have current sheets (Gourgouliatos et al. 2012). The non-relativistic MHD equations are linearized around the background velocity and the magnetic field structure of the jet. The resulting linear equations are solved numerically inside the jet. We find that introduction of current-sheet-free magnetic field significantly improves jet stability relative to unmagnetized jets or magnetized jets with current sheets at their surface. This particularly applies to the fundamental pinch and kink modes - they become completely suppressed in a wide range of long wavelengths that are known to become most pernicious to jet stability when the evolution enters the non-linear regime. The reflection modes, both for the pinch and kink instability, also become progressively more stable with increased magnetization.

Widely Tunable High-Power Two-Color VECSELs for New Wavelength Generation

We report a collinear two-color vertical external cavity surface emitting laser (VECSEL), which with minimal changes is suitable for intracavity type I second harmonic generation (SHG), type II sum frequency generation (SFG), and type II difference frequency generation (DFG). The demonstrated two-chip cavity provides high-brightness, independently tunable two color, orthogonally polarized intracavity lasing modes. VECSEL chips with emission around 970 and 1185 nm allow the generation of a wide range of wavelengths in the visible and long-wavelength spectral range. The open cavity, collinear geometry, provided a platform for efficient SHG yellow output, SHG blue output, and SFG of green output in the same cavity while only changing the nonlinear crystals. In all cases for visible light generation, a lithium triborate nonlinear crystal is introduced in the cavity region where the fundamental beams are overlapped. Simultaneous SHG and SFG in the same cavity resulting in overlapped outputs consisting of two visible colors are also demonstrated. By using two VECSEL chips with similar gain peaks, intracavity terahertz generation by DFG is demonstrated for the first time in a T-cavity VECSEL geometry. A custom-made tilted periodically poled lithium niobate nonlinear crystal generated DFG signal around 1 THz.

Rapid Determination of Alkalinity (Ammonia Content) in Para Rubber Latex Using Portable and Fourier Transform-Near Infrared Spectrometers

Ammonia (NH3) is the main preservative that is added to field and concentrated latices to prevent the deterioration of properties and, in serious cases, coagulation of latex. Almost all factories monitor NH3 content or alkalinity during processing, as it is an important parameter for trading purposes. Alkalinity is determined by the standard analytical method, acid-based titration, as detailed in ISO 125:2011(E) Natural Rubber Latex Concentrate – Determination of Alkalinity. This method requires a skilled analyst and also the use of chemicals. In addition, the titration involves the subjective determination of the end-point, which may vary with each analyst. Near infrared (NIR) spectroscopy, a rapid, non-chemical and repeatable method, was used in this work. Calibration equations for predicting alkalinity were constructed from the relationship between the absorbance spectra of latex (measured using a portable NIR and a Fourier transform (FT)-NIR spectrometer) and the alkalinity content of the latex. It was found that the best equation obtained using the portable and the FT-NIR spectrometer could be used to predict alkalinity in latex with a coefficient of determination, standard error of prediction and ratio of standard error of validation to the standard deviation of 0.63, 0.101% and 1.62, and 0.97, 0.027% and 6.07, respectively. From the statistics testing for performance measurement as detailed in ISO12099:2010, NIR-predicted values were no different from actual values at the 95% confident interval. Moreover, the best equation obtained from the more reliable calibration achieved using the FT-NIR spectrometer is attributed to the longer wavelength range.

Characteristic features of colloid centers in the absorption spectra of preliminarily heated zinc oxide powders irradiated by protons

The regularities of changes in the diffusion reflection spectra of unheated and heated ZnO powders in the range of 0.3–2.5 μm are studied after heating at temperatures from 400 to 1000°C under 100-keV proton exposure. A shift to the long-wavelength range and an increase in the absorption-band intensity in the ultraviolet region with increasing preliminary-heating temperature are established. An explanation for the optical degradation of thermally treated ZnO powders under the action of accelerated protons is proposed. This degradation is determined by the absorption of intrinsic point defects and colloid centers.

Spectral and temporal characteristics of a ZnSe:Fe2+ laser pumped by a non-chain HF(DF) laser at room temperature

Spectral and temporal characteristics of a ZnSe:Fe2+ laser with a nonselective resonator pumped at room temperature by the radiation of a pulse-periodical electrodischarge HF(DF) laser are studied. It was established that the spectral distributions of the energy and peak power of ZnSe:Fe2+ laser generation depend on a spectral composition of the pumping radiation. The spectra exhibit a line structure with spectral intervals between neighboring lines δλ ≈ 6.8 ÷ 8.6 nm. The shape of the ZnSe:Fe2+ laser generation pulse is wavelength dependent. In a short-wavelength range, the pulse has the form of a peak with a duration of ~5 ns at half-maximum. At a longer wavelength, the peak is accompanied by a ‘tail’. The duration and amplitude of the tail increase with wavelength, in a long-wavelength spectrum range, the peak actually becomes unnoticeable on a background of the ‘tail’. The spectral dependence of the ZnSe:Fe2+ laser generation pulse’s shape affects the positions of the energy and peak power maxima on the wavelength axis. The dynamics of ZnSe:Fe2+ laser generation under the pumping by the pulsed HF(DF) laser is discussed.