Redshift Of Frequency Research Articles

Terahertz multichannel notch filter consisting of four T-shaped cavities based on parallel-plate waveguide

We propose herein a terahertz (THz) multichannel notch filter consisting of four T-shaped cavities based on a parallel-plate waveguide (PPWG). For such devices, the resonance frequency depends on the separation b of the PPWG and the width w of the grooves. The full width at half maximum (FWHM) of the resonance peak in the power-transmission spectrum varies with the ratio w/b and the resonance frequency f0. When the ratio increases, the resonance frequency redshifts and the FWHM widens. The successive passage of THz waves through the four PPWG-based T-shaped cavities reveals four resonance frequencies. In other words, the device serves as a tunable monochromatic THz multichannel notch filter. Optimizing the THz multichannel notch filter diminishes the effect of the undesired weak resonant modes and gives a smooth power-transmission spectrum. To better display the multichannel notch filter, the electrical-field distribution at four resonance frequencies are shown. Finally, the optimized THz multichannel notch filter operates not only in the lower THz range but also in the higher THz range. This filter may have potential applications in optical communications and wavelength division multiplexing.

Finite-temperature correlations in a two-dimensional electron gas within a dynamical self-consistent mean-field approximation

We have studied the effect of temperature on density-density correlations in a two-dimensional quantum electron gas by using the dynamical Singwi-Tosi-Land-Sjölander (STLS) theory. Static correlation functions viz. structure factor S(q; T), pair-correlation function g(r; T), and static density susceptibility χ(q, 0; T) are calculated over a wide range of temperature and electron density. The inclusion of the dynamics of correlations is found to cause a strong peak in S(q; T) at q ∼ 2.5kF for low densities. Correspondingly, there develops a sharp peak in χ(q, 0; T) that diverges below a critical density. This divergence is interpreted as a precursor of transition to finite-temperature Wigner crystal state. Rise in temperature tends to oppose the crystallization, with critical transition density decreasing with temperature. It is rather found that the dynamical correction to the conventional (static) STLS theory becomes small at a sufficiently high temperature. We have also calculated the plasmon dispersion for the GaAs quantum well based electron system and found that, except at ultra-low densities, agreement with the experimental data of Hirjibehedin et al. is reasonably good. Our results show a red shift in plasmon frequency over the predictions of the STLS and dynamical Hubbard approaches.

Temperature-Dependent Raman Scattering of Large Size Hexagonal Bi2Se3 Single-Crystal Nanoplates

Bi 2 Se 3 has extensive application as thermoelectric materials. Here, large-scale Bi 2 Se 3 single-crystal hexagonal nanoplates with size 7.50–10.0 μ m were synthesized successfully by hydrothermal method. X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM) were used to characterize the Bi 2 Se 3 nanoplates, which confirm the single-crystal quality and smooth surface morphology with large size. Micro-Raman spectra over a temperature range of 83–603 K were furthermore used to investigate the lattice dynamics of Bi 2 Se 3 nanoplates. Both 2A g 1 and 1E g 2 modes shift evidently with reduced temperature. The line shape demonstrates a significant broadening of full width at half maximum (FWHM) and red-shift of frequency with increased temperature. The temperature coefficient of A 1 g 1 , E g 2 , A 1 g 2 modes were determined to be −1.258 × 10 − 2 cm − 1 /K, −1.385 × 10 − 2 cm − 1 /K, −2.363 × 10 − 2 cm − 1 /K, respectively. Such low temperature coefficient may favor the obtaining of a high figure of merit (ZT) and indicate that Bi 2 Se 3 nanoplates were used as excellent candidates of thermoelectric materials.

Thermal conductivity of normal and deuterated water, crystalline ice, and amorphous ices.

The effect of deuteration on the thermal conductivity κ of water, crystalline ice, and amorphous ices was studied using the pressure induced amorphization of hexagonal ice, ice Ih, to obtain the deuterated, D2O, forms of low-density amorphous (LDA), high-density amorphous (HDA), and very-high density amorphous (VHDA) ices. Upon deuteration, κ of ice Ih decreases between 3% and 4% in the 100-270 K range at ambient pressure, but the effect diminishes on densification at 130 K and vanishes just prior to amorphization near 0.8 GPa. The unusual negative value of the isothermal density ρ dependence of κ for ice Ih, g = (d ln κ/d ln ρ) T = -4.4, is less so for deuterated ice: g = -3.8. In the case of the amorphous ices and liquid water, κ of water decreases by 3.5% upon deuteration at ambient conditions, whereas κ of HDA and VHDA ices instead increases by up to 5% for pressures up to 1.2 GPa at 130 K, despite HDA's and VHDA's structural similarities with water. The results are consistent with significant heat transport by librational modes in amorphous ices as well as water, and that deuteration increases phonon-phonon scattering in crystalline ice. Heat transport by librational modes is more pronounced in D2O than in H2O at low temperatures due to a deuteration-induced redshift of librational mode frequencies. Moreover, the results show that κ of deuterated LDA ice is 4% larger than that of normal LDA at 130 K, and both forms display an unusual temperature dependence of κ, which is reminiscent of that for crystals (κ ∼ T -1), and a unique negative pressure dependence of κ, which likely is linked to local-order structural similarities to ice Ih.

Structural and dielectric properties of ion beam deposited titanium oxynitride thin films

Titanium oxynitride ( $${\hbox {TiO}}_{x}{\hbox {N}}_{y}$$ ) thin films were fabricated by ion beam-assisted sputtering deposition. Effects of oxygen contribution, assisting ion energy ( $$E_{\mathrm{a}}$$ ), assisting ion beam current ( $$I_{\mathrm{a}}$$ ) on the microstructure and dielectric behavior of the films were analyzed. The results show that increasing O content made the films to turn from fcc-TiN (111)-oriented to fcc $${\hbox {TiO}}_{x}{{\hbox {N}}}_{y}$$ (220)-oriented. Proper $$E_{\mathrm{a}}$$ and low $$I_{\mathrm{a}}$$ can enhance the (220) orientation in $${\hbox {TiO}}_{x}{{\hbox {N}}}_{y}$$ thin films. The increase in oxygen content leads to the red-shift of plasmonic resonant frequency and makes the films more dielectric. Higher $$E_{\mathrm{a}}$$ and $$I_{\mathrm{a}}$$ make the $${\hbox {TiO}}_{x}{{\hbox {N}}}_{y}$$ films more metallic. Atomic composition is an important factor underlying the results. The study provides a method to control the plasmonic properties of oxynitride films in a wide range by atomic composition and assisting ions.

Labyrinth Metasurface Absorber for Ultra‐High‐Sensitivity Terahertz Thin Film Sensing

In this work, a labyrinth metasurface sensor operating at the low‐frequency edge of the THz band is presented. Its intricate shape leads to a high electric field confinement on the surface of the structure, resulting in ultrasensitive performance, able to detect samples of the order of tens of nanometers at a wavelength of the order of millimeters (i.e., five orders of magnitude larger). The sensing capabilities of the labyrinth metasurface are evaluated numerically and experimentally by covering the metallic face with tin dioxide (SnO2) thin films with thicknesses ranging from 24 to 345 nm. A redshift of the resonant frequency is observed as the analyte thickness increases, until reaching a thickness of 20 μm, where the response saturates. A maximum sensitivity of more than 800 and a figure of merit near 4500 nm−1 are achieved, allowing discriminating differences in the SnO2 thickness of less than 25 nm, and improving previous works by a factor of 35. This result can open a new paradigm of ultrasensitive devices based on intricate metageometries overcoming the limitations of classical metasurface sensor designs based on periodic metaatoms.

A density functional study on synthetic polymer–amino acid interaction

Interaction of four synthetic polymers viz., poly- $$\upvarepsilon $$ -caprolactone (PCL), polyglycolide (PGA), polylactic acid (PLA) and Poly(lactic-co-glycolic) acid (PLGA) used as protein delivery vectors with a few amino acids have been studied by using density functional theory. Association geometries of polymer–amino acid adduct are modelled in a vacuum and in four solvents. Nature and strength of interaction have been analyzed in terms of interaction energy and thermochemical parameters of adducts as well as vibrational frequency shifts upon adduct formation. Results suggest comprehensive stability of adducts in the gas phase. Progressive destabilization of adducts with increasing polarity of solvent is observed. Redshifts in vibrational frequencies of X-H bonds ( $$\hbox {X}= \hbox {H}$$ donor in hydrogen bonding) upon adduct formation are noticed. The study asserts the potentiality of the considered synthetic polymers as an amino acid carrier. Synopsis Synthetic polymers can form stable complexes with amino acids and are potential vectors for protein delivery.

