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Dielectric Properties of Tissues
Published in Charles Polk, Elliot Postow, CRC Handbook of Biological Effects of Electromagnetic Fields, 2019
Kenneth R. Foster, Herman P. Schwan
The Kramers-Kronig relations follow from very general assumptions, the principal ones being linearity of response and causality (i.e., no effect preceding a stimulus). They apply to dielectric materials with any arbitrary' distribution of relaxation times, and even hold in the presence of resonance-type behavior such as is found in optical spectra. The principal limitation is that the applied field must be of sufficiently low amplitude that the response is linear — which is practically always the case in dielectric measurements on biological materials. A second limitation — that the properties of the system do not vary with time — is not strictly true for most biological preparations, but is nearly always an excellent approximation for the timescale in which dielectric measurements are carried out.
Stratified Media for Novel Optics, Perfect Transmission and Perfect Coherent Absorption
Published in Shyamal Bhadra, Ajoy Ghatak, Guided Wave Optics and Photonic Devices, 2017
In most of the current MMs, negative refraction is accompanied by substantial losses, more so at higher frequencies. The origin of these losses can be traced to the fact that the refractive index becomes negative close to electromagnetic resonances where the absorption is high. The presence of large absorption is associated with large dispersion via the Kramers–Kronig relations. In MMs at lower frequencies, such strong dispersion was exploited to show the feasibility of slow light [9]. Achieving large delays in the higher-frequency range can thus pose a challenging problem. In an analogous fashion, achieving the desired SR at higher frequencies is also threatened by high losses. In recent studies, two specific examples, namely, the delay devices and MM lens were considered, in order to make a quantitative assessment of the effects of losses [15,27]. The parameters of a recently reported MM [42] were used by fitting the experimental data for the permeability to a causal Lorentz-type model, while the interpolated experimental data for the permittivity were used.
Dynamic behavior
Published in Roderic S. Lakes, Viscoelastic Solids, 2017
Optical properties of materials (specifically the electric polarizability) are also governed by the Kramers–Kronig relations. Materials can exhibit resonant absorption of light due to microresonance on the atomic scale. For example, transparent materials such as glass and water exhibit resonant absorption of ultraviolet light due to resonance of electrons in the material. These materials are transparent (they have a small loss term) to visible light, which has lower frequencies than that of the resonant absorption. The dispersion of visible light manifests itself as the rainbow and in the spectral colors seen in light refracted through diamonds.
Performance comparison of commonly used photoacoustic tomography reconstruction algorithms under various blurring conditions
Published in Journal of Modern Optics, 2022
As the phase velocity of acoustic waves varies with frequency, the acoustic attenuation is often followed by the dispersion of those waves. As a result of that, the shape of a propagating pulse changes [18]. The Kramers–Kronig relations connect the frequency-dependent attenuation and the dispersion [32]. A general dispersion relation satisfying Equation (2) can be written as [33] where is the frequency-dependent wave number. The corresponding time-independent wave equation becomes The solution to Equation (4) can be obtained using the Green's function approach. The appropriate Green's function in 3D becomes [7] where and are the source and field points, respectively. The second exponential term in Equation (5) is responsible for attenuation and dispersion of the acoustic wave while propagating through the medium.
DFT study of electronic and optical properties of CH3NH3SnI3 perovskite
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2020
Roozbeh Sabetvand, Mohammad Ebrahim Ghazi, Morteza Izadifard
In our DFT calculations for study the optical behavior of CH3NH3SnI3 perovskite, the real (ε1) and imaginary (ε2) parts of dielectric function, absorption coefficient, refractive index, energy loss function and reflectivity were calculated by the GW approximation due to its appropriate electronic results. The imaginary part of the dielectric function was calculated from momentum matrix elements between the occupied and unoccupied wave functions. Figure 8 shows the real and imaginary parts of the dielectric function as a function of incidence energy. The calculated imaginary part shows a significant increase at an energy of about 1.38 eV with a rather sharp peak around 2.18 eV and then decreases. By using the imaginary part of dielectric function and Kramers-Kronig relation, the real part of dielectric function was calculated. These optical parameters show the polarizability of the atomic structure. It can be observed that the real part of dielectric constant reaches a maximum around the band gap energy and then decreases until it becomes nearly constant. The static dielectric constant ε1(ω = 0) is about 4.51.
Modelling relativistic effects in momentum-resolved electron energy loss spectroscopy of graphene
Published in Radiation Effects and Defects in Solids, 2018
K. Lyon, D. J. Mowbray, Z. L. Miskovic
Any good model of graphene's conductivity should be able to replicate behaviour in the low-energy region ( eV) where graphene famously has a frequency-independent absorption ratio due to the linear dispersion of its bands, as well as in the region ranging from the visible to the ultraviolet, where the inter-band transitions yield broad peaks in the energy loss spectra. For these purposes a hybrid model was developed, termed the extended hydrodynamic (eHD) model, consistent with both the number of electrons per carbon atom and the Kramers–Kronig relations. Another natural way to look into graphene's electrical properties is to perform density functional theory (DFT) calculations, in particular, time-dependent DFT in the frequency domain (TDDFT-ω), to compute the conductivity (7–10). These computations can be used side-by-side with the theoretical model to aid in understanding our model's applicability to graphene's EELS spectra (6).