共查询到20条相似文献,搜索用时 31 毫秒
1.
《Waves in Random and Complex Media》2013,23(3):197-211
Abstract Diffuse reflection of a narrow collimated light beam from a slab of a dense random medium is considered theoretically and experimentally for the case of the source and the detector fibres dipped into the random medium, as is the case for an endoscopic catheter. By use of the diffusion approximation, a simple and physically clear analytical expression is derived for the distribution of the diffuse reflectance of the light beam along the surface of the random medium slab. We include the effects of reflections from the top and bottom surfaces of the slab. The analytical expression derived predicts that, in the case of small absorption, the relative reflectance is a universal function defined by the geometrical parameters and is independent of specific properties of the random medium. The theoretical prediction is found to show good agreement with the results of measurements. 相似文献
2.
The refractive index of a dielectric medium comprising both passive and inverted components in its permittivity was determined using two methods: (i) in the time-domain, a finite-difference algorithm to compute the frequency-domain reflectance from reflection data for a pulsed plane wave that is normally incident on a dielectric half-space and (ii) in the frequency-domain, the deflection of an obliquely incident Gaussian beam on transmission through a dielectric slab. The dielectric medium was found to be an active medium with a negative real part for its refractive index. Thereby, a recent controversy in the scientific literature was resolved. 相似文献
3.
《Journal of Quantitative Spectroscopy & Radiative Transfer》1986,35(3):187-212
An analytical solution is developed for the radiation field engendered by illuminating an absorbing/anisotropically scattering planar medium with an azimuthally unsymmetric incident flux. The boundaries of the medium are allowed to possess both specular and diffuse reflection characteristics. The problem is mathematically reduced to solving a system of azimuthally independent equations which may be analyzed by many existing techniques. A solution scheme is formulated in terms of singular eigenfunctions and solved using a modified FN method. Computations are performed for the azimuthally symmetric case of irradiation by parallel rays at oblique incidence. Several different phase functions are considered and results are presented for the discrete eigenvalues, the heat flux distributions within the medium, the reflectance and transmittance of the slab and the angular distribution of the intensities at the boundaries. 相似文献
4.
5.
A criterion for evaluating beam quality is discussed.By dividing the mediumdisturbances into the ordered and random part, introducing the time-averaged variance ofphase distortion and fitting the wavefront with Zernike polynomials,a simplified formula forevaluating beam quality in a turbulent mixing medium is derived. 相似文献
6.
Mikhail I. Charnotskii 《Waves in Random and Complex Media》1994,4(3):243-273
Asymptotic analysis of on-axis intensity fluctuation variance of a finite-sized light beam is performed starting from the functional integral representation for the field in a random medium. Collimated, diverging and converging beams with different diffraction properties are considered. A complete set of asymptotes is obtained using the main and additional coherence channels approach to the fourth moment of field analyses. We study the conditions of weak and strong fluctuation regimes for all these types of beams and obtain relatively simple formulae for on-axis intensity variance, which are valid for the case of stratified turbulence. It is shown that scintillations in the vicinity of the beam focus are governed by double-scattering processes for weak scintillations and double scattering from coherence channels for strong fluctuations. 相似文献
7.
We consider the propagation of p-polarized electromagnetic waves obliquely incident on stratified random dielectric media. Using the invariant imbedding method generalized to random media and applying the random phase approximation, we derive a simple analytical expression of the localization length and calculate the disorder-averaged reflectance and transmittance and the fluctuations of the localization length and the reflectance as functions of the incident angle. We also calculate the disorder-averaged intensity profile of the magnetic field inside the random medium. We find that within the random phase approximation, the p wave can be delocalized and transmitted completely at a certain critical incident angle, which is bigger than the Brewster angle in the uniform case. 相似文献
8.