Electromagnetically induced transparency in a spin-orbit coupled Bose-Einstein condensate.

The artificial field can be generated by properly arranging pulsed magnetic fields interacting with a Bose-Einstein condensate (BEC), which can be widely used to simulate the phenomena of traditional condensed matter physics, such as spin-orbit (SO) coupling and the neutral atom spin Hall effect. The introduction of SO coupling in a BEC will alter its optical properties. Eletromagnetically induced transparency (EIT) is a powerful tool that can change and detect the properties of an atomic medium in a nondestructive way. It is important and interesting to study EIT properties and to investigate the effects of SO coupling on EIT. In this paper, we investigate EIT in a SO-coupled BEC. Not only is the transparency existing, but the real and imaginary parts of the susceptibility have an additional red frequency shift, which is linearly proportional to the strength of the SO coupling. By using this unconventional, sensitive EIT spectrum, SO coupling can be detected and its strength can be accurately measured according to the frequency shift.

Unconventional lattice dynamics in few-layer h-BN and indium iodide crystals* *Project supported by the Major Program of Aerospace Advanced Manufacturing Technology Research Foundation from NSFC and CASC, China (Grant No. U1537204), the National Key Research and Development Program of China (Grant No. 2017YFA0206301), and the National Natural Science Foundation of China (Grant No. 51702146).

Unusual quadratic dispersion of flexural vibrational mode and red-shift of Raman shift of in-plane mode with increasing layer-number are quite common and interesting in low-dimensional materials, but their physical origins still remain open questions. Combining ab initio density functional theory calculations with the empirical force-constant model, we study the lattice dynamics of two typical two-dimensional (2D) systems, few-layer h-BN and indium iodide (InI). We found that the unusual quadratic dispersion of flexural mode frequency on wave vector may be comprehended based on the competition between atomic interactions of different neighbors. Long-range interaction plays an essential role in determining the dynamic stability of the 2D systems. The frequency red-shift of in-plane Raman-active mode from monolayer to bulk arises mainly from the reduced long-range interaction due to the increasing screening effect.

Unusual spin-wave dynamics in core-shell magnetic nanodisks

We investigated the spin dynamics of a vortex state in a core-shell magnetic nanodisk driven by an oscillating field applied perpendicular to the disk plane by means of micromagnetic simulations. The nanodisk comprises a Py (Fe0.2Ni0.8) core of 100 nm in radius, surrounded by a 50 nm thick Fe shell. Fourier transform analyses show that the Py core and the Fe shell dominate spin-wave oscillation at the fundamental and higher order radial modes, respectively. For oscillating driving field tuned to the fundamental eigenfrequency, the Py/Fe interface effectively confines spin-wave excitation in the Py core region. This effect leads to significantly more rapid vortex core (VC) reversal in comparison to homogeneous disks. Our work demonstrates that the higher order modes can drive much faster VC reversal than the fundamental mode, in sharp contrast to the results obtained in homogeneous disks. With excitation levels up to 30 mT, we find strong nonlinear spin-wave dynamics in the system, which results in mode frequency redshifting, therefore the observation of the most rapid VC reversals below eigenfrequencies and VC switching in wide ranges of frequencies.

Ferromagnetic resonance of a two-dimensional array of nanomagnets: Effects of surface anisotropy and dipolar interactions

We develop an analytical approach for studying the FMR frequency shift due to dipolar interactions and surface effects in two-dimensional arrays of nanomagnets with (effective) uniaxial anisotropy along the magnetic field. For this we build a general formalism on the basis of perturbation theory that applies to dilute assemblies but which goes beyond the point-dipole approximation as it takes account of the size and shape of the nano-elements, in addition to their separation and spatial arrangement. The contribution to the frequency shift due to the shape and size of the nano-elements has been obtained in terms of their aspect ratio, their separation and the lattice geometry. We have also varied the size of the array itself and compared the results with a semi-analytical model and reached an agreement that improves as the size of the array increases. We find that the red-shift of the ferromagnetic resonance due to dipolar interactions decreases for smaller arrays. Surface effects may induce either a blue-shift or a red-shift of the FMR frequency, depending on the crystal and magnetic properties of the nano-elements themselves. In particular, some configurations of the nano-elements assemblies may lead to a full compensation between surface effects and dipole interactions.

Temperature-dependent layer breathing modes in two-dimensional materials

Relative out-of-plane displacements of the constituent layers of two-dimensional materials give rise to unique low-frequency breathing modes. By computing the height-height correlation functions from molecular dynamics simulations, we show that the layer breathing modes (LBMs) can be mapped consistently to vibrations of a simple linear chain model. Our calculated thickness dependence of LBM frequencies for few-layer (FL) graphene and molybdenum disulfide $({\mathrm{MoS}}_{2})$ are in excellent agreement with available experiments. Our results show a redshift of LBM frequency with an increase in temperature, which is a direct consequence of anharmonicities present in the interlayer interaction. We also predict the thickness and temperature dependence of LBM frequencies for FL hexagonal boron nitride. Our Rapid Communication provides a simple and efficient way to probe the interlayer interaction for layered materials and their heterostructures with the inclusion of anharmonic effects.

Nonadiabatic effects in the molecular high-order harmonic generation from two-center molecular ions

The nonadiabatic effects in the molecular high-order harmonic generation (MHHG) from H2+ and its isotope have been theoretically investigated beyond the Born-Oppenheimer approximations. It is found that (i) due to the asymmetric contributions of the harmonic emission on the rising and the falling parts of the laser field, the frequency red-shifts in the MHHG can be found. As the initial vibrational state or the pulse intensity increases, the red-shifts of the harmonics are decreased and even the blue-shifts of the harmonics can be found in the higher pulse intensity with the higher vibrational state. With the increase of the pulse duration, the frequency red-shifts and the blue-shifts in the MHHG are decreased and enhanced, respectively. The shifts of the harmonics from the heavy nuclei are decreased compared with those from the light nuclei. (ii) Due to the coupling of the electron-nuclear dynamics, the non-odd harmonics can be produced at the larger internuclear distance. With the increase of the initial vibrational state or the pulse duration, the generations of the non-odd harmonics can be enhanced. As the pulse intensity increases, the intensities of the non-odd harmonics are first increased and then decreased at the much larger pulse intensity. Due to the slower nuclear motion, the generations of the non-odd harmonics are decreased as the nuclear mass increases. (iii) With the increase of the alignment angle of the molecule ions, the nonadiabatic effects in MHHG (including the shifts of the harmonics and the generations of the non-odd harmonics) are decreased.

Deciphering Anomalous Raman Features of Regioregular Poly(3-hexylthiophene) in Ordered Aggregation Form

Poly(3-hexylthiophene) (P3HT), being a prototypic conjugated polymer, bears a high charge mobility that is sensitive to its packing configuration in the condensed phase. Despite its extensive experimental study with X-ray diffraction, its specified packing structure still remains stymied. This study searched for possible structures of crystalline P3HT and identified the one that holds a simulated Raman spectrum most approximate to the experimental one of ordered P3HT aggregates in the frozen solvent. The spectral correspondence shows that the Raman-active C–C stretch peak exhibits a red shift in frequency, while the C═C stretch peak displays a blue shift as the layer planarity of P3HT is relaxed. Moreover, the C═C peak splits into two when adjacent thiophene rings in the P3HT chain hold a dihedral angle of 22° with respect to each other. This study demonstrates that Raman spectroscopy plus first-principles simulations can serve as a powerful tool to resolve fine structures of molecular crystals.

