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Yemelyan Rybakov
Yemelyan Rybakov

Optical Properties Of Solids

Many-body effects are known to play a crucial role in the electronic and optical properties of solids and nanostructures. Nevertheless, the majority of theoretical and numerical approaches able to capture the influence of Coulomb correlations are restricted to the linear response regime. In this work, we introduce an approach based on a real-time solution of the electronic dynamics. The proposed approach reduces to the well-known Bethe-Salpeter equation in the linear limit regime and it makes it possible, at the same time, to investigate correlation effects in nonlinear phenomena. We show the flexibility and numerical stability of the proposed approach by calculating the dielectric constants and the effect of a strong pulse excitation in bulk h-BN.

Optical Properties of Solids

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It is shown that the interaction between macroscopic, nondissipative media, and time-varying electromagnetic fields can be described by a time-averaged potential function. From this function it is possible to derive phenomenologically the tensors that describe any of the usual electro- and magneto-optic effects for electric and magnetic fields of any frequency. In addition, these same potential functions describe the various optical nonlinearities like harmonic generation in potassium dihydrogen phosphate, and harmonic generation by electric quadrupole and magnetic dipole nonlinearities.

We demonstrate rapid characterization of complex optical properties of solids via dual-comb spectroscopy (DCS) in the near-infrared region. The fine spectral structures in the complex refractive index of an Er:YAG are successfully deduced using the developed system and Fourier analysis. Moreover, simultaneous determination of the refractive index and the thickness is demonstrated for a silicon semiconductor wafer through the use of multireflected echo signals. The results indicate the potential of DCS as a powerful measurement tool for the rapid and full characterization of solid materials.

"This book is well organized and can be of great benefit for teaching the subject of optical properties of solid materials to undergraduate students. The text can also be used as handy reference material for researchers that need more understanding on optical properties." Prof. Pieter Stroeve, University of California, Davis, USA

This has now just been achieved in the lab of Majed Chergui at EPFL within the Lausanne Centre for Ultrafast Science, in collaboration with the theory groups of Angel Rubio (Max-Planck Institute, Hamburg) and Pascal Ruello (Université de Le Mans). Publishing in Science Advances, the international team shows, for the first time, control of excitonic properties using acoustic waves. To do this, the researchers launched a high-frequency (hundreds of gigahertz), large-amplitude acoustic wave in a material using ultrashort laser pulses. This strategy further allows for the dynamical manipulation of the exciton properties at high speed.

The ab-initio calculation of optical absorption spectra of nano-structures and solids is a formidable task. The current state-of-the-art is based on many-body perturbation theory: one solves the Bethe-Salpeter equation (BSE) using the one-body Green's function obtained from the GW approximation. Resonances, corresponding tobound electron-hole pairs called excitons, which have energies inside the gap, can then appear in the spectrum.This procedure is a well-established method for yielding macroscopic dielectric tensors which are generally in good agreement with experiment. Unfortunately, solving the BSE involves diagonalizing a large matrix making this method computationally very expensive.

A stringent test for any approximate xc-kernel is in its ability to treatmaterials with strongly bound excitons. In these cases a new resonant peakappears in the bandgap itself and represents the bound state of an electron-holepair. Perhaps the most studied test case for this phenomenon is the ionic solid LiF.Other excitonic materials which have also attracted attention and are consideredparticularly difficult to treat are the noble gas solids. Plotted in the first column of Fig. 2 are theresults for three materials of this class: LiF, solid Ar and Ne.What is immediately clear is that the bootstrap procedure, which gave only aslight shift of spectral weight forGe, now gives rise to an entirely new bound excitonic peak inside thegap in all three cases. The location of the peak, which corresponds to theexcitonic binding energy, is also very well reproduced for all these materials.

In its current form, the new diffuser could be used to calibrate a wide range of imaging systems, but the researchers believe that their mechanism could ultimately lead to holographic video screens or to tunable optical devices with applications in imaging, sensing, and photography.

The same would be true, Heshmat argues, of other types experimental materials whose refractive indices change in response to either light or an electric field. And optical or electrical activation would broaden the range of applications for tunable optical devices.

The Normal incidence Diffuse Optical Scatter Instrument (NDOSI) is a new facility for measuring the hemispherical reflectance and transmittance of materials under direct and diffuse illumination in the visible range (400 nm to 800 nm). The measured quantities are used to investigate the diffuse optical properties of materials; these parameters are critical for understanding the interaction of light with optically diffusive materials.

NDOSI will provide calibration reference materials, solid and liquid tissue mimicking phantoms, measurements of scattering, absorption, reflectance, and transmittance of materials such as tissues, plant leaves, suspended solids and plankton for databases, model validation, and scales (e.g., turbidity). 041b061a72


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