共查询到20条相似文献,搜索用时 15 毫秒
1.
The optical trapping characteristics of highly focused higher-order radially polarized beams (R-TEMp1*) acting on a Rayleigh particle are studied theoretically. Numerical results show that as the order p of beam increases and the numerical aperture NAo of the objective decreases, the axial trap distance increases but the trap depth and maximum restoring force decreases. In a limit of NAo = 1, three higher-order R-TEMp1* beams of p = 1, 2, 3, like the fundamental lowest-order radially polarized beam of p = 0, can three-dimensionally trap a particle to the focus but the axial trap stiffness decreases with the increase of p. When NAo = 0.95, the focus is still a stable trap point for the two beams of p = 0 and 1 but it becomes an unstable trap point for the two beams of p = 2 and 3. The trap stability is also discussed for higher-order radially polarized beam illumination. 相似文献
2.
Xiaoqing Li 《Optics Communications》2009,282(1):7-1239
The influence of atmospheric turbulence on the propagation of superimposed partially coherent Hermite-Gaussian (H-G) beams is studied in detail. The closed-form propagation equation of superimposed partially coherent H-G beams through atmospheric turbulence is derived. It is shown that the turbulence accelerates the evolution of three stages which superimposed partially coherent H-G beams undergo. The turbulence results in a beam spreading and a decrease of the maximum intensity. However, the larger the beam number M, the beam order m, the separate distance xd, and the smaller the beam correlation length σ0 are, the less the power focusability of superimposed partially coherent H-G beams is affected by the turbulence. Specially, superimposed partially coherent H-G beams are less sensitive to turbulence than superimposed fully ones, and than partially coherent H-G beams if the beam power focusability and the maximum intensity are taken as beam criterions. However, the maximum intensity of superimposed partially coherent H-G beams is less sensitive or more sensitive to turbulence than that of superimposed Gaussian Schell-model (GSM) beams depending on σ0. 相似文献
3.
Based on the vectorial Rayleigh diffraction integrals, the analytical expression for the spectral intensity of a vectorial nonparaxial ultrashort pulsed Gaussian beam diffracted at a circular hard-aperture is derived. The effect of f-parameter (f = 1/k0w0) on the spectral anomalies near phase singularities of the vectorial nonparaxial ultrashort pulsed beams is studied. It is shown that the spectral switch near the phase singularity of diffracted vectorial nonparaxial ultrashort pulsed beam still exists beyond paraxial regime, but disappears when the f-parameter is larger than a certain value. 相似文献
4.
推导出部分相干厄米-双曲正弦-高斯(H-ShG)光束通过湍流大气的平均光强和角扩展的解析表示式,并用以研究了部分相干H-ShG光束在湍流中的平均光强分布演化和角扩展. 结果表明,折射率结构常数C2n的增加和空间相关长度σ0的减小都会加速演化过程. 引入相对角扩展来定量描述光束抗拒湍流的能力. 空间相关长度σ0,束腰宽度w0和双曲
关键词:
平均光强分布演化
角扩展
湍流大气
部分相干厄米-双曲正弦-高斯(H-ShG)光束 相似文献
5.
Based on the generalized Rayleigh diffraction integral, the spectral shifts and spectral switches of nonparaxial partially coherent beams diffracted at an aperture are studied. Our main attention is paid to the effect of beam nonparaxiality on the spectral switches of partially coherent beams. It is shown that the spatial coherent parameter β, trunction parameter δ and waist width-to-central wavelength ratio w0/λ0 affect the behavior of spectral switches. Only when β, δ and w0/λ0 satisfy certain conditions, can the difference between the nonparaxial and paraxial results be negligible. The discrepancy arises physically from the change of the spectral modifier by using the paraxial approximation. 相似文献
6.
The propagation of a partially coherent Hermite–Gaussian beam in a slant turbulent channel is studied. The analytical formula for the quantum degree of polarization of a partially coherent Hermite–Gaussian beam is derived based on the quantum Stokes operators and the generalized Huygens–Fresnel principle. It is shown that the zenith angle slightly affects the polarization degree of partially coherent Hermite–Gaussian beams, and the changes of polarization degree are affected by the coherence length, the detection photon-number and the ground refractive index structure parameter. Furthermore, the numerical simulations show that a partially coherent Hermite–Gaussian beam with bigger coherence length, higher photon-number level, lower beam order, shorter wavelength is less affected by the turbulence. These results indicate that one can choose the partially coherent Hermite–Gaussian beam with bigger coherence length, lower beam order, shorter wavelength, higher detection photon-number and set the size of transmitting aperture w0 as about 0.065 m to improve the performance of a polarization-encoded free-space quantum communication system. 相似文献
7.
The definition of second-order intensity moments in the spatial domain and spatial frequency domain has been generalized for the case that the linear gain or absorbing media are included, where the wave number is generally complex. The formula for beam propagation M2-factor of partially coherent beams in linear gain or absorbing media has been given. The partially coherent flat-topped Schell-model beam is taken as an example. The closed-form expression for the beam propagation M2-factor of partially coherent flat-topped beam in gain or absorbing media has been derived. The changes of the M2-factor in media have been discussed with numerical examples. It can be shown that the M2-factor of flat-topped Schell-model beams in gain or absorbing media depends on the coherent parameter β, the coherent length σ0, the beam order M, the propagation distance B, the imaginary part of the wave number Ki, as well as the real part of the wave number Kr. 相似文献
8.
We analyze the finite temperature behavior of the Sakai-Sugimoto model, which is a holographic dual of a theory which spontaneously breaks a U(Nf)L × U(Nf)R chiral flavor symmetry at zero temperature. The theory involved is a 4 + 1 dimensional supersymmetric SU(Nc) gauge theory compactified on a circle of radius R with anti-periodic boundary conditions for fermions, coupled to Nf left-handed quarks and Nf right-handed quarks which are localized at different points on the compact circle (separated by a distance L). In the supergravity limit which we analyze (corresponding in particular to the large Nc limit of the gauge theory), the theory undergoes a deconfinement phase transition at a temperature Td = 1/2πR. For quark separations obeying L > Lc ? 0.97 ∗ R the chiral symmetry is restored at this temