P. P. Kircheva

Bio: P. P. Kircheva is an academic researcher from Georgi Nadjakov Institute of Solid State Physics. The author has contributed to research in topics: Four-wave mixing & Dipole. The author has an hindex of 3, co-authored 10 publications receiving 35 citations.

Kramers-Kronig relations in FWM spectroscopy

Abstract: The Kramers-Kronig (K-K) relations are verified for the third-order non-linear optical susceptibilities which are described by known expressions, obtained by the density matrix formalism. Possible applications of the K-K relations in non-linear Raman spectroscopy techniques are also discussed. Coupling of the real and imaginary parts of CARS excitation and probe profiles, as well as a relation between the spectra of two variants of OHD-RIKE are demonstrated. Some cases are shown when the susceptibility does not obey the K-K relations. This result is not a consequence of a break of the causality principle.

Two-photon resonance four-wave mixing spectroscopy in polar media

Abstract: The effect of the permanent electric-dipole moments of polar media on the two-photon resonance four-wave mixing (TPR FWM) as described by chi (3)( omega 4= omega 1- omega 2+ omega 3) is studied. The spectra corresponding to different processes of TPR FWM are modelled for a three-level system. It is demonstrated that the difference- and sum-frequency TPR FWM spectroscopy is sensitive to the difference between permanent dipole moments of the two levels coupled by the TPR.

Optical four-wave mixing in bulk polymers

Abstract: A multiplex CARS scheme is used to study the bulk polymers poly-(methyl methacrylate) (PMMA) and poly-(vinyl toluene) (PVT). A coherent signal resulting from optical four-wave mixing is obtained with electronic and Raman-resonant parts. The magnitude of the third-order optical susceptibility tensor of these polymers is experimentally estimated. Significant enhancement of the total susceptibility by the Raman resonances is observed for PVT.

Photorefractive optical damage in protonated LiNbO3 waveguides

Abstract: The photorefractive optical damage in annealed proton-exchanged LiNbO3 planar waveguides is studied The pfotorefractive thresholds for the wavelengths 5145 and 6471 nm are determined Also, the light-induced refractive index change in a wide range of powers is determined An interesting and unexpected photorefractive behaviour of the samples in q-state (metastable at room temperature) and s-state (steady-state) is described An analysis of the light self action in s-type waveguides is performed and used for estimating the waveguide conductivity

On the Phase Match in Multiplex CARS of Condensed Media

Abstract: The phase-matching conditions in condensed-media multiplex CARS are analyzed for the case where a spectral region is recorded according to variation of the pump frequency. The possibility of eliminating the influence of the phase mismatch on the recorded CARS spectra and of experimentally evaluating the third-order electronic susceptibility tensor is discussed.

Physics of negative refractive index materials

Abstract: In the past few years, new developments in structured electromagnetic materials have given rise to negative refractive index materials which have both negative dielectric permittivity and negative magnetic permeability in some frequency ranges. The idea of a negative refractive index opens up new conceptual frontiers in photonics. One much-debated example is the concept of a perfect lens that enables imaging with sub-wavelength image resolution. Here we review the fundamental concepts and ideas of negative refractive index materials. First we present the ideas of structured materials or meta-materials that enable the design of new materials with a negative dielectric permittivity, negative magnetic permeability and negative refractive index. We discuss how a variety of resonance phenomena can be utilized to obtain these materials in various frequency ranges over the electromagnetic spectrum. The choice of the wave-vector in negative refractive index materials and the issues of dispersion, causality and energy transport are analysed. Various issues of wave propagation including nonlinear effects and surface modes in negative refractive materials (NRMs) are discussed. In the latter part of the review, we discuss the concept of a perfect lens consisting of a slab of a NRM. This perfect lens can image the far-field radiative components as well as the nearfield evanescent components, and is not subject to the traditional diffraction limit. Different aspects of this lens such as the surface modes acting as the mechanism for the imaging of the evanescent waves, the limitations imposed by dissipation and dispersion in the negative refractive media, the generalization of this lens to optically complementary media and the possibility of magnification of the near-field images are discussed. Recent experimental developments verifying these ideas are briefly covered. (Some figures in this article are in colour only in the electronic version)

Efficient numerical approach to the evaluation of Kramers–Kronig transforms

Abstract: A method is presented to deal with the numerical evaluation of Kramers–Kronig transforms (the Hilbert transforms of even and odd functions on the positive real axis). The general Hilbert transform is also treated. The functions involved must be continuous on the integration interval with suitable asymptotic behavior for large values of the argument and must have an appropriate functional form in the vicinity of the singularity of the integrand of the transform. The approach is based on a specialized Gaussian quadrature technique that uses the weight function log x-1. This choice allows the region in the vicinity of the singularity to be swept into the quadrature weights and abscissa values. Application to the Lorentzian and Gaussian line profiles is discussed.

Generalized Kramers-Kronig relations in nonlinear optical- and THz-spectroscopy

Abstract: Kramers?Kronig (K?K) relations have constituted one of the principal tools in the optical spectroscopy for the assessment of the optical properties of media from measured spectra. The underlying principle for the existence of the K?K relations is causality. Thanks to the K?K relations we have achieved a better understanding of both macroscopic and microscopic properties of media.Recently, various kinds of modified K?K relations have been presented in the literature. Such relations have been applied, e.g. to the nonlinear optical properties of polymers. A typical advantage of these generalized K?K relations is that the measured data do not need to be manipulated as in the case of the traditional K?K relations. Hence, the accuracy of the inverted data on linear or nonlinear optical properties of media becomes higher.A novel way to utilize generalized K?K relations is related to the measurement and correction of terahertz spectra in the time-domain reflection spectroscopy. Terahertz spectroscopy is nowadays one of the most rapidly developing fields in modern physics with applications being related to, e.g. security at the airports or inspection of pharmaceutical tablets. While recording THz spectra it is also possible to perform a chemical mapping of species. Therefore, correctness of the spectrum is of crucial importance for the identification of different species. This is possible by the generalized K?K relations. In this review paper we consider advances of K?K relations both in nonlinear optical and THz spectroscopy.

Kramers-Kronig relations and sum rules in nonlinear optical spectroscopy.

Abstract: The full potential of the Kramers–Kronig relations and sum rules for nonlinear susceptibilities has unfortunately drawn relatively little attention in nonlinear optical spectra analysis. In this feature article a simple treatment of an anharmonic oscillator model in description of the nonlinear susceptibility of media and holomorphic properties of the nonlinear susceptibility were utilized. Using such concepts, conventional Kramers–Kronig, multiply-subtractive Kramers–Kronig, and generalized Kramers–Kronig dispersion relations can be derived. We demonstrate how in practice the variety of different Kramers–Kronig relations mentioned above, as well as various sum rules, can be applied in nonlinear optical spectra analysis. As an example we treat the third-harmonic wave generation spectrum from a polymer.

An algorithm for the nonlinear optical susceptibilities of dipolar molecules, and an application to third harmonic generation

Abstract: It is well known that a simple transformation of the electric dipole interaction provides a convenient means for ascertaining the effects of permanent dipoles on the optical behaviour of systems with a response dominated by two energy levels. By establishing the general validity of the procedure for parametric processes such as harmonic generation, it is shown how the detailed structure of the optical susceptibilities associated with arbitrary forms of optical nonlinearity can be ascertained by an algorithmic method, based on a novel interpretation of the relevant quantum electrodynamical Feynman diagrams. Application of the algorithm to second and third harmonic generation illustrates its usefulness and simplicity, whilst also providing new results and revealing features, related to the role of permanent dipoles, which have not hitherto been apparent.