A Theory of Gravity and General Relativity based on Quantum Electromagnetism

Based on first principles solutions in a unified framework of quantum mechanics and electromagnetism we predict the presence of a universal attractive depolarisation radiation (DR) Lorentz force (F) between quantum entities, each being either an IED matter particle or light quantum, in a polarisable dielectric vacuum. Given two quantum entities i = 1, 2 of either kind, of characteristic frequencies , masses and separated at a distance r0, the solution for F is , where is the susceptibility and πλ is the reduced linear mass density of the vacuum. This force F resembles in all respects Newton’s gravity and is accurate at the weak F limit; hence ℊ equals the gravitational constant G. The DR wave fields and hence the gravity are each propagated in the dielectric vacuum at the speed of light c; these can not be shielded by matter. A test particle µ of mass m0 therefore interacts gravitationally with all of the building particles of a given large mass M at r0 apart, by a total gravitational force F = −GMm0/(r0)2 and potential V = −∂F/∂r0. For a finite V and hence a total Hamiltonian H = m0c2 + V, solution for the eigenvalue equation of µ presents a red-shift in the eigen frequency ν = ν0(1 − GM/r0c2) and hence in other wave variables. The quantum solutions combined with the wave nature of the gravity further lead to dilated gravito optical distance r = r0/(1 − GM/r0c2) and time t = t0/(1 − GM/r0c2), and modified Newton’s gravity and Einstein’s mass energy relation. Applications of these give predictions of the general relativistic effects manifested in the four classical test experiments of Einstein’s general relativity (GR), in direct agreement with the experiments and the predictions given based on GR.

Identifying the perfect absorption of metamaterial absorbers

We present a detailed analysis of the conditions that result in unity absorption in metamaterial absorbers to guide the design and optimization of this important class of functional electromagnetic composites. Multilayer absorbers consisting of a metamaterial layer, dielectric spacer, and ground plane are specifically considered. Using interference theory, the dielectric spacer thickness and resonant frequency for unity absorption can be numerically determined from the functional dependence of the relative phase shift of the total reflection. Further, using transmission line theory in combination with interference theory we obtain analytical expressions for the unity absorption resonance frequency and corresponding spacer layer thickness in terms of the bare resonant frequency of the metamaterial layer and metallic and dielectric losses within the absorber structure. These simple expressions reveal a redshift of the unity absorption frequency with increasing loss that, in turn, necessitates an increase in the thickness of the dielectric spacer. The results of our analysis are experimentally confirmed by performing reflection-based terahertz time-domain spectroscopy on fabricated absorber structures covering a range of dielectric spacer thicknesses with careful control of the loss accomplished through water absorption in a semiporous polyimide dielectric spacer. Our findings can be widely applied to guide the design and optimization of the metamaterial absorbers and sensors.

Composition- and Temperature-Resolved Raman Shift of Silicon.

We formulated the composition and temperature dependence of the Si and Si1-xGe x Raman shift from the perspectives of bond order-length-strength correlation and local bond average approach. It is verified that the Raman shift Δω varies in the form of Δω ∝ zE1/2/ d, with inclusion of bond length d and energy E changing with temperature and composition. Numerical reproduction of the thermally induced Si1-xGe x phonon softening indicates that bond thermal expansion and energy loss dictate the frequency redshift, which resulted in quantitative information on the bond energy and the reference frequencies from which the Raman shifts proceed. Observations not only gain deeper insight into the mechanism of the Raman shift but also demonstrate the revealing power of Raman technique for the bonding thermodynamics.

Femtosecond coherent nuclear dynamics of excited tetraphenylethylene: Ultrafast transient absorption and ultrafast Raman loss spectroscopic studies.

