Frequency Dependence Of Spectrum Research Articles

Prediction of Frequency-Dependent Optical Spectrum for Solid Materials: A Multioutput and Multifidelity Machine Learning Approach.

The frequency-dependent optical spectrum is pivotal for a broad range of applications from material characterization to optoelectronics and energy harvesting. Data-driven surrogate models, trained on density functional theory (DFT) data, have effectively alleviated the scalability limitations of DFT while preserving its chemical accuracy, expediting material discovery. However, prevailing machine learning (ML) efforts often focus on scalar properties such as the band gap, overlooking the complexities of optical spectra. In this work, we employ deep graph neural networks (GNNs) to predict the frequency-dependent complex-valued dielectric function across the infrared, visible, and ultraviolet spectra directly from the crystal structures. We explore multiple architectures for the spectral multioutput representation of the dielectric function and utilize various multifidelity learning strategies, such as transfer learning and fidelity embedding, to address the challenges associated with the scarcity of high-fidelity DFT data. Additionally, we model key solar cell absorption efficiency metrics, demonstrating that learning these parameters is enhanced when integrated through a learning bias within the learning of the frequency-dependent absorption coefficient. This study demonstrates that leveraging multioutput and multifidelity ML techniques enables accurate predictions of optical spectra from crystal structures, providing a versatile tool for rapidly screening materials for optoelectronics, optical sensing, and solar energy applications across an extensive frequency spectrum.

The spectrum of Ih ice using terahertz time-domain spectroscopy.

Here, we report the frequency-dependent spectrum of ice Ih in the range of 0.2-2THz. We confirm the presence of a feature that blue-shifts from around 1.55-1.65THz with a decreasing temperature from 260 to 160K. There is also a change in the trend of the refractive index of ice corresponding to a dispersion, which is also around 1.6THz. The features are reproduced in data acquired with three commercial terahertz time-domain spectrometers. Computer-simulated spectra assign the feature to lattice translations perpendicular to the 110 and 1̄10 planes of the ice Ih crystal. The feature's existence should be recognized in the terahertz measurements of frozen aqueous solution samples to avoid false interpretations.

A flashing beacon in axion inflation: recurring bursts of gravitational waves in the strong backreaction regime

The coupling between a pseudo-scalar inflaton and a gauge field leads to an amount of additional density perturbations and gravitational waves (GWs) that is strongly sensitive to the inflaton speed. This naturally results in enhanced GWs at (relatively) small scales that exited the horizon well after the CMB ones, and that can be probed by a variety of GW observatories (from pulsar timing arrays, to astrometry, to space-borne and ground-based interferometers). This production occurs in a regime in which the gauge field significantly backreacts on the inflaton motion. Contrary to earlier assumptions, it was later shown that this regime is characterized by an oscillatory behavior of the inflaton speed, with a period of O ( 5 ) e-folds. Bursts of GWs are produced at the maxima of the speed, imprinting nearly periodic bumps in the frequency-dependent spectrum of GWs produced during inflation. This can potentially generate correlated peaks appearing in the same or in different GWs experiments. While recent lattice studies show that the inclusion of inflaton gradients can modify significantly the dynamics of this system in the strong backreaction regime, this is not the case for the first oscillation or two of the inflaton speed, so that we expect our results to be robust for modes that were excited during that epoch.

Estimation of Electrical Spectra of Irregular Embedded Samples

In order to interpret spectral induced polarization (SIP) data measured in the field for the purpose of mineral exploration, laboratory investigations are necessary that establish relationships between electrical parameters and mineral properties. For massive ores, and in particular for seafloor massive sulfides (SMSs), samples may be difficult to obtain, and it is often not desired to cut cylindrical plugs out of the available hand pieces. We suggest a method to obtain the electrical spectra of hand pieces from measurements on the samples embedded in a non-polarizing medium, in our case quartz sand. As such, destroying potentially precious samples is not necessary. The frequency-dependent phase spectrum of the sample is calculated by dividing the bulk spectrum with a so-called dilution factor, which is obtained from numerical simulation and has a real and constant value. We evaluate the method with a set of SMS samples where conventional cylindrical plugs are available. We can estimate the phase shift maximum of 73% of the samples with a deviation less than 50% from the reference. The estimation quality slightly decreases if the dilution factor is approximated by the volumetric share of the sample. We consider the performance acceptable if the general difficulty to obtain reproducible and representative laboratory measurements for massive sulfides is taken into account.

