Interference Oscillations Research Articles

Application of a B-spline model dielectric function to infrared spectroscopic ellipsometry data analysis

The flexibility of the Kramers–Kronig-consistent basis spline (B-spline) function provides advantages over traditional physics-based oscillator functions for analyzing infrared spectroscopic ellipsometry data. Oscillator functions require that the user identify the spectral location of every absorption, choose the correct oscillator line shape, and choose the correct number of oscillators (which can be difficult for organic films that have many overlapping absorption peaks). The user must also choose starting parameters sufficiently close to the final values so that the regression fit converges. Weak absorptions can be difficult to identify when they overlap with strong absorption features, or they can be obscured when there are large interference oscillations in the spectroscopic ellipsometry data. The B-spline function’s flexibility allows the user to model the infrared, dielectric-function line shape for any material, and it requires far less user input and judgment to obtain the dielectric function of a material. The user must simply define the spacing of control nodes, which can usually be obtained from a reference dielectric function or by a preliminary fit to a spectral region where the material is transparent. A “wavelength-range-expansion” fit can then be used to obtain the dielectric function for the remaining spectral range.

Combined interpolation, scale change, and noise reduction in spectral analysis

The authors present a simple, convenient, and accurate noise-reduction approach for interpolating spectra, in particular, for converting those available as discrete points equally spaced in wavelength, acquired, for example, by a photodiode-array detector, to equivalent spectra equally spaced in energy, as needed for analysis. Based on continuum mathematics, the algorithm uses Gaussian kernels and capitalizes on the fact that trapezoidal-rule integration is accurate to fourth order in the ratio of point separation to width when applied to Gaussian functions. The approach can be expanded to perform differentiation and other operations. Examples include false-data verification, wavelength-to-energy conversion of near-bandgap interference oscillations of a GaN film, and calculation of the second energy derivative of monolayer MoS2 in the exciton region.

On the determination of alloy composition using optical spectroscopy in MOVPE grown InGaN layers on Si(111)

Abstract Alloy composition of InxGa1-xN layer is investigated using photoluminescence (PL), and photoluminescence excitation (PLE) techniques. InGaN layers were grown on Si(111) substrates using metal organic vapour phase epitaxy (MOVPE) technique under variable flow of either ammonia (NH3) or trimethyl gallium (TMGa) with corresponding V/III ratios in the range of 3.6 × 103 to 1.2 × 104. We observed that though room temperature PL measurements give an indication about the growth of InGaN, it doesn't help in the estimation of band gap or alloy composition of layer. It is restricted mainly by the presence of multiple peaks in the PL spectra which is rather severe at low temperature. A comparison of PL and reflectivity spectra confirmed that such peaks are attributed to interference oscillations and are not related with multiple compositions of InGaN. Further, in such a scenario PLE is found to be quite effective where the measured PLE spectrum is consisted of a clear onset related to InGaN layer in the sub band gap region of GaN. It is quite helpful in a precise determination of band gap and hence the alloy composition of InGaN layer. The indium composition is estimated to be in the range of 3.5% ≤ x ≤ 20.5%, however its trends with respect to V/III ratio as a function of NH3 or TMGa flow are found to be rather opposite.

Quantized Fermi arc mediated transport in Weyl semimetal nanowires

We study longitudinal transport in Weyl semimetal nanowires, both in the absence and in the presence of a magnetic flux threading the nanowires. We identify two qualitatively different regimes of transport with respect to the chemical potential in the nanowires. In the "surface regime", for low doping, most of the conductance occurs through the Fermi-arc surface states, and it rises in steps of one quantum of conductance as a function of the chemical potential; furthermore, with varying flux the conductance changes in steps of one quantum of conductance with characteristic Fabry-P\'erot interference oscillations. In the "bulk-surface regime", for highly-doped samples, the dominant contribution to the conductance is quadratic in the chemical potential, and mostly conditioned by the bulk states; the flux dependence shows clearly that both the surface and the bulk states contribute. The two aforementioned regimes prove that the contribution of the Fermi-arc surface states is salient and, therefore, crucial for understanding transport properties of finite-size Weyl semimetal systems. Last but not least, we demonstrate that both regimes are robust to disorder.