The modulation of phase and amplitude of an electromagnetic wave in a ferrite by a longitudinal, time-dependent (sinusoidally modulated) magnetic field has been investigated. In the case of an infinite medium, an analytical expression for the amplitude envelope of the wave is obtained and it is found that the envelope differs appreciably from the sinusoidal form. In the case of a ferrite slab, the amplitude of the transmitted and the reflected components of the magnetic vector have been evaluated by an expansion method. Numerical calculations indicate, as expected, that for a uniform plane wave incident on a ferrite sheet of finite thickness, the transmitted beam is strongly amplitude modulated. 相似文献
9.
M. V. Kabanov 《Russian Physics Journal》1966,9(5):11-13
A quantitative assessment of effects due to the interference of waves scattered by an ensemble of particles is given. It is shown that in the propagation of a collimated beam of light in a turbid medium, interference effects give rise to an additional stretching of the scattering indicatrix for a particle system, as compared with the scattering indicatrix for a single particle. The interference tongue obtained is concentrated in a very small angle in the direction of the light beam incident on the particle system. (Over a wide range of experimental conditions this angle is of the order of a few minutes.)Formulas are obtained for the instrumental scattering coefficient in the case of light scattering by volumes in the form of a sphere, disk, or rod. The analytical form of the formulas obtained indicates the possibility of concentration effects in experiments in a turbid medium. 相似文献
10.
Petrov NI 《Optics letters》2004,29(5):421-423
The reflection and transmission of tightly focused azimuthally and linearly polarized electromagnetic wave beams with subwavelength spot size at a dielectric interface are investigated. A substantial increase of the reflectance of a light beam that is normally incident from a higher-index medium to a lower-index medium and a decrease of the reflectance of a beam that is incident from a lower-index medium to a higher-index dielectric medium in comparison with the Fresnel reflectance are predicted. 相似文献
11.
S. E. Skipetrov S. S. Chesnokov S. D. Zakharov M. A. Kazaryan V. A. Shcheglov 《JETP Letters》1998,67(9):635-639
The time correlation function of light reflected diffusely from a semi-infinite randomly inhomogeneous medium is calculated
with allowance for the acceleration of the scatterers in the field of the laser beam incident on the medium. An analytical
expression is found for the characteristic coherence time due to the ponderomotive action of light. It is shown that even
with laser radiation power densities of the order of 1–10 W laser-acceleration effects substantially alter the character of
the time autocorrelation function of the scattered light and must be taken into account in theoretical calculations.
Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 611–615 (10 May 1998) 相似文献
12.
SpectralChangesofPartiallyCoherentLightTransmittedfromaDielectricSlabCHENHongping;LINQiang;LUXuanhui;WANGShaomin(Departmentof... 相似文献
13.
以高斯光束为例,研究了非线性介质表面振幅调制型缺陷对光场的调制。基于广义惠更斯-菲涅尔衍射积分并采用泰勒级数展开方法,建立了高斯光束经过表面存在缺陷的非线性介质后的传输模型,得到了受缺陷调制光束经非线性介质后的光强分布解析式,研究了缺陷尺寸和光束在非线性介质中产生的附加相移大小对光强分布的影响,结果表明介质表面的缺陷尺寸越大,介质内产生的附加相移越大,光场受到的调制越严重。分别考虑了非线性折射率为正值和为负值的情况,研究发现相应的折射率值导致光束发生会聚或发散现象,并由于缺陷调制的作用,在光束传输过程中始终存在一光强极值点,且随着附加相移绝对值的增大光强极值点的峰值也随之增强。 相似文献
14.
Negative and positive Goos–Hänchen shifts of a light beam transmitted from an indefinite medium slab
We study the lateral shift of a light beam transmitted from an indefinite medium slab. The analytic expressions for the transmission
coefficient and the lateral shift are obtained by using the stationary-phase method, and different conditions for the occurrence
of a negative lateral shift are deduced. It is shown that the thickness of the indefinite medium strongly affects the properties
of the lateral shift. For a larger thickness, the sign of the lateral shift depends only on the signs of the permittivity
and permeability, while, for a smaller thickness, the sign of the lateral shift is determined by the combined effect of the
incident angle of the light, the signs of the permittivity and permeability, and the thickness of the indefinite medium slab.
