Temporal analysis of tissue displacement induced by a transient ultrasound radiation force

One of the stress sources that can be used in dynamic elastography imaging methods is the acoustic radiation force. However, displacements of the medium induced by this stress field are generally not fully understood in terms of spatial distribution and temporal evolution. A model has been developed based on the elastodynamic Green's function describing the different acoustic waves generated by focused ultrasound. The function is composed of three terms: two far-field terms, which correspond to a purely longitudinal compression wave and a purely transverse shear wave, and a coupling near-field term which has a longitudinal component and a transverse component. For propagation distances in the shear wavelength range, the predominant term is the near field term. The displacement duration corresponds to the propagation duration of the shear wave between the farthest source point and the observation point. This time therefore depends on the source size and the local shear modulus of the tissue. Evolution of the displacement/time curve profile, which is directly linked to spatial and temporal source profiles, is computed at different radial distances, for different durations of force applications and different shear elastic coefficients. Experimental results performed with an optical interferometric method in a homogeneous tissue-mimicking phantom agreed with the theoretical profiles.     

Near-field characteristics of highly non-paraxial subwavelength optical fields with hybrid states of polarization

The vectorial structure of an optical field with hybrid states of polarization(So P) in the near-field is studied by using the angular spectrum method of an electromagnetic beam. Physical images of the longitudinal components of evanescent waves are illustrated and compared with those of the transverse components from the vectorial structure. Our results indicate that the relative weight integrated over the transverse plane of the evanescent wave depends strongly on the number of the polarization topological charges. The shapes of the intensity profiles of the longitudinal components are different from those of the transverse components, and it can be manipulated by changing the initial So P of the field cross-section. The longitudinal component of evanescent wave dominates the near-field region. In addition, it also leads to three-dimensional shape variations of the optical field and the optical spin angular momentum flux density distributions.     

Soliton generation via continuous stokes acoustic self-scattering of hypersonic waves in a paramagnetic crystal

A new mechanism is proposed for continuous frequency down-conversion of acoustic waves propagating in a paramagnetic crystal at a low temperature in an applied magnetic field. A transverse hypersonic pulse generating a carrier-free longitudinal strain pulse via nonlinear effects is scattered by the generated pulse. This leads to a Stokes shift in the transverse hypersonic wave proportional to its intensity, and both pulses continue to propagate in the form of a mode-locked soliton. As the transverse-pulse frequency is Stokes shifted, its spectrum becomes narrower. This process can be effectively implemented only if the linear group velocity of the transverse hypersonic pulse equals the phase velocity of the longitudinal strain wave. These velocities are renormalized by spin-phonon coupling and can be made equal by adjusting the magnitude of the applied magnetic field. The transverse structure of the soliton depends on the sign of the group velocity dispersion of the transverse component. When the dispersion is positive, planar solitons can develop whose transverse component has a topological defect of dark vortex type and longitudinal component has a hole. In the opposite case, the formation of two-component acoustic “bullets” or vortices localized in all directions is possible.     

傍轴光束在单轴晶体中传输的矢量性质

研究了垂直光轴入射的傍轴光束在单轴晶体中传输的矢量性质.通过角谱的方法得到了寻常光（o光）和非常光（e光）在晶体中传输的严格解，并得到了傍轴近似下的矢量传输模型.利用这一模型，分析得到了垂直光轴偏振的入射光束在晶体中只激发o光，而平行光轴偏振的入射光束在晶体中同时激发e光和二阶小量的o光. 关键词： 傍轴传输 单轴晶体 矢量效应     

Polarimetric Measurement of Plasma Poloidal Magnetic Field via Heterodyne Phase Shift Methods

The rotation of the plane of polarization of radiation propagating through a magnetized plasma (Faraday effect) yields the line integral of the electron density times the longitudinal magnetic field component. A commonly used technique for this measurement senses the change in intensity of a laser beam after passing through a linear polarizer. Two methods often employed to facilitate detection are 1) to mix the transmitted beam with a frequency-offset reference beam to allow heterodyne detection and 2) to oscillate the polarization direction of the laser beam. In addition to being sensitive to spurious amplitude variations, such amplitude measurements are sensitive to small polarization ellipticities introduced by optical components as well as by transverse magnetic fields within the plasma. By the addition of a quarter wave plate, the Faraday rotation can alternatively be sensed as a phase shift of the heterodyne beat of two frequency-offset input beams relative to the case of no plasma. This scheme has the advantage of phase modulation over amplitude modulation, i.e., independence of absolute amplitude and weak dependence on amplitude change. We demonstrate with Jones matrix algebra how the measured phase shift depends only weakly on imperfections and angular alignments of the optical components. Moreover, the phase shifts can be increased more than an order of magnitude by deliberate modifications in the basic optical configuration at a sacrifice of comparable amounts of the amplitude modulation of the carrier from which the phase shift is determined.     