Experimental and theoretical study of valence electronic structure of tetrabromomethane by (e,2e) electron momentum spectroscopy

We report a measurement of the valence orbital momentum profiles of tetrabromomethane (${\mathrm{CBr}}_{4}$) using symmetric noncoplanar $(e,2e)$ experiments at the impact energies of about 600 and 1200 eV. The experimental momentum profiles for the individual orbitals $5{t}_{1}, 13{t}_{2}, 5e, 12{t}_{2}$, and $9{a}_{1}$ and the branching ratio of $5{t}_{1}$ to $13{t}_{2}$ and $5e$ to $13{t}_{2}$ are obtained and compared with two kinds of calculations under the plane-wave impulse approximation. One is theoretical momentum profiles that have been calculated at the equilibrium geometry, the other is those that involve vibrational effects using a thermal sampling molecular dynamics method. The calculations considering molecular vibrations are in better agreement with experiment than the equilibrium geometry calculations, indicating the important role of nuclear motions on the valence orbital electronic structures of ${\mathrm{CBr}}_{4}$. The distorted-wave effects are observed in the experimental momentum profiles of $5{t}_{1}, 5e$, and $12{t}_{2}$ which display dynamic dependencies on the impact energies. A multicenter interference or bond oscillation effect has been observed from the momentum profile ratios of $5{t}_{1}$ to $13{t}_{2}$ and $5e$ to $13{t}_{2}$ which has direct information about the bond length of a molecule.

Optical characterizations of nanoporous anodic alumina for thickness measurements using interference oscillations

Highly-ordered nanoporous anodic alumina (NAA) with varying thicknesses were prepared by changing the anodization time from 10 min to 10 h, using a two-step electrochemical oxidation in oxalic acid. Three optical characterizations, namely, Ultraviolet–Visible–Near Infrared (UV–VIS–NIR), Photoluminescence (PL), and Reflective interference spectra were used to measure NAA’s thicknesses. All three types of spectra displayed oscillations due to the optical interferences which were used for the extraction of NAA thickness using two approaches (i) Fabry–Perot fringe equation and (ii) Bragg’s interference condition equation. Both these approaches yielded the matching thicknesses. The results are coincident for all three measurement techniques. The obtained thicknesses are in good agreement with the cross-sectional SEM images showcasing the feasibility of such approaches. Measurements were carried out on NAA up to ∼10μm thickness beyond which no oscillations were observed. Out of the three, reflective interference spectroscopy turns out to be an inexpensive alternative for the NAA thickness measurement. Such nondestructive optical characterization methods are important from the device development point of view for chemical, optical and biosensing applications.

Tailoring the synthesis of tantalum-based thin films for biomedical application: Characterization and biological response

The aim of this study was to tailor the deposition parameters of magnetron sputtering to synthetize tantalum oxide (TaxOy) films onto commercially pure titanium (cpTi) surface. The structural and optical properties, morphology, roughness, elemental chemical composition and surface energy were assessed. The impact of TaxOy films on initial Streptococcus sanguinis adhesion was investigated. The morphology and spreading of pre-osteoblastic (MC3T3-E1) cells on a crystalline tantalum oxide film were evaluated. TaxOy films with estimated thickness of 600 nm and different structures (amorphous or crystalline) were produced depending on the various oxygen flow rates and parameters used. X-ray diffraction analysis revealed that the 8 O2 sccm (600 °C/400 W) group showed crystallization corresponding to the β-Ta2O5 phase. Optical analysis showed that the 4 O2 sccm (200 °C 300 W) to 8 O2 sccm (600 °C 300 W) groups and 10 O2 sccm (200 °C 300 W) group presented regular and large-amplitude interference oscillations, suggesting high optical homogeneity of the films. The crystalline β-Ta2O5 coating showed higher roughness and surface energy values than the other groups (P < .05) and was biocompatible. Compared with cpTi, the amorphous and crystalline tantalum oxide films did not increase bacterial adhesion (P > .05). By tailoring the deposition parameters, we synthetized a crystalline β-Ta2O5 coating that improved titanium surface properties and positively affected cell spreading and morphology, making it a promising surface treatment for titanium-based implants.

