Propagation of a stochastic electromagnetic Gaussian Schell-model beam through an optical system in turbulent atmosphere

A generalized diffraction integral formula for stochastic electromagnetic beams propagating through an optical system in turbulent atmosphere is derived with the help of tensor method. Some analyses are illustrated by a numerical example relating to changes in the average intensity and the degree of polarization of an electromagnetic Gaussian Schell-model beam propagating through a double-lenses system. It is shown that the optical system has strong influence on the propagation properties of the beam. The method used in this paper can be widely applied to the propagation of astigmatic beams through an optical system in turbulent atmosphere.     

Stochastic electromagnetic beams for LIDAR systems operating through turbulent atmosphere

With the help of the generalized Huygens–Fresnel integral and the ABCD matrix approach a bistatic LIDAR system involving a rough target at a distant location in a turbulent atmosphere is modeled. The system operates by means of an optical beam which has arbitrary spectral composition, and states of coherence and polarization. The rough target is modeled as a combination of a Gaussian mirror and a thin phase screen which induces phase perturbations of the components of the electric field. The analytical form of the cross-spectral density matrix of the returned beam is determined, from which the effect of the rough target on the spectral density (intensity) and polarization of the returned wave is analyzed. E. Watson’s work performed while assigned to the Ladar and Optical Communications Institute, University of Dayton, Dayton, OH 45469, USA.     

Flat-topped spectrum obtained from partially coherent hollow Gaussian Schell-model beams passing through a slit

The anomalous spectral behaviors of partially coherent polychromatic hollow Gaussian Schell-model beams (HGSMBs) diffracted by a slit are investigated. Besides spectral switches, asymmetrical dual-peak spectral split and multi-peak spectral split are observed. In the vicinity of the positions where spectral switches occur, flat-topped spectrum can be obtained, the bandwidth of which is adjustable by changing the transverse coherence width and the observation position. The flat-topped spectrum has potential application for creating probe beams in optical metrology systems and generating broadband optical sources in fiber optic sensor interface systems.     

Illustration of a bimodal system in Intralipid-20% by polarized light scattering: experiments and modeling

Intralipid suspensions behave like phantoms of human tissues concerning their light scattering properties. We present experimental measurements of the angular distribution of polarized light scattering at various incidence angles on Intralipid-20%. A comparison of the absolute values of these measurements with simulations using a vector radiative transfer model (N-flux) developed for multilayered media demonstrates a stratified structure of the samples with a double distribution of the size of scatterers. This result is confirmed by polarimetry imaging.     

Considerations about Z-scan sensitivity improvement: theory versus experiments

This paper reports an experimental demonstration of the improvement of the Z-scan technique’s sensitivity. It is shown that this sensitivity can be multiplied by a factor equal to almost 400 with the help of simple binary diffractive elements. Such a possibility was actually predicted theoretically in one of our previous papers. In this study, the interpretation is investigated in a wider context taking into account the definition of the signal normalisation as introduced by Z-scan and the well-known eclipsing Z-scan (EZ-scan) experiments. In particular, advantages and drawbacks are compared, by looking at the normalised or the unnormalised aperture transmission.     

Changes in the coherence of quasi-monochromatic electromagnetic Gaussian Schell-model beams propagating through turbulent atmosphere

Based on the extended Huygens-Fresnel principle, the mutual coherence function of quasi-monochromatic electromagnetic Gaussian Schell-model (EGSM) beams propagating through turbulent atmosphere is derived analytically. By employing the lateral and the longitudinal coherence length of EGSM beams to characterize the spatial and the temporal coherence of the beams, the behavior of changes in the spatial and the temporal coherence of those beams is studied. The results show that with a fixed set of beam parameters and under particular atmospheric turbulence model, the lateral coherence of an EGSM beam reaches its maximum value as the beam propagates a certain distance in the turbulent atmosphere, then it begins degrading and keeps decreasing along with the further distance. However, the longitudinal coherence length of an EGSM beam keeps unchanging in this propagation. Lastly, a qualitative explanation is given to these results.     