Wave spectrum analysis for southern Caspian Sea

The general form of spectrum S(ω)=Aω−pexp(−Bω−q) was used in the present study to mathematically describe the spectrum of the Caspian Sea waves, and its fit to observations was obtained by assigning different values to pair (p,q). For this purpose, five spectral models, including Donelan, Pierson-Moskowitz (PM), Krylov, Davidan, and Neumann were considered corresponding to (4,4), (5,4), (7,4), (6.5,5.5), and (6,2). In the first step, the default coefficients were found to lack the required accuracy, therefore, they were optimized. The performance comparison of these five models showed that the spectral width index (ν) provides a good insight into the performance of the models such that the Krylov model exhibited the highest accuracy for narrow spectra (ν < 0.4). For broad (ν > 0.45) and medium-width spectra (0.4 < ν < 0.45), the Donelan model and PM model yielded the best result, respectively. The highest rate of error was related to the Davidan model; therefore, it was excluded from the study. The Neumann model had similar performance to the Donelan model. Additionally, it was observed that the models are sometimes incapable of accurately estimating the spectrum in high frequency band. In validation of Philips (∼ω−5) and Toba (∼ω−4) theories, the frequency dependence of spectrum in the equilibrium range was found to take the form of ω−7, ω−4, and ω−5 in narrow, broad and medium-width spectra, respectively.

Spatial frequency-dependent pulse-heightspectrum and method for analyzing detector DQE(f) from ensembles of single X-rayimages.

Scintillators and photoconductors used in energy integrating detectors (EIDs) have inherent variations in their imaging response to single-detected X-rays due to variations in X-ray energy deposition and secondary quanta generation and transport, which degrades DQE(f). The imaging response of X-ray scintillators to single X-rays may be recorded and studied using single X-ray imaging (SXI) experiments; however, no method currently exists for relating SXI experimental results to EID DQE(f). This work proposes a general analytical framework for computing and analyzing the DQE(f) performance of EIDs from single X-ray image ensembles using a spatial frequency-dependent pulse-height spectrum. A spatial frequency (f)-dependent gain, , is defined as the Fourier transform of the imaging response of an EID to a single-detected X-ray. A f-dependent pulse-height spectrum, , is defined as the 2D probability density function of over the complex plane. is used to define a f-dependent Swank factor, AS (f), which fully characterizes the DQE(f) degradation due to single X-ray noise. AS (f) is analyzed in terms of its degradation due to Swank noise, variations in the frequency-dependent attenuation of , and noise in which occurs due to variations in the asymmetry in each single X-ray's imaging response. Three example imaging systems are simulated to demonstrate the impact of depth-dependent variation in , remote energy deposition, and a finite number of secondary quanta, on , AS (f), MTF(f), and NPS(f)/NPS(0), which are computed from ensembles of single X-ray images. The same is also demonstrated by simulating a realistic imaging system; that is, a Gd2 O2 S-based EID. Using the latter imaging system, the convergence of AS (f) estimates is investigated as a function of the number of detected X-rays per ensemble. Depth-dependent variation resulted in AS (f) degradation exclusively due to depth-dependent optical Swank noise and the Lubberts effect. Conversely, the majority of AS (f) degradation caused by remote energy deposition and finite secondary quanta occurred due to variations in . When using input X-ray energies below the K-edge of Gd, variations in the frequency-dependent attenuation of accounted for the majority of AS (f) degradation in the GOS-based EID, and very little Swank noise and variations in were observed. Above the K-edge, however, AS (f) degradation due to Swank noise and variations in greatly increased. The convergence of AS (f) was limited by variation in ; imaging systems with more variation in required more detected X-rays per ensemble. An analytical framework is proposed that generalizes the pulse-height spectrum and Swank factor to arbitrary f. The impact of single X-ray noise sources, such as the Lubberts effect, remote energy deposition, and finite secondary quanta on detector performance, may be represented using , and quantified using AS (f). The approach may be used to compute MTF(f), NPS(f), and DQE(f) from ensembles of single X-ray images and provides an additional tool to analyze proposed EID designs.

Spectral Properties of Clipping Noise

One serious disadvantage of any multicarrier-modulation technique such as orthogonal frequency division multiplexing (OFDM) is its high peak-to-average-power ratio (PAPR) which might lead to signal clipping in several scenarios. To maximize the transmit data rate, it is important to take this non-linear distortion into account. The most common approach is based on the Bussgang theorem, which splits the distortion in a correlated part, represented by a linear damping factor, and uncorrelated additive noise. However, there are two aspects that are not correctly considered by the Bussgang theorem. Firstly, clipping noise shows a frequency-dependent power spectrum which depends on the clipping probability. Secondly, some of the clipping noise power is located outside of the transmission bandwidth, so that it does not influence the transmission quality. In this work, the Bussgang theorem is reviewed in detail and the exact power spectral density of the uncorrelated clipping noise is approximated to determine the signal-to-noise power ratio on every subcarrier separately. Although it is shown that the frequency dependence within the transmission bandwidth is relatively small, at least 36% of the uncorrelated noise power, depending on the clipping level, lays outside of the transmission band. Monte Carlo simulations validate that a simple expression for the power spectral density allows to calculate the symbol error probability of an OFDM transmission system that suffers from clipping. Furthermore, the newly found result can be used to optimize bit allocation tables in bit loading algorithms or to calculate the channel capacity.

Frequency-dependent transport properties in disordered systems: A generalized coherent potential approximation approach

To predict transport properties of disordered systems, especially ac transport properties, one has to calculate the disorder average of the correlation of the multiple Green's function at different energies. To avoid brute force calculation, diagrammatic perturbation expansion must be used along with the coherent potential approximation (CPA). In this paper, we develop a theoretical formalism based on the nonequilibrium Green's function that maps the average of the correlation of the multiple Green's function into an average over a single generalized Green's function. After the mapping, this formalism is structurally very similar to the CPA and completely eliminates the need to perform diagrammatic expansion. As a demonstration of our theory, the dynamic conductance, frequency-dependent shot noise under dc bias, and frequency-dependent noise spectrum under ac bias in the presence of Anderson disorder are calculated by directly taking the disorder average of the generating function of full counting statistics (FCS) within the CPA. Our numerical results on dynamic conductance, frequency-dependent shot noise under dc bias, and frequency-dependent noise spectrum under ac bias show remarkable agreement with that obtained by the brute force calculation. The phase diagram in the frequency versus disorder strength plane has been efficiently calculated using the generalized FCS-CPA method.

The planar Multipole Resonance Probe: a functional analytic approach

Active Plasma Resonance Spectroscopy (APRS) is a well known diagnostic method, where a radio frequency probe is immersed into a plasma and excites plasma oscillations. The response of the plasma is recorded as frequency dependent spectrum, in which resonance peaks occur. By means of a mathematical model plasma parameters like the electron density or the electron temperature can be determined from the detected resonances.The majority of all APRS probes have in common, that they are immersed into the plasma and perturb the plasma due to the physical presence of the probe. Thus, they are invasive and can at least influence the homogeneity of the plasma. To overcome this problem, the planar Multipole Resonance Probe (pMRP) was invented, which can be integrated into the chamber wall of a plasma reactor.Within this paper, the first analytic model of the pMRP is presented, which is based on a cold plasma description of the electrons. The general admittance of the probe-plasma system is derived by means of functional analytic methods and a complete orthonormal set of basis functions. Explicit spectra for an approximated admittance including a convergence study are shown. The determined resonance frequencies are in good agreement with former simulation results.

Current fluctuations across a nano-pore

The frequency-dependent spectrum of current fluctuations through nano-scale channels is studied using analytical and computational techniques. Using a stochastic Nernst–Planck description and neglecting the interactions between the ions inside the channel, an expression is derived for the current fluctuations, assuming that the geometry of the channel can be incorporated through the lower limits for various wave-vector modes. Since the resulting expression turns out to be quite complex, a number of further approximations are discussed such that relatively simple expressions can be used for practical purposes. The analytical results are validated using Langevin dynamics simulations.

Joint effect of the tube sizes and Fe-filling process on microwave dielectric properties of carbon nanotubes

To explore the effect of tube sizes and filling process on dielectric properties of carbon nanotubes (CNTs), CNTs with various diameters and lengths were applied to synthesize Fe-filled CNTs through wet-chemical method. The permittivities of the composites of CNTs and paraffin rely on the dispersion state (agglomeration) of CNTs in polymer matrices. During the filling process, CNTs with diverse tube parameters and dispersibility experienced different structural transformations, and different iron oxides (either Fe3O4 or Fe2O3) particles (depending on the dispersibility of CNTs) were observed adhering to walls of CNTs after filling. The discrepancies along with the dispersibility of CNTs per se alter the order of the permittivity (from high to low) within samples. The smallest agglomerates and the largest permittivity were obtained for CNTs with diameters of 10–20 nm and lengths of 10–30 μm, both before and after the filling of CNTs. The role that the shape parameters played in the dielectric properties during the whole filling process was analyzed. The frequency-dependent permittivity spectrum was also examined and it obeyed Jonscher’s universal dielectric relaxation law.

Probing PM2.5 with terahertz wave

The goal of continuous ambient participate monitoring has been accomplished by the use of terahertz (THz) wave. The frequency-dependent spectrum and absorbance of the particulate matter (PM2.5) were measured in the range of 0–10 THz. The PM2.5 concentration ρ was calculated according to the sampling time and air flow. With the increase of ρ, the THz wave amplitude gradually decreased and the absorbance A of PM2.5 increased. The relationship between r and A can be described mathematically through ρ ∝ A0.5. Our results demonstrate that the terahertz wave could be a valuable tool to monitor and inspect the PM2.5 concentration.

Response to “Comment on ‘Intrinsic dielectric frequency dependent spectrum of a single domain tetragonal BaTiO3’” [J. Appl. Phys. 113, 126104 (2013)

Response to “Comment on ‘Intrinsic dielectric frequency dependent spectrum of a single domain tetragonal BaTiO3’” [J. Appl. Phys. 113, 126104 (2013)

Flow over a flat plate with uniform inlet and incident coherent gusts

AbstractThe flow over a flat plate at a Reynolds number of 750 is numerically investigated via fine large-eddy simulation (LES), first at normal ($90\textdegree $) and then at oblique ($45\textdegree $) incidence flow direction with a uniform steady inlet. The results are in complete agreement with the direct numerical simulation (DNS) and experimental data, thereby serving as a validation for the present simulations. For the normal ($90\textdegree $) uniform inflow case, coherent vortices are alternately shed from both leading edges of the plate, whereas for the oblique ($45\textdegree $) uniform inflow case the vortices shed from the two sides of the plate interact strongly resulting in a quasi-periodic force response. The normal flat plate is then analysed with an incident gust signal with varying amplitude and time period. For these incident coherent gust cases, a reference test case with variable coherent inlet is first studied and the results are compared to a steady inlet simulation, with a detailed analysis of the flow behaviour and the wake response under the incident gust. Finally, the flat plate response to 16 different gust profiles is studied. A transient drag reconstruction for these incident coherent gust cases is then presented based on a frequency-dependent transfer function and phase spectrum analysis.

A review of terahertz plasmonics in subwavelength holes on conducting films

In this paper, we present a review of experimental studies of terahertz plasmonic transmission properties through subwavelength holes patterned in conducting films. The frequency-dependent transmission spectrum reveals resonant behavior with an anomalously high peak transmission which is mediated by the excitation of surface plasmon polaritons. We show how terahertz time-domain spectroscopy has been utilized to determine the resonant transmission effects of hole shape, dielectric properties of materials, and thickness of the arrays. Enhanced terahertz transmission was also observed through a single hole, accompanied by annular periodic corrugations. In addition to metals films, we review films comprised of highly doped semiconductors and superconductors. We finally review various modulation schemes to actively control or manipulate the resonant terahertz transmission using external stimuli such as thermal, optical, and electrical fields. This body of work is used to provide perspective on how manipulation of terahertz radiation via surface plasmon polaritons could affect next-generation terahertz photonic devices.

Intrinsic dielectric frequency dependent spectrum of a single domain tetragonal BaTiO3

The intrinsic dielectric frequency dependent spectrum of single domain barium titanate (BaTiO3) at room temperature is investigated by considering the vibration of phonons and the conductivity of the tetragonal system in a wide frequency range up to THz. The proposed model combines Debye type of dissipation, soft mode theory, and the influence of conductivity on the dielectric loss to obtain a more precise dielectric frequency spectrum. The calculated results were compared with experimental data on single domain nanocrystals of BaTiO3, both free standing and suspended in a low dielectric medium. The comparisons provide insight into the mechanism for the dielectric behavior, which can be extended to apply to a range of composites that comprise single domain dielectrics embedded in continuous media. At the lower frequency range, conductivity plays a dominant role in the contribution to the dielectric loss along both a- and c-axes, while the phonon vibration controls the dielectric behavior of the system at higher frequency range. When the conductivity of the system increases, the dielectric loss increases below the MHz range, with such an effect diminishing when the frequency reaches the GHz regime.

Joint spectral-timing modelling of the hard lags in GX 339−4: constraints on reflection models

The X-ray variations of hard state black hole X-ray binaries above 2 keV show 'hard lags', in that the variations at harder energies follow variations at softer energies, with a time-lag \tau depending on frequency \nu approximately as \tau \propto \nu^{-0.7}. Several models have so far been proposed to explain this time delay, including fluctuations propagating through an accretion flow, spectral variations during coronal flares, Comptonisation in the extended hot corona or a jet, or time-delays due to large-scale reflection from the accretion disc. In principle these models can be used to predict the shape of the energy spectrum as well as the frequency-dependence of the time-lags, through the construction of energy-dependent response functions which map the emission as a function of time-delay in the system. Here we use this approach to test a simple reflection model for the frequency-dependent lags seen in the hard state of GX 339-4, by simultaneously fitting the model to the frequency-dependent lags and energy spectrum measured by XMM-Newton in 2004 and 2009. Our model cannot simultaneously fit both the lag and spectral data, since the relatively large lags require an extremely flared disc which subtends a large solid angle to the continuum at large radii, in disagreement with the observed Fe K\alpha emission. Therefore, we consider it more likely that the lags > 2 keV are caused by propagation effects in the accretion flow, possibly related to the accretion disc fluctuations which have been observed previously.

NMR Force Microscopy of Ammonium Sulfate with 12 μm Resolution

ABSTRACTPosition-dependent force-detected NMR measurements on a 25x15x7 μm3 single crystal of ammonium sulfate (NH4)2SO4 were performed at room temperature in a sample-on-oscillator configuration. Force signals were detected with 12 μm resolution in a one-dimensional scan. Measurements of NMR relaxation times T2* = 1.5 ± 0.3 μs, T2 = 44 ± 2 μs, and T1 = 5.6 ± 0.7 s were obtained in an 8-T magnetic field, revealing an unexpected frequency-dependent fluctuation spectrum at room temperature.

Classical Fano oscillator

Starting from the quantum-mechanical Fano-Anderson Hamiltonian, we derive classical equations of motion for coordinates associated with the discrete and continuum states. The frequency-dependent absorption spectrum associated with this classical system exhibits the same Fano line shape as the quantum-mechanical system when appropriate correspondences between classical and quantum variables are made. In the time domain, the response of this classical Fano oscillator depends upon the asymmetry parameter $q$ that appears in the expression for the Fano line shape. In particular, under the influence of impulsive driving of the system, the discrete oscillator's phase changes by $\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2$ as $q$ increases from zero (maximum asymmetry in the frequency domain) to $\ensuremath{\mp}\ensuremath{\infty}$ (minimum asymmetry). Previously published ultrafast-laser-pulse-driven coherent-phonon oscillations in degenerate $p$-type Si [K. Kato et al., Jpn. J. Appl. Phys. 48, 100205 (2009)] are discussed in light of these theoretical results.

Fast 3D chemical exchange saturation transfer (CEST) imaging of the human brain

Chemical exchange saturation transfer magnetic resonance imaging can detect low-concentration compounds with exchangeable protons through saturation transfer to water. This technique is generally slow, as it requires acquisition of saturation images at multiple frequencies. In addition, multislice imaging is complicated by saturation effects differing from slice to slice because of relaxation losses. In this study, a fast three-dimensional chemical exchange saturation transfer imaging sequence is presented that allows whole-brain coverage for a frequency-dependent saturation spectrum (z-spectrum, 26 frequencies) in less than 10 min. The approach employs a three-dimensional gradient- and spin-echo readout using a prototype 32-channel phased-array coil, combined with two-dimensional sensitivity encoding accelerations. Results from a homogenous protein-containing phantom at 3T show that the sequence produced a uniform contrast across all slices. To show translational feasibility, scans were also performed on five healthy human subjects. Results for chemical exchange saturation transfer images at 3.5 ppm downfield of the water resonance, so-called amide proton transfer images, show that lipid signals are sufficiently suppressed and artifacts caused by B(0) inhomogeneity can be removed in postprocessing. The scan time and image quality of these in vivo results show that three-dimensional chemical exchange saturation transfer MRI using gradient- and spin-echo acquisition is feasible for whole-brain chemical exchange saturation transfer studies at 3T in a clinical time frame.