We consider the model of two coupled oscillators with Kerr nonlinearities in the rotating-wave approximation. We demonstrate that for a certain set of parameters of the model, the multi-photon ...transitions occur between many pairs of the oscillator states simultaneously. Also, the position of the multi-photon resonances does not depend on the coupling strength between two oscillators. We prove rigorously that this is a consequence of a certain symmetry of the perturbation theory series for the model. In addition, we analyse the model in the quasi-classical limit by considering the dynamics of the pseudo-angular momentum. We identify the multi-photon transitions with the tunnelling transitions between the degenerate classical trajectories on the Bloch sphere.
Colloidal semiconductor nanoplatelets exhibit strong quantum confinement for electrons and holes as well as excitons in one dimension, while their in-plane motion is free. Because of the large ...dielectric contrast between the semiconductor and its ligand environment, the Coulomb interaction between electrons and holes is strongly enhanced. By means of one- and two-photon photoluminescence excitation spectroscopy, we measure the energies of the 1S and 1P exciton states in CdSe nanoplatelets with thicknesses varied from 3 up to 7 monolayers. By comparison with calculations, performed in the effective mass approximation with account of the dielectric enhancement, we evaluate exciton binding energies of 195–315 meV, which is about 20 times greater than that in bulk CdSe. Our calculations of the effective Coulomb potential for very thin nanoplatelets are close to the Rytova-Keldysh model, and the exciton binding energies are comparable with the values reported for monolayer-thick transition metal dichalcogenides.
This study is dedicated to the wind waves parameters distribution on the Black Sea with a focus on coastal areas. Wind waves were simulated using the SWAN model forced by the NCEP CFSR and NCEP CFSv2 ...reanalyses for the period 1979–2016. An unstructured computational grid with resolution varying between 15 km and 200 m was applied. Modelling results were validated against in situ and remote sensing data. For the entire sea, distributions of total and seasonal maxima of wave parameters are analyzed. Maximal significant wave heights exceed 8.5 m and are found in the southwestern areas of the sea. Maximal wave lengths (over 160 m) and periods (over 10.5 s) are located in the eastern part of the sea. Total and seasonal time series extracted for 12 locations along the coasts of the Black Sea were studied. Maximal values of significant wave heights (over 6 m) correspond to areas along the northeastern and southwestern coasts of the Black Sea and the southern coast of the Crimean Peninsula. Maximal values of wave lengths and wave periods (over 140 m and 10 s) are located along the northeastern and southern coasts of the Black sea.
•SWAN model is applied for the Black Sea using CFSR winds and an unstructured grid.•Validation against laser altimeter data, buoy measurements and satellite altimetry data showed adequate performance.•Spatial distribution of seasonal and total wave parameters maxima is assessed.•Detailed wave statistics for 12 points located along the coasts is demonstrated.
We numerically study the effect of structural asymmetry in a plasmonic metamaterial made from gold nanowires. It is reported that optically inactive (i.e., optically dark) particle plasmon modes of ...the symmetric wire lattice are immediately coupled to the radiation field, when a broken structural symmetry is introduced. Such higher order plasmon resonances are characterized by their subradiant nature. They generally reveal long lifetimes and distinct absorption losses. It is shown that the near-field interaction strongly determines these modes.
Thin layers of silicon nanocrystals (SiNC) in oxide matrix with optimized parameters are fabricated by the plasma-enhanced chemical vapor deposition. These materials with SiNC sizes of about 4.5 nm ...and the SiO
barrier thickness of 3 nm reveal external quantum yield (QY) close to 50% which is near to the best chemically synthetized colloidal SiNC. Internal QY is determined using the Purcell effect, i.e. modifying radiative decay rate by the proximity of a high index medium in a special wedge-shape sample. For the first time we performed these experiments at variable temperatures. The complete optical characterization and knowledge of both internal and external QY allow to estimate the spectral distribution of the dark and bright NC populations within the SiNC ensemble. We show that SiNCs emitting at around 1.2-1.3 eV are mostly bright with internal QY reaching 80% at room temperature and being reduced by thermally activated non-radiative processes (below 100 K internal QY approaches 100%). The mechanisms of non-radiative decay are discussed based on their temperature dependence.
A detailed study of novel intermetallic superconductor Mo4Ga20Sb (Tc = 6.6 K) by means of 69,71Ga nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spectroscopy was performed to ...gain insight into the superconducting pairing mechanism and gap characteristics, as well as into the normal-state electronic properties on a microscopic scale. An unexpected step-like increase of the 69Ga1 NQR linewidth was observed at Tc with decreasing temperature, which may indicate coexistence of several superconducting phases on a microscopic scale with slightly different Tc values. Above 140 K, the NQR linewidth decreases rapidly with increasing temperature due to possible oscillatory and/or rotational modes of the gallium cluster network at the elevated temperatures. Nuclear spin lattice relaxation rate 1/T1 of 69Ga nuclei in the Ga1 position follows Korringa law characteristic for the compounds with good metallic conductivity. The calculated Korringa ratio S< 1 indicates the presence of antiferromagnetic correlations in Mo4Ga20Sb, in agreement with the magnetic susceptibility data. Below Tc, a pronounced intensive Hebel-Slichter peak was observed indicating s-wave superconductivity without point or line nodes in the k-space (full gap s-wave superconductivity). We have demonstrated that the best fit of the experimental data is achieved using two s-wave superconducting gaps of 13 K and 6 K with relative weights of 0.8 and 0.2, respectively. The obtained weighted average value of 11.6 K is consistent with the specific heat value of Δ = 12.05 K.
•NQR linewidth grows with decreasing temperature, demonstrating a pronounced step at TC.•Hebel-Slichter peak below TC clearly evidences for the s-wave superconductivity.•Antiferromagnetic correlations were established both by the value of Korringa ratio and by the narrowing of Hebel-Slichter peak.•Two s-wave gaps model fits best to the experimental data with gaps of 13 K and 6 K.•Long period oscillations of the spin echo intensity with the frequency of ∼ 2E4 rad/s were observed on spin-spin relaxation curves.
We are reporting on laser microstructuring of thin nanocrystalline diamond membranes, for the first time. To demonstrate the possibility of microstructuring, we fabricated a diamond membrane, of 9 μm ...thickness, with a two-dimensional periodic array of closely located chiral elements. We describe the fabrication technique and present the results of the measurements of the infrared transmission spectra of the fabricated membrane. We theoretically studied the reflection, transmission, and absorption spectra of a model structure that approximates the fabricated chiral metamembrane. We show that the metamembrane supports quasiguided modes, which appear in the optical spectra due to grating-assisted diffraction of the guided modes to the far field. Due to the C4 symmetry, the structure demonstrates circular dichroism in transmission. The developed technique can find applications in infrared photonics since diamond is transparent at wavelengths >6 μm and has record values of hardness. It paves the way for creation of new-generation infrared filters for circular polarization.
Several improvements have been introduced for the Fourier modal method in the last fifteen years. Among those, the formulation of the correct factorization rules and adaptive spatial resolution have ...been crucial steps towards a fast converging scheme, but an application to arbitrary two-dimensional shapes is quite complicated.We present a generalization of the scheme for non-trivial planar geometries using a covariant formulation of Maxwell's equations and a matched coordinate system aligned along the interfaces of the structure that can be easily combined with adaptive spatial resolution. In addition, a symmetric application of Fourier factorization is discussed.
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