Propagation characteristics of stochastic electromagnetic array beams

A stochastic electromagnetic array beam that is generated by an electromagnetic Gaussian Schell-model source is introduced by using tensor method. An analytical expression for the cross-spectral density matrix of the stochastic electromagnetic array beam propagating in a paraxial ABCD optical system is derived after performing vector integration. Some numerical calculations are illustrated for the propagation characteristics of such an array beam in free space and fractional Fourier transform (FRFT) system as application examples.     

Fringe waves radiated by a half-plane for the boundary conditions of Neumann

The fringe waves of the physical theory of diffraction are obtained in terms of Fresnel integrals for a half-plane satisfying the Neumann boundary condition. The approximate expressions of the radiated waves are also evaluated for sufficiently large wavenumbers. The fields are plotted and compared numerically.     

Change in degree of coherence of partially coherent electromagnetic beams propagating through atmospheric turbulence

We study the change in the degree of coherence of partially coherent electromagnetic beam (so called electromagnetic Gaussian Schell-model beam). It is shown analytically that with a fixed set of source parameters and under a particular atmospheric turbulence model, an electromagnetic Gaussian Schell-model beam propagating through atmospheric turbulence reaches its maximum value of coherence after the beam propagates a particular distance, and the effective width of the spectral degree of coherence also has its maximum value. This phenomenon is independent of the used turbulence model. The results are illustrated by numerical curves.     

Comparison of gold and silver dispersion laws suitable for FDTD simulations

We report optical planar waveguide formation and modal characterization in β-BaB₂O₄ crystals by Cu²⁺-ion implantation at an energy of 3.0 MeV and doses of ∼ 10¹⁴ ions/cm². The prism-coupling method was used to investigate the dark-mode property at wavelengths of 633 nm and 1539 nm. The refractive-index profile of the waveguide was reconstructed by an effective refractive index method. The modal analysis indicates that the fields of TM modes can be well restricted in the guiding region, which means the formation of a non-leaky waveguide in the crystal. The results show that the β-BaB₂O₄ waveguides may be used in the application of high efficiency frequency conversion. PACS 61.80.Jh; 42.70.Mp; 42.65.Wi     

Random lasing in porous scattering medium

The spectrum behavior evolution and the threshold of random lasing depending on the way of photon walk randomization in an active random medium were investigated. The following three ways of photon walk randomization were implemented: multiple light scattering by corundum and silica particles embedded into a solid polymer solution of dye (astrafloxin), multiple light reflection at sub-millimeter extensive air pores (mean diameter 200 μm) produced in the medium, and the combined action of both these effects. The most effective lasing is observed in the case of an active medium with air pores and scattering particles in the interpore space. Such a combined porous scattering medium acts as a network of dielectric waveguides transmitting effectively the random light. This spatial structure of the random active medium significantly increases the photon path in the medium, thereby promoting photon multiplication due to stimulated emission. In this combined medium the random lasing reveals the narrowest spectrum, the lowest threshold, and the highest density of spectral energy in the spectrum maximum.     

Nonlinear coherent backscattering

We theoretically examine diffusive transport and coherent backscattering of waves in dilute disordered media with weak nonlinearity. Depending on the type of nonlinearity, the coherent backscattering interference is either reduced or enhanced, as compared to the linear case.     

Directional terahertz radiation through groove-assisted hole array

The authors theoretically demonstrate the terahertz beam shaping with metallic subwavelength holes array surrounded by concentric periodic grooves. High transmission and directional radiation can be obtained simultaneously for the resonant excitation of the surface wave in the combined structure. Finite-difference time-domain simulation results shows that the transmission mainly depends on the lattice constant of hole array and the features of incident surface around it, while the far-field angle distribution is dominated by the details of the output surface. This compact beam shaping structure is hoped to serve as a basic device for future terahertz systems.     

Ultrafast dynamics of surface electromagnetic waves in nanohole array on metallic film

We study the ultrafast dynamics of surface electromagnetic waves photogenerated on aluminum film perforated with subwavelength holes array by means of transient photomodulation with ∼100 fs time resolution. We observed a pronounced blueshift of the resonant transmission band that reveals the important role of plasma attenuation in the dynamics and that is inconsistent with plasmon–polariton mechanism of extraordinary transmission. The transient photomodulation spectra were successfully modeled within the Boltzmann equation approach for the electron–phonon relaxation dynamics, involving non-equilibrium hot electrons and quasi-equilibrium phonons.     