Strain Analysis of CdTe on InSb Epitaxial Structures Using X-ray-Based Reciprocal Space Measurements and Dynamical Diffraction Simulations

The structural properties of CdTe layers grown by molecular beam epitaxy on (001) InSb substrates were assessed using high-resolution x-ray scattering methods. Triple-axis diffraction measurements and dynamical diffraction theory were employed to determine the strain in the epitaxial layer and at the interface for CdTe layer thicknesses of 1.0 μm and 1.8 μm. Reciprocal space maps were generated around the 004, 115, 224 and 335 reciprocal lattice points (RLP). An alignment procedure is described which ensured all reflections were aligned to the same zone axes. (335) reflections exhibited greater sensitivity to strain relaxation than (115) reflections due to a larger in-plane component; so these were used to better determine the strain state of the layers. Both of our 1.0-μm and 1.8-μm thick samples exhibited no relaxation, showing that pseudomorphic layers formed. Additionally, interference oscillations in the (004) ω:2θ scan of the 1.0-μm CdTe sample were observed. These oscillations were replicated through dynamical diffraction simulations by including a ∼1-nm highly strained interfacial layer. These results are consistent with an earlier electron microscopy study that detected a similar strained layer at a defect-free CdTe/InSb interface. The CdTe lattice parameter and Poisson ratio are determined to be 0.648201 ± 0.000002 nm and 0.408, respectively.

Graded pitch profile for the helicoidal broadband reflector and left-handed circularly polarizing cuticle of the scarab beetle Chrysina chrysargyrea

The cuticle of the beetle Chrysina chrysargyrea reflects left-handed polarized light in the broad spectral range from 340 to 1000 nm. Interference oscillations in the experimental Mueller-matrix spectroscopic ellipsometry data reveal that transparent materials comprise the cuticle. A spectral analysis of the interference oscillations makes evident that the pitch profile across the cuticle is graded. The graded pitch and effective refractive indices are determined through non-linear regression analysis of the experimental Mueller matrix by using a cuticle model based on twisted biaxial dielectric slices. Non-uniformity in cuticle thickness as well as in pitch profile near the cuticle surface account for depolarizance of the Mueller matrix. Transmission electron microscopy supports the reliability of the results.

Detecting the Oxidation of Zircaloy Claddings by Infrared Interference

As the expected life of dry cask storage installations increases, it becomes increasingly desirable to monitor the state and performance of the cask internals to ensure that they continue to safely contain the radioactive materials in the fuel. One aspect of this task is the monitoring of oxidation of the cladding. With this consideration in mind, Zircaloy-4 (Zr-4) cladding samples were exposed to air at 500[Formula: see text]C for various duration times to create thin corrosion oxide layers on the surface. The surfaces of the oxidized samples were then systematically scanned by Fourier Transform Infrared (FT-IR) spectroscopy to achieve the infrared (IR) interference spectra and study the relationship between the optical interference and the various thicknesses of the oxide layers. The profiles of the oxide layers were verified througth cross-sectional examination by Scanning Electron Microscopy. The IR interference patterns varied with oxide layer thickness, enabling the determination of oxide layer thickness of values, including half micron thick. Further analysis demonstrated that the interference oscillation period and the oscillation amplitude decreased with increasing oxide layer thickness. Combined with a physical model that describes the optical interference, the interference spectra were directly correlated to the oxide layer thickness quantitatively. The study provides the basis for an accurate, nondestructive and sensitive method to monitor the degree of zirconium-based cladding corrosion due to oxidation.

Coherent electron emission from O2 in collisions with fast electrons

Absolute double differential cross sections (DDCS) of secondary electrons emitted in ionization of O2 by fast electrons have been measured for different emission angles. Theoretical calculations of atomic DDCS were obtained using the first Born approximation with an asymptotic charge of Z T = 1. The measured molecular DDCS were divided by twice the theoretical atomic DDCS to detect the presence of interference effects which was the aim of the experiment. The experimental to theoretical DDCS ratios showed clear signature of first order interference oscillation for all emission angles. The ratios were fitted by a first order Cohen-Fano type model. The variation of the oscillation amplitudes as a function of the electron emission angle showed a parabolic behaviour which goes through a minimum at 90°. The single differential and total ionization cross sections have also been deduced, besides the KLL Auger cross sections. In order to make a comparative study, we have discussed these results along with our recent experimental data obtained for N2 molecule.

Electron impact ionization of O2 and the interference effect from forward–backward asymmetry

Absolute double differential cross sections (DDCSs) of secondary electrons emitted from O2 under the impact of 7 keV electrons were measured for different emission angles between 30° and 145° having energies from 1–600 eV. The forward–backward angular asymmetry was observed from angular distribution of the DDCS of secondary electrons. The asymmetry parameter, thus obtained from the DDCS of two complementary angles, showed a clear signature of interference oscillation. The Cohen–Fano model of Young type electron interference at a molecular double slit is found to provide a good fit to the observed oscillatory structures. The present observation is in qualitative agreement with the recent results obtained from photoionization.

Interference effect in e-emission spectrum from a molecular (N2) double slit in collisions with fast electrons

SynopsisDouble differential cross sections (DDCS) of electrons ejected in ionization of N2 by fast electrons have been measured. Signature of interference effect was revealed in the DDCS ratio (N2/2N). Interference oscillation was also confirmed from the asymmetry parameter, determined from the DDCS of two complementary angles. Traces of second order effect was also observed from the asymmetry parameter.

Improvement of the measurement accuracy of the spectral method for evaluation parameters of the optically transparent thin films

In this paper the ways of increasing the measurement accuracy of the parameters of the optically transparent thin films using the spectral method are described. The proposed plotting of the envelope curves for the minima and maxima of the interference oscillations in a film and additional filtering of the spectral data, allows improving the accuracy of determining the extrema position and, consequently, increasing the accuracy of the thickness measurements of the deposited coatings.

High-order-harmonic generation in an elliptically polarized laser field: analytic form of the electron wave packet

An analytic expression for the electron wave packet (EWP) describing high-order harmonic generation (HHG) by atoms in an intense laser field with small ellipticity is derived quantum mechanically in the tunneling limit within the time-dependent effective range theory. This result is valid over a wide interval of returned electron energies in the HHG plateau region and generalizes the previous result for HHG rates obtained by Frolov M V et al (2012 Phys. Rev. A 86 063406) only for the HHG plateau cutoff region. It is shown that the most important difference from the case of a linearly polarized field originates from a nonzero electron energy at the moment of ionization in an elliptically polarized field that, in turn, results in the dependence of ionization factor in the EWP on the returning electron energy. Our analytic result for the EWP averaged over interference oscillations in the HHG spectrum is applied for analysis of the laser wavelength scaling of the HHG yield induced by an elliptically polarized midinfrared laser field as well as for the improvement of the recently suggested method of elliptic HHG spectroscopy for retrieving both the energy and angular dependence of the photorecombination cross section of the target atom (see Frolov M V et al 2016 Phys. Rev. A 93 031403).

