Normal Reflection Research Articles

Mid-infrared absorption properties of green wood

There is a lack of quantitative data on the penetration depth and the amount of energy absorbed by green wood under infrared (IR) radiation. This lack of knowledge is a potential barrier to the development of IR heating as an alternative to soaking as a means of warming logs prior to peeling in the manufacture of plywood. Experimental measurements of normal hemispherical spectral reflectance and transmittance over the range 550–5,500 cm−1 wavenumbers on four wood species, beech, birch, Douglas-fir and spruce have brought new knowledge on mid-infrared absorption properties of green wood and removed some uncertainties. For instance, it is not possible to deliver energy deeper than up to 0.3 mm below the wood surface because 70–90 % of all incident IR radiation on the wood surface is absorbed in this layer. Some wood features, such as surface quality, the presence of knots and of free water in wood (the latter two having a more significant effect) influence the amount of energy absorbed. These results illustrate that IR radiation can heat the surface layers, but then heat penetrates deeper into the inside layers of wood by conduction.

Wong–Zakai approximation of solutions to reflecting stochastic differential equations on domains in Euclidean spaces

In this paper, we study the Wong–Zakai approximation of the solution to the stochastic differential equation on a domain D in a Euclidean space with normal reflection at the boundary. We prove the Lp convergence of the approximation in C([0,T]→D̄) under some general conditions on D.

Electron and hole transmission through superconductor — Normal metal interfaces

We have investigated the transmission of electrons and holes through interfaces between superconducting aluminum (T c = 1.2K) and various normal non-magnetic metals (copper, gold, palladium, platinum, and silver) using Andreev-reflection spectroscopy at T = 0.1K. We analysed the point contacts with the modified BTK theory that includes Dynes’ lifetime as a fitting parameter Γ in addition to superconducting energy gap 2Δ and normal reflection described by Z. For contact areas from 1 nm2 to 10000nm2 the BTK Z parameter was 0.5, corresponding to transmission coefficients of about 80%, independent of the normal metal. The very small variation of Z indicates that the interfaces have a negligible dielectric tunneling barrier. Fermi surface mismatch does not account for the observed transmission coefficient.

An ultrasonic methodology for determining the mechanical and geometrical properties of a thin layer using a deconvolution technique

An ultrasonic method is proposed for simultaneously determining the thickness, density, sound velocity, and attenuation of a thin layer from a reflection spectrum at normal incidence. The normal theoretical reflection spectrum of a thin layer is established as a function of three dimensionless parameters to reduce the number of independent parameters. The inverse algorithm, using the least squares method, is adopted to determine the dimensionless parameters, and the corresponding convergence zones are investigated. The measured reflection spectrum at normal incidence is obtained using Wiener filtering, and spectral extrapolations following Wiener filtering are applied to obtain the time-of-flights by identifying the overlapping pulse-echoes inside the thin layer and the superposing pulse-echoes from the reference material and front surface of the specimen. The thickness of the thin layer can then be calculated and as initial estimate for the inverse algorithm. The density, sound velocity, and attenuation are then determined by the measured thin layer thickness and determined dimensionless parameters. Two 500μm stainless steel and aluminum plates were immersed in coupling water and a 5MHz flat transducer was applied. The relative errors of measured thickness, density, and sound velocity were less than 6%, and the ultrasound attenuation was close to its true value. The validity of the proposed technique was verified.

Quantitative Correlations Between the Coverage and the Normal Incidence Differential Reflectance for Bromide Adsorbed on a Polycrystalline Platinum Rotating Disk Electrode

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Structural, morphology and optical properties of ITO/PEDOT:PSS and ITO/Ag nanoparticles/PEDOT:PSS thin films

In order to investigate the effects and optical properties in the mid-infrared region, thin films of PEDOT:PSS were spin-coated on ITO glasses and ITO/Ag nanoparticles substrates. Scanning Electron Microscopy, SEM, has been applied to investigate the surface morphology and measure the size of nanoparticles. Dielectric and optical coefficients such as refractive index, n, and extinction coefficient, k, were calculated by applying Kramers-Kronig dispersion relations to the experimentally recorded near normal reflection FTIR spectra. Different analyses confirm that Ag nanoparticles are present in the structure of the synthesized thin films. The recorded Raman spectra have shown that the intensity enhancement of Raman peaks of PEDOT:PSS increases due to the plasmon resonance of Ag colloidal silver nanoparticles layer and optical properties investigation has confirmed the electrical conductivity increase in the mid-infrared region.

