(PR) crystals are photoconductive, specific doped electrooptics materials. When an optical beam propagates in a PR crystal, charge carriers are excited from photosensitive impurities into the conduction band where they move due to drift and diffusion and eventually are trapped in darker regions of the light intensity pattern. The resulting space-charge redistribution produces the spatially nonuniform internal electric field which in turn modulates the material refractive index through the linear electro-optic effect. Under an appropriate polarity of an external voltage, a local variation in the index of refraction leads to self-focusing or self-defocusing effect of an optical beam. This allows forming bright or dark soliton states when the nonlinearity compensates exactly the diffraction spreading of a light beam. As a result, the beam propagates in a PR medium without changing its transverse profile. For one-dimensional bright spatial solitons, a polarized optical beam in the form of a narrow stripe is launched at the entrance of a crystal together with orthogonally polarized background illumination. In the case of dark soliton, a black notch is superimposed on an otherwise uniform background illumination. Both types of solitons can be generated in the same crystal by reversing the bias voltage direction.Due to a possibility of soliton creation at very low laser power levels, and their potential applications in all optical switching devices, PR screening solitons have been subjected to intensive theoretical and experimental works for the last two decades. So far, the formation of (1 + 1)D solitary waves has been demonstrated experimentally in various PR materials such as ferroelectrics (SBN) [3][4][5], (LiNbO 3 ) [11,12], sillenities (BTO) [6,7], (BSO) [13] centrosymmetric paraelectrics (KLTN) [8], semiconductors (InP:Fe) [9,14], and (CdZnTe) [15] both in geometry with bulk PR crystals and in planar waveguides [10,16]. AbstractIn the present paper, the problem of one-dimensional screening photorefractive solitons is reconsidered in the context of the accordance of soliton solutions with the Kukhtarev-Vinetskii model. In all theoretical and experimental works dealing with the analysis of such type solitons, one assumes that under the slowly varying approximation for the optical field amplitude the reduced form of photorefractive rate equations can be employed. In this work, we point out that the crucial and commonly accepted approximation within this scheme has a limited range of applicability as regards dark solitons. This author proposes a relatively simple modification of the standard saturable photorefractive response formula to obtain the plausible self-consistent solutions. The improved solutions for screening black solitons have been derived and discussed by comparison with standard solutions.
A general approach to the approximate analytical solution of photorefractive transport equations for arbitrary fringe contrast is presented. The method based partially on the perturbative scheme permits us to find the stationary space–charge field distribution inside a photorefractive crystal for a two-wave mixing (TWM) geometry with a DC electric field. The method can be employed for various band transport models. In the present work two distinct electron–hole conduction models are investigated. The solutions are compared with the results of numerical calculations and good agreement is found.
Części 1÷9 i 1÷10 Eurokodu 3 oraz PN-EN ISO 5817:2014 uporządkowały reguły projektowania konstrukcji stalowych narażonych na zmęczenie. Podane klasy zmęczeniowe FAT określono wg jednakowego algorytmu dla badanych elementów poziomu jakości B i C. W artykule wykazano, że algorytmu tego nie można stosować dla elementów starzonych oraz z niezgodnościami spawalniczymi, karbami poziomu jakości D i większymi. Wymagania dotyczące zmęczenia z części 1÷9 Eurokodu 3, podobnie jak udarności z części 1÷10, nie powinny dotyczyć konstrukcji eksploatowanych.
Abstract:In our work we propose a novel method of analysis of photorefractive transport equations. The method based on a perturbative approach can be used in the case of two wave mixing and four wave mixing geometry, i.e. for the samples illuminated by interference patterns. Presented approach can be employed for a broad range of material and experimental parameters, particularly for arbitrary depth of light modulation pattern. The approximate analytical solution is compared with results of numerical calculations and a good agreement practically in every case was found. PACS
Nonlinear transport of hot electrons in semi-insulating GaAs / AlGaAs quantum wells significantly affects their photorefractive properties. In case of two waves mixing, this influence consists, among others, in an increased shift of photorefractive grating relative to light intensity distribution. The influence of nonlinear transport on grating recording time is less examined experimentally and theoretically. This study compares numerical and analytical solutions describing grating dynamics in approximation of small fringe contrast. The influence of nonlinear electron mobility on space-charge field was examined depending on external electric field intensity and on the grating constant. It was found that in the electric field range below 20 kV/cm, the nonlinear transport of electrons does not shorten the grating generation time.
with wave mixing process in PR media considered recording of a photorefractive grating resulting from the interference of two plane waves. In the mid-1990's theoretical predictions after soon confirmed experimentally revealed the possibility of generating photorefractive spatial solitonsoptical beams that propagate in PR media without spatial divergence. Solitary waves are formed when the intensity dependent changes in refractive index cause self-focusing nonlinearity (for bright solitons) or self-defocusing nonlinearity (for dark solitons) which exactly balances the beam diffraction. Relative ease of experiments and possibility of obtaining PR solitons at very low optical power caused led to extensive studies of such beams.So far, four different types of solitons have been discovered: quasi-steady-state solitons, screening solitons, photovoltaic solitons and screening-photovoltaic solitons. The greatest interest was focused on screening solitons 1D-slab beams confined only in one transverse direction. This kind of self-trapping beams, which can exist in the steady state regime in biased non-photovoltaic photorefractive crystals, was predicted in works [2, 3] and then observed in various PR materials. Photorefractive nonlinearity as a macroscopic phenomenon allows supporting spatial solitons at very low optical power levels. This feature, however, is associated with a slow response time of PR materials. The first theoretical time-dependent analysis concerning self-focusing in PR media were performed by Zozulya and Anderson [4,5], Soon after, Fressengeas et al. [6] presented the explicit wave propagation equation including temporal behavior of spatial solitons. In all mentioned papers [2-6] the starting point was the K-V band transport model, where analogous simplifying assumptions were formulated to determine the refractive index variation. The solution given in [6] was regarded as able to describe screening and photovoltaic bright solitons Abstract The validity of the commonly used time-dependent wave equation describing the propagation of screening one-dimensional solitons in photorefractive materials is discussed. Concentrating attention on temporal development of the space-charge field, we show that the widely used standard solution follows from a phenomenological description, which is consistent with the band transport model equations only in specific cases. The exact analytical solution for the localized optical beam is derived within the microscopic model under a low contrast approximation. The numerical modeling of photorefractive response to an arbitrary contrast is performed and compared with standard solutions. The range of applicability of the macroscopic approach for three basic classes of photorefractive crystals is discussed.
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