Transition radiation of seismic waves at the atmosphere-earth interface

The method of contour integration is used for solving the problem of transition radiation of elastic waves by a mass source, which travels uniformly in a gas normal to the interface between homogeneous gaseous and elastic halfspaces and disappears at the time it touches the solid surface. We have obtained asymptotic formulas for the field of transition radiation, which hold true near the fronts of longitudinal and transverse spherical waves and a conical wave. An exact analytical expression for the field of transition radiation has been obtained for the observation points located on the source trajectory extension in a solid. The influence of interaction between longitudinal and transverse waves, which occurs on the surface of elastic medium, on the space distribution of the field of transition radiation is analyzed. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 10, pp. 1210–1223, October, 1997.     

High-aperture binary axicons for the formation of the longitudinal electric field component on the optical axis for linear and circular polarizations of the illuminating beam

Diffraction of uniformly polarized laser beams with vortex phase singularity is theoretically analyzed using the plane wave expansion. It is shown that for a high numerical aperture, an intense longitudinal electric field component is formed on the optical axis in this case. It is numerically demonstrated that an analogous effect is ensured for diffraction of a conventional Gaussian beam from asymmetric binary axicons. The field intensity on the optical axis can be varied either by rotating the optical element or by changing the direction of polarization of radiation.     

非均匀光纤中梯度项对模式的影响

对弱导非均匀折射率分布光纤的严格理论分析表明,显然在纵向场分量的标量波动方程中能够保留▽δ梯度项,但同时在横向场分量的方程中必须略去梯度项.并以TM模为例,给出了梯度项影响的具体表达式;消除了TM与TE模式之间的简并,明确了单模工作条件.     

Effect of a longitudinal wave on the self-resonant motion of electrons

The effect of the longitudinal component of an electromagnetic wave on the self-resonant motion of electrons in waveguides is considered. The equation of motion for electrons in a plane, longitudinal-transverse, circularly polarized wave has been solved by the method of successive approximations with allowance for the longitudinal integral of motion present in such a wave. The expansion parameter is the ratio of the amplitudes of the longitudinal and transverse components of the wave. For the case n < 1 the longitudinal field of the wave does not change the nature of the electron motion, but only induces weak oscillations about a solution that does not include the longitudinal field. For n > 1 with spontaneous phasing of the electrons a criterion is obtained for neglecting the effect of the longitudinal field, and numerical estimates are given.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 66–71, September, 1978.     

Optical vortices in low-mode fibers: III. Dislocation reactions, phase transitions, and topological birefringence

Topological birefringence of waves in optical fibers resulting from the spin-orbit interaction in the field of optical vortices is manifested, as a rule, in the form of Rytov-Magnus unified optical effect. At the same time, the field transformations caused by this effect are not explicitly related to the evolution of phase dislocations of longitudinal and transverse components of the electric and the magnetic fields. This relation can be provided by the dislocation reactions proposed by Berry. As opposed to the Berry’s approach, where dislocation reactions at the wavefront surface are considered, it is suggested in this work that topological reactions at the specific characteristic surface of the wave field formed by the coordinate representation of the transverse components of the Poynting vector be considered. Using the action of topological birefringence in a low-mode optical fiber as an example, it is shown that the course of a topological reaction in a vector optical field is accompanied by rigorous conservation of the total topological index of the characteristic surface and does not depend on the presence of an interface (where topological charges can originate and annihilate). The total topological index of a dislocation reaction is found to be equal to the absolute value of the sum of the topological charge and the spirality of the vector wave field.     

Coherence Volume of an Optical Wave Field with Broad Frequency and Angular Spectra

We consider the sizes of a region in a three-dimensional space in which an optical wave field excites mutually coherent perturbations. We discuss the conditions under which the length of this region along the direction of propagation of the wave field and, correspondingly, its volume are determined either by the width of the frequency spectrum of the field or by the width of its angular spectrum, or by the parameters of these spectra simultaneously. We obtain expressions for estimating extremely small values of the coherence volume of the fields with a broad frequency spectrum and an extremely broad angular spectrum. Using the notion of instantaneous speckle-modulation of the wave field, we give a physical interpretation to the occurrence of a limited coherence volume of the field. The length of the spatiotemporal coherence region in which mutually coherent perturbations occur at different times is determined. The coherence volume of a wave field that illuminates an object in high-resolution microscopy with frequency broadband light is considered. The conditions for the dominant influence of the angular or frequency spectra on the longitudinal length of the coherence region are given, and the conditions for the influence of the frequency spectrum width on the transverse coherence of the wave field are examined. We show that, when using fields with broad and ultrabroad spectra in high-resolution microscopy, this influence should be taken into account.     

Optimization of focusing of linearly polarized light

We show that, by adding a π-phase shift to one-half of a linearly polarized beam, the roles of the transversal and longitudinal field components of the focused beam are interchanged, resulting in better focusing of the longitudinal component in the direction perpendicular to the phase jump line. For this component the scheme produces a spot with FWHM >15% smaller than a spot generated with either linearly or radially polarized light for any NA. The scheme has a similar advantage when applied to circularly polarized light, and it holds for both a plane wave and a realistic case of a Gaussian incident beam. This technique may find applications when using recording media responsive to the longitudinal field only, particularly in read/write for optical storage where the resolution in one transverse dimension is most important.     

A novel design for high gain lens antennas with homogeneous media

A high gain lens antenna is designed by using a new transformation different from the discrete optical transformation. The antenna is composed of two blocks. Each block is made of homogenous and anisotropic materials, and thus can be easily achieved by metamaterial. The numerical results based on full wave simulation indicate that the antenna can be used to realize highly directive radiation beam, and the direction of radiation beam can be controlled artificially by changing the geometry parameters of the device. The electromagnetic field in the transformation region can be either stretched or compressed along both transverse and longitudinal directions by varying the geometry parameters in the virtual space while the distribution of electromagnetic field outside the antenna is little influenced. Moreover, effective medium theory is applied to realize such an antenna with isotropic materials. Also, the multi-beams antenna is investigated. It is indicated that this antenna can generate multi-collimated beams radiating at the desired angles.     

SRS generation of anti-Stokes radiation under phase quasi-matching conditions

New methods of generation of anti-Stokes radiation of stimulated Raman scattering in media with changeable parameters of the third-order nonlinearity χ⁽³⁾ along the longitudinal coordinate are proposed. The conditions of obtaining phase quasi-matching in different media and reaching the maximum efficiency of conversion into the anti-Stokes component of stimulated scattering are determined by numerical methods. The dependence of the efficiency of energy transfer from the pump wave to the anti-Stokes wave on the ratio of intensities of the pump and Stokes waves at the input of the medium is studied. The models of the media in which the efficiency of generation of anti-Stokes radiation exceeds 30% are obtained. The results of the investigation can be used for the development of new efficient nonlinear optical devices upconverting the laser radiation frequency.     

What type of coherence of the optical field is observed in the Michelson interferometer

The types and corresponding coherence functions of the optical field are considered depending on the frequency and angular spectra of the field. The main concepts of the theory of coherence effects in the interference experiment with the amplitude splitting of the initial field are discussed. It is shown that, in strict correspondence with the theory of coherence of the random wave fields, the Michelson interferometer reveals manifestations of the transverse and longitudinal spatial (rather than temporal, as it is commonly adopted) coherence of the optical field; the purely temporal coherence of the optical field is revealed only under special conditions of the interference experiment. Beyond these conditions, either spatial or spatiotemporal coherence is revealed.     

Conditions of similarity of the cross-spectra of pseudosound pressures in turbulence

Nondimensional cross-spectra of pseudosound wall-pressures in turbulence are considered in the framework of model fields of progressively increasing complexity. Conditions for the existence of the similarity of the nondimensional cross-spectra for frequency and spacing are studied; factors that lead to the violation of this similarity are considered. It is shown that, for the similarity of both longitudinal and transverse spectra, a wide distribution of the field energy in the wave numbers is necessary in combination with a small dispersion of the transport velocity of the wave components of the field. For the longitudinal spectrum, this condition is also a sufficient one. For the similarity of the transverse spectrum, the wave-number spectrum of the field should have some special form. The absence of the field energy in some region of the wave space leads to the violation of similarity of the cross-spectra at the corresponding frequencies. The results of the calculations of the longitudinal and transverse spectra are presented. The calculations are performed in the framework of the proposed model; they illustrate the effects of different parameters of the field on the properties of the cross-spectra.     

Sharper focus for a radially polarized light beam

We experimentally demonstrate for the first time that a radially polarized field can be focused to a spot size significantly smaller [0.16(1)lambda(2)] than for linear polarization (0.26lambda(2)). The effect of the vector properties of light is shown by a comparison of the focal intensity distribution for radially and azimuthally polarized input fields. For strong focusing, a radially polarized field leads to a longitudinal electric field component at the focus which is sharp and centered at the optical axis. The relative contribution of this component is enhanced by using an annular aperture.     

Angular momentum of gaussian light beams

Analysis of the orbital angular momentum of paraxial light beams shows that a key role in the formation of this quantity is played by phase relations between longitudinal and transverse radiation fields. When a light beam is circularly polarized or has a helical wave front, the azimuthal component of the Poynting vector and the density of orbital angular momentum prove to be non-zero. In the case of circularly polarized radiation, the azimuthal component of the Poynting vector and the density of the orbital angular momentum can change the sign at different points in the cross section of the light beam, while the total orbital momentum of the beam remains quantized.     

Can a linearly polarized light beam polarize atomic spin?

Lax et al. [Phys. Rev. 11 (1975) 1365] discovered that a light beam in vacuum is not a transverse wave but does have a longitudinal field component. We investigate atomic and molecular electric dipole transitions induced by such a light beam, in particular, linearly polarized in a transverse plane. We derive the selection rules and the transition rates for various quantization axes using the paraxial approximation up to the first order of 1/kw, where k is the wave number and w is the transverse size of the light beam. The light beam is able to yield atomic spin polarization in the direction perpendicular to both the optical axis and the transverse electric field, and its magnitude is approximately 1/kw times that generated by a circularly polarized light wave with the similar intensity.