In addition, the lateral shift at the transmitted resonance points is found to be enhanced greatly, and the sign of the lateral
shift around the transmitted resonance points is strongly dependent on the permittivity and permeability of the indefinite
medium. Some examples are given to validate the obtained theoretical results.
PACS 41.20.Jb; 42.25.Gy; 42.25.Bs; 78.20.Ci; 78.68.+m 相似文献
15.
《Waves in Random and Complex Media》2013,23(1):86-103
Propagation and localisation of cylindrical waves in a two-dimensional (2D) isotropic and homogeneous random medium is studied using the stochastic functional approach. By expanding the random permittivity fluctuation in the form of a Wiener integral equation, and representing the wave fields by a linear combination of outgoing and incoming waves, the scalar Helmholtz equation is solved in the cylindrical coordinates system. An analytical expression of the cylindrical wave is derived and demonstrates the localisation phenomenon, as well as the wavenumber fluctuation in the random medium. Comparing with the waves in non-random medium, the wave transfer equation between plane wave and cylindrical wave in random medium shows an additional exponential factor to indicate the modulation effect owing to the medium randomness in both the amplitude and phase. Numerical simulations are presented to illustrate the functional dependence of the localisation phenomena. 相似文献
16.
A two-beam random interferometer is demonstrated where coupling is facilitated by a scattering medium. A modulation observed in the normalized second-order intensity frequency correlation of the transmitted light is attributed to the relative temporal delay of the two beams and is insensitive to beam alignment and spacing. 相似文献
17.
A. P. Vinogradov A. A. Zyablovsky A. V. Dorofeenko A. A. Pukhov 《Applied Physics A: Materials Science & Processing》2012,107(1):89-92
The problem of light propagation through a slab of gain medium is studied in terms of the transfer matrix pole distribution.
Identifying the lasing by the existence of the poles in the upper half-space of the complex frequencies, we demonstrate that
under the condition of total internal reflection lasing may be observed at a finite thickness of the slab; further increase
in the thickness results in the quenching of lasing. However, in the latter case the amplification of reflected wave is possible. 相似文献
18.
Vasil P. Yashchuk O. Prygodiuk V. Koreniuk E. Tikhonov V. Bezrodny 《Applied physics. B, Lasers and optics》2008,92(4):593-597
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. 相似文献
19.
M. Cheng R. Chen S. Feng 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,50(1):81-85
The lateral shift of a light beam at the surface of an anisotropic
metamaterial slab is investigated. Analytical expressions of the lateral
shifts are derived using the stationary-phase method, in the case that total
reflection does and does not occur at the first interface. The sign of the
lateral shift in two situations is discussed, and the necessary conditions
for the lateral shift to be positive or negative are given. It is shown that
the thickness and physical parameters of the anisotropic metamaterial slab,
as well as the incident angle of the light beam, strongly affect the
properties of the lateral shift, and numerical results validate these
conclusions. The effect of a lossy metamaterial on the lateral shift is also
investigated. A restriction on the thickness of the slab is obtained, which
is necessary for the stationary-phase method to remain valid. 相似文献
20.
Positive and Negative Lateral Shifts from an Anisotropic Metamaterial Slab Backed by a Metal
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The lateral shift of a light beam at the surface of an anisotropic metamaterial (AMM) slab backed by a metal is investigated. Analytical expressions of the lateral shifts are derived using the stationary-phase method, in the case that total reflection does and does not occur at the first interface. The sign of the lateral shift in two situations is discussed, and the necessary conditions for the lateral shift to be positive or negative are given. It is shown that the thickness and physical parameters of the AMM slab and the incident angle of the light beam strongly affect the properties of the lateral shift. Numerical results validate these conclusions. The lossy effect of the metamaterial on the lateral shift is also investigated. 相似文献