Spin blockade and coherent dynamics of high-spin states in a three-electron double quantum dot

Asymmetry in a three-electron double quantum dot (DQD) allows spin blockade, when spin-3/2 (quadruplet) states and spin-1/2 (doublet) states have different charge configurations. We have observed this DQD spin blockade near the (1,2)-(2,1) charge transition using a pulsed-gate technique and a charge sensor. We then use this spin blockade to detect Landau-Zener-St\"uckelberg (LZS) interference and coherent oscillations between the spin quadruplet and doublet states. Such studies add to our understandings of coherence and control properties of three-spin states in a double dot, which in turn would benefit the explorations into various qubit encoding schemes in semiconductor nanostructures.

Resonance states and beating pattern induced by quantum impurity scattering in Weyl/Dirac semimetals.

Currently, Weyl semimetals (WSMs) are drawing great interest as a new topological nontrivial phase. When most of the studies concentrated on the clean host WSMs, it is expected that the dirty WSM system would present rich physics due to the interplay between the WSM states and the impurities embedded inside these materials. We investigate theoretically the change of local density of states in three-dimensional Dirac and Weyl bulk states scattered off a quantum impurity. It is found that the quantum impurity scattering can create nodal resonance and Kondo peak/dip in the host bulk states, remarkably modifying the pristine spectrum structure. Moreover, the joint effect of the separation of Weyl nodes and the Friedel interference oscillation causes the unique battering feature. We in detail an- alyze the different contribution from the intra- and inter-node scattering processes and present various scenarios as a consequence of competition between them. Importantly, these behaviors are sensitive significantly to the displacement of Weyl nodes in energy or momentum, from which the distinctive fingerprints can be extracted to identify various semimetal materials experimentally by employing the scanning tunneling microscope.

Light transmission of liquid crystal domains formed by polycarbonate surface

We investigate the optical transmission of a monochromatic laser beam passing through an individual domain of a nematic liquid crystal formed by the polycarbonate surface. The domain has the radial orientation structure with a disclination line on the polymer surface, which gradually transforms to “pseudoplanar” alignment at the coherence distance ξ from the surface. The dependence of the optical transmittance on an electric field applied to the domain are different at various light polarization directions relative to the disclination lines and, in the absence of an analyzer, are accompanied by interference oscillations. To explain the results obtained, a domain is considered to be a gradient lens with the refractive index variable in the plane perpendicular to the laser beam and the resulting deflecting effect that was collected at the beam path through the liquid crystal layer.

Repelling, binding, and oscillating of two-particle discrete-time quantum walks

In this paper, we investigate the effects of particle–particle interaction and static force on the propagation of probability distribution in two-particle discrete-time quantum walk, where the interaction and static force are expressed as a collision phase and a linear position-dependent phase, respectively. It is found that the interaction can lead to boson repelling and fermion binding. The static force also induces Bloch oscillation and results in a continuous transition from boson bunching to fermion anti-bunching. The interplays of particle–particle interaction, quantum interference, and Bloch oscillation provide a versatile framework to study and simulate many-particle physics via quantum walks.

An unambiguous identification of 2D electron gas features in the photoluminescence spectrum of AlGaN/GaN heterostructures

A fast and non-destructive method for probing the true signatures of 2D electron gas (2DEG) states in AlGaN/GaN heterostructures is presented. Two broad features superimposed with interference oscillations are observed in the low temperature photoluminescence (PL) spectrum. The two features are identified as the ground and excited 2DEG states which are confirmed by comparing the PL spectra of as-grown and top barrier layer etched samples. Broad PL features disappear at a certain temperature along with the associated interference oscillations. Furthermore, the two broad PL features depicts specific temperature and excitation intensity dependencies which make them easily distinguishable from the bandedge excitonic or defect related PL features. The presence of strong interference oscillations associated with the 2DEG PL features is explained by considering the localized generation of PL signal at the AlGaN/GaN heterointerface. Finally, a large value of the polarization induced electric field of ~1.01 MV cm−1 is reported from PL measurements for AlGaN/GaN HEMT structures. It became possible only when the true identification of 2DEG features was made possible by the proposed method.