Ultrafast torsional dynamics plays an important role in the photoinduced excited state dynamics. Tetraphenylethylene (TPE), a model system for the molecular motor, executes interesting torsional dynamics upon photoexcitation. The photoreaction of TPE involves ultrafast internal conversion via a nearly planar intermediate state (relaxed state) that further leads to a twisted zwitterionic state. Here, we report the photoinduced structural dynamics of excited TPE during the course of photoisomerization in the condensed phase by ultrafast Raman loss (URLS) and femtosecond transient absorption (TA) spectroscopy. TA measurements on the S1 state reveal step-wise population relaxation from the Franck-Condon (FC) state → relaxed state → twisted state, while the URLS study provides insights on the vibrational dynamics during the course of the reaction. The TA spectral dynamics and vibrational Raman amplitudes within 1 ps reveal vibrational wave packet propagating from the FC state to the relaxed state. Fourier transformation of this oscillation leads to a ∼130 cm-1 low-frequency phenyl torsional mode. Two vibrational marker bands, Cet=Cet stretching (∼1512 cm-1) and Cph=Cph stretching (∼1584 cm-1) modes, appear immediately after photoexcitation in the URLS spectra. The initial red-shift of the Cph=Cph stretching mode with a time constant of ∼400 fs (in butyronitrile) is assigned to the rate of planarization of excited TPE. In addition, the Cet=Cet stretching mode shows initial blue-shift within 1 ps followed by frequency red-shift, suggesting that on the sub-picosecond time scale, structural relaxation is dominated by phenyl torsion rather than the central Cet=Cet twist. Furthermore, the effect of the solvent on the structural dynamics is discussed in the context of ultrafast nuclear dynamics and solute-solvent coupling.

Geodetic methods to determine the relativistic redshift at the level of 10^{-18} in the context of international timescales: a review and practical results

The frequency stability and uncertainty of the latest generation of optical atomic clocks is now approaching the one part in 10^{18} level. Comparisons between earthbound clocks at rest must account for the relativistic redshift of the clock frequencies, which is proportional to the corresponding gravity (gravitational plus centrifugal) potential difference. For contributions to international timescales, the relativistic redshift correction must be computed with respect to a conventional zero potential value in order to be consistent with the definition of Terrestrial Time. To benefit fully from the uncertainty of the optical clocks, the gravity potential must be determined with an accuracy of about 0.1,hbox {m}^{2},hbox {s}^{-2}, equivalent to about 0.01 m in height. This contribution focuses on the static part of the gravity field, assuming that temporal variations are accounted for separately by appropriate reductions. Two geodetic approaches are investigated for the derivation of gravity potential values: geometric levelling and the Global Navigation Satellite Systems (GNSS)/geoid approach. Geometric levelling gives potential differences with millimetre uncertainty over shorter distances (several kilometres), but is susceptible to systematic errors at the decimetre level over large distances. The GNSS/geoid approach gives absolute gravity potential values, but with an uncertainty corresponding to about 2 cm in height. For large distances, the GNSS/geoid approach should therefore be better than geometric levelling. This is demonstrated by the results from practical investigations related to three clock sites in Germany and one in France. The estimated uncertainty for the relativistic redshift correction at each site is about 2 times 10^{-18}.

Equatorial coordination of uranyl: Correlating ligand charge donation with the Oyl-U-Oyl asymmetric stretch frequency

In uranyl coordination complexes, UO2(L)n2+, uranium in the formally dipositive [O=U=O]2+ moiety is coordinated by n neutral organic electron donor ligands, L. The extent of ligand electron donation, which results in partial reduction of uranyl and weakening of the U=O bonds, is revealed by the magnitude of the red-shift of the uranyl asymmetric stretch frequency, ν3. This phenomenon appears in gas-phase complexes in which uranyl is coordinated by electron donor ligands: the ν3 red-shift increases as the number of ligands and their proton affinity (PA) increases. Because PA is a measure of the enthalpy change associated with a proton-ligand interaction, which is much stronger and of a different nature than metal ion-ligand bonding, it is not necessarily expected that ligand PAs should reliably predict uranyl-ligand bonding and the resulting ν3 red-shift. Here, ν3 was measured for uranyl coordinated by ligands with a relatively broad range of PAs, revealing a surprisingly good correlation between PA and ν3 frequency From computed ν3 frequencies for bare UO2 cations and neutrals, it is inferred that the effective charge of uranyl in UO2(L)n2+ complexes can be reduced to near zero upon ligation by sufficiently strong charge-donor ligands. The basis for the correlation between ν3 and ligand PAs, as well as limitations and deviations from it, are considered. It is demonstrated that the correlation evidently extends to a ligand that exhibits polydentate metal ion coordination.