Quantitative Correlations between the Normal Incidence Differential Reflectance and the Coverage of Adsorbed Bromide on a Polycrystalline Platinum Rotating Disk Electrode

Normal incidence reflectance spectra (λ = 635 nm) data were acquired from a Pt disk of a rotating Pt

Temperature dependence of anisotropic mode splitting induced by birefringence in an InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser studied by reflectance difference spectroscopy

The temperature dependence of the mode splitting induced by birefringence in an InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser has been studied by reflectance difference spectroscopy at different temperatures ranging from 80 to 330 K. The anisotropic broadening width and the anisotropic integrated area of the cavity mode under different temperatures are also determined. The relation between the mode splitting and the birefringence is obtained by theoretical calculation using a Jones matrix approach. The temperature dependence of the energy position of the cavity mode and the quantum well transition are also determined by nearly normal reflectance and photoluminescence, respectively.

Preproduction prediction of the optical normalized reflectance transient of MOVPE-grown AlGaAs films using transfer matrix method

This paper reported on the prediction and analysis of the optical normalized reflectance (NR) transient of AlGaAs multi layers and Distributed Bragg Reflector (DBR) using transfer matrix method (TMM). The simulation result correlated well with the measured NR transient of grown samples. Deviations of AlxGa1⿿xAs composition between the expected and real grown sample were predicted successfully. Smaller optical oscillation amplitude in the first DBR layer was predicted and could be used as an indication of Al composition. Special characteristics of NR transient within the first three pairs of DBR were also clarified using the calculated transmissivity changing with the growth thickness. TMM simulation of NR transient was thus shown to be a convenient and reliable pre-production technique, also not restricted to the AlGaAs material.

The Josephson current through a long quantum wire

The dc Josephson current through a long SNS junction receives contributions from both Andreev bound states localized in the normal region and scattering states incoming from the superconducting leads. We show that in the limit of a long junction, this current, at low temperatures, can be expressed entirely in terms of properties of the Andreev bound states at the Fermi energy: the normal and Andreev reflection amplitudes at the left- and right-hand S–N interfaces. This has important implications for treating interactions in such systems.

Bottom-up Fabrication and Optical Characterization of Dense Films of Meta-Atoms Made of Core–Shell Plasmonic Nanoparticles

In an attempt to fabricate low index metamaterials by a bottom-up approach, meta-atoms constituted of silica-coated silver nanoparticles are assembled by a Langmuir-Schaefer technique into thin films of large area and well-controlled thickness. The silica shells ensure a constant distance between the silver cores, hence providing a constant coupling of the localized surface plasmon resonance (LSPR) of the nanoparticles in the assembled composite material. The optical response is studied by normal angle spectral reflectance measurements and by variable angle spectroscopic ellipsometry. The normal incidence data are described well in the framework of a single effective Lorentz oscillator model. The resonance of the assembled material is blue-shifted and shows no significant broadening with respect to the absorption band of the individual nanoparticles. The observation of these two effects is enabled by the core-shell structure of the meta-atoms that prevents aggregation of the metallic cores. The ellipsometry study confirms the general behavior and reveals the natural birefringence of the few-layer materials. The amplitude of the observed resonance is weaker than expected from the Maxwell-Garnett mixing rule. This well-characterized system may constitute a good model for numerical simulations.

Determining building interior structures using compressive sensing

We consider imaging of the building interior structures using compressive sensing (CS) with applications to through-the-wall imaging and urban sensing. We consider a monostatic synthetic aperture radar imaging system employing stepped frequency waveform. The proposed approach exploits prior information of building construction practices to form an appropriate sparse representation of the building interior layout. We devise a dictionary of possible wall locations, which is consistent with the fact that interior walls are typically parallel or perpendicular to the front wall. The dictionary accounts for the dominant normal angle reflections from exterior and interior walls for the monostatic imaging system. CS is applied to a reduced set of observations to recover the true positions of the walls. Additional information about interior walls can be obtained using a dictionary of possible corner reflectors, which is the response of the junction of two walls. Supporting results based on simulation and laboratory experiments are provided. It is shown that the proposed sparsifying basis outperforms the conventional through-the-wall CS model, the wavelet sparsifying basis, and the block sparse model for building interior layout detection.

Formation of Interface Bound States on a Graphene-Superconductor Junction in the Presence of Charge Inhomogeneities

Interface bound states have been theoretically predicted to appear at isolated graphene-superconductor junctions. These states are formed at the interface due to the interplay between virtual Andreev and normal reflections and provide long range superconducting correlations on the graphene layer. We describe in detail the formation of these states from combining the Dirac equation with the Bogoliubov de Gennes equations of superconductivity. On the other hand, fluctuations of the low energy charge density in graphene have been confirmed as the dominating type of disorder. For analyzing the effect of disorder on these states we use a microscopic tight binding model. We show how the formation of these states is robust against the presence of disorder in the form of electron charge inhomogeneities in the graphene layer. We numerically compute the effect of disorder on the interface bound states and on the local density of states of graphene.

Reflectância de banda larga em recém-nascidos com presença de emissões otoacústicas evocadas por transiente

To characterize the normal values of acoustic energy reflectance obtained with pure-tone stimulation in neonates prior to hospital discharge. Seventy-seven infants (37 girls and 40 boys) were evaluated by measuring acoustic reflectance using pure-tone stimuli, transient-evoked otoacoustic emissions, and tympanometric probe frequencies of 226 Hz and 1 kHz. At low frequencies (258-750 Hz), greater energy reflectance was observed, while at medium frequencies (1-3 kHz), greater energy absorption was observed. There was no difference between ears or between genders. Normal energy reflectance values were obtained for the studied population. The data indicate a reflectance curve with a distinct configuration for the studied age.

Normal reflection at superconductor - normal metal interfaces due to Fermi surface mismatch

Electrons can be reflected at an interface between two metals because of a dielectric barrier or different properties of the Fermi surface. Andreev reflection allows to directly measure normal reflection when one of the metals is a superconductor. We have investigated normal reflection at interfaces between non-magnetic normal metals and superconducting Nb (Tc = 9.2K) and Al (Tc = 1.2K). The distribution of the values of the relative strength of the interface barrier, Z, for a number of contacts of a specific metal combination shows a well-defined peak which can be attributed to Fermi surface mismatch. Our reflection coefficients are generally smaller than those predicted theoretically or those derived from proximity-effect studies of normal-superconductor bi-layers.

Spin polarization versus lifetime effects at point contacts between superconducting niobium and normal metals

Point-contact Andreev reflection spectroscopy is used to measure the spin polarization of metals but analysis of the spectra has encountered a number of serious challenges, one of which is the difficulty to distinguish the effects of spin polarization from those of the finite lifetime of Cooper pairs. We have recently confirmed the polarization-lifetime ambiguity for Nb-Co and Nb-Cu contacts and suggested to use Fermi surface mismatch, the normal reflection due to the difference of Fermi wave vectors of the two electrodes, to solve this dilemma. Here we present further experiments on contacts between superconducting Nb and the ferromagnets Fe and Ni as well as the noble metals Ag and Pt that support our previous results. Our data indicate that the Nb - normal metal interfaces have a transparency of up to about 80% and a small, if not negligible, spin polarization.

Use of an ultrasonic reflectance technique to examine bubble size changes in dough

Bread quality largely depends on the manner in which bubbles are created and manipulated in the dough during processing. We have developed an ultrasonic reflectance technique to monitor bubbles in dough, even at high volume fractions, where near the bubble resonances it is difficult to make measurements using transmission techniques. A broadband transducer centred at 3.5 MHz in a normal incidence wave reflection set-up is used to measure longitudinal velocity and attenuation from acoustic impedance measurements. The technique is illustrated by examining changes in bubbles in dough due to two very different physical effects. In dough made without yeast, a peak in attenuation due to bubble resonance is observed at approximately 2 MHz. This peak diminishes rapidly and shifts to lower frequencies, indicative of Ostwald ripening of bubbles within the dough. The second effect involves the growth of bubble sizes due to gas generated by yeast during fermentation. This process is experimentally challenging to investigate with ultrasound because of very high attenuation. The reflectance technique allows the changes of the velocity and attenuation during fermentation to be measured as a function of frequency and time, indicating bubble growth effects that can be monitored even at high volume fractions of bubbles.

Unified Formalism of Andreev Reflection at a Ferromagnet/Superconductor Interface

We present a unified formalism of Andreev reflection of a partial polarized current at a ferromagnet/superconductor interface instead of assuming a linear combination of unpolarized and polarized currents. The Andreev reflection is limited by the states of minority spins and the extra majority spins become evanescent wave. We further study the effects of spin polarization, inelastic scattering, and interfacial scattering on the Andreev reflection, normal reflection, and transmitted probabilities in equilibrium as well as under a bias. Our model, which reduces to those of Blonder, Tinkham, and Klapwijk, Mazin, and Dynes in three limiting cases, provides a significantly better description of the experimental results.

Substrate surface polariton splitting due to thin zinc oxide and aluminum nitride films presence

Surface polariton (SP) is the non-radiative interface electromagnetic mode, propagating along the interface between two media, if one of them is absorbing (metal, semiconductor or dielectric with the strong absorption bands) and exponentially decaying out of the interface. The introduction of a transition layer at this interface results in the shift and broadening of SP. This effect can be used to obtain film parameters (thickness and optical constants) in the region of SP existence. Zinc oxide (ZnO) films (100–300nm thick) have been prepared on the LiF and CaF2 substrates and aluminum nitride films (40 and 400nm thick) have been prepared on sapphire substrates. The SP spectra have been measured by attenuated total reflection (ATR) technique. IFS66v (BRUKER) infrared Fourier-transform spectrometer was used for ATR and near normal incidence external reflection spectral measurements. Angular dependencies of the absorption bands positions in the ATR spectra give the dispersion of SP. The measured SP dispersion is compared with one calculated using the film parameters obtained by dispersion analysis of the external reflectivity spectra. The splitting of the dispersion curves of substrate SP was found. It is due to the resonance interaction of substrate SP with the film optical phonons. This splitting is proportional to the square root of the film thickness. For ZnO films on CaF2 “long range” SPs were observed. These effects allow estimate film complex dielectric function in the region under consideration.

Tunneling conductance in gapped graphene-based f-wave superconductor N/S and N/I/S junctions

Proximity effect by a f-wave superconductor on top of the graphene grown on a substrate-induced bandgap leads to a two-dimensional relativistic Dirac–Bogoliubov–de Gennes Hamiltonian with f-type pair coupling, that electrons are treated as massive Dirac particles. In this work, we aim to study the influence of two types of f-wave superconductivity on normal tunneling conductance in gapped graphene-based superconductor junctions. Using the explicit obtained forms of DBdG spinors for normal, insulator and superconductor regions in the N/S and N/I/S structures, the normal and Andreev reflection coefficients are exactly found. Finally, the behavior of normal conductance spectra in terms of bias voltage, electrostatic potentials and energy gap is investigated via the BTK formalism. Because of energy gap of graphene we find our results for N/I/S structure in the thin barrier limit with large voltage V0 applied across the barrier region.