Generalized tensor ABCD law for an elliptical Gaussian beam passing through an astigmatic optical system in turbulent atmosphere

The propagation of an elliptical Gaussian beam (EGB) through an astigmatic ABCD optical system in a turbulent atmosphere is investigated. An analytical formula for the average intensity of an EGB and a generalized tensor ABCD law for the generalized complex curvature tensor are derived. As an application example, we derived an analytical formula for the average intensity of an elliptical flat-topped beam propagating through an astigmatic ABCD optical system in a turbulent atmosphere. As a numerical example, the focusing properties of an EGB focused by a thin lens in a turbulent atmosphere are studied. It is found that the focused beam at the focal plane becomes a circular Gaussian beam when the atmospheric turbulence is strong enough, and the beam width of the circular Gaussian beam is determined by atmospheric turbulence strength, focal length of the thin lens, and wavelength of the initial beam but is independent of the initial beam widths (i.e., initial intensity distribution).     

Self-trapping of weak optical beams in photorefractive materials

Intensity-independent self-trapping of optical beams in photorefractive crystals was found recently. But we show that due to the existence of dark conductivity in some photorefractive materials or coherent and non-coherent background irradiance, the self-trapping of weak optical beams in photorefractive materials, such as SBN, becomes intensity dependent. The threshold condition of the applied external de electric field for self-trapping is determined as function of the material parameters as well as the initial transverse sizes and relative intensities of the beams.     

Propagation of partially polarized, partially coherent beams in uniaxial crystals orthogonal to the optical axis

The propagation of partially polarized and partially coherent beams in uniaxial crystals orthogonal to the optical axis is investigated. The analytical formulae for the elements of the cross-spectral density matrix of partially polarized and partially coherent beams propagating through uniaxial crystals orthogonal to the optical are derived. The numerical results show that the degree of polarization decreases with the increase of the ratio of extraordinary to ordinary refractive indices at a certain propagation distance, and the influence of uniaxial crystals on the degree of coherent is not so evident. And the beams spread more rapidly in the direction parallel to the optical axis than orthogonal to the optical axis in positive crystal with the propagation distance increasing.     

双曲余弦高斯列阵光束通过湍流大气传输的角扩展及方向性

 采用积分变换,推导出了双曲余弦高斯（ChG）列阵光束通过湍流大气传输的二阶矩束宽和角扩展的解析公式,给出了ChG列阵光束与一束高斯光束具有相同角扩展的条件。研究表明:相干合成的ChG列阵光束的角扩展比非相干合成的小,但是,非相干合成的ChG列阵光束的角扩展受湍流影响比相干合成ChG光束小;相干合成情况下,ChG列阵光束的角扩展随离心参数、束腰宽度和相对子光束间距的变化均出现振荡,但在湍流中的振荡减弱,非相干合成情况下,ChG列阵光束的角扩展与相对子光束间距和光束数无关。     

Cluster growth in an ablation plume propagating through a buffer gas

A phenomenological mixed-propagation model that describes the expansion of an ablation plume through a buffer gas is introduced. Selected experiments including LaMnO₃ and tin ablation in oxygen, as well as tungsten ablation in argon, are analysed. For given ablation conditions the expansion parameters required to model the growth of clusters in the expanding plasma plume are deduced and the average asymptotic size of the clusters is calculated and compared (for tungsten) with the size of clusters measured by transmission electron microscopy.     

双曲余弦高斯列阵光束通过湍流大气传输的角扩展及方向性

采用积分变换,推导出了双曲余弦高斯（ChG）列阵光束通过湍流大气传输的二阶矩束宽和角扩展的解析公式,给出了ChG列阵光束与一束高斯光束具有相同角扩展的条件。研究表明:相干合成的ChG列阵光束的角扩展比非相干合成的小,但是,非相干合成的ChG列阵光束的角扩展受湍流影响比相干合成ChG光束小;相干合成情况下,ChG列阵光束的角扩展随离心参数、束腰宽度和相对子光束间距的变化均出现振荡,但在湍流中的振荡减弱,非相干合成情况下,ChG列阵光束的角扩展与相对子光束间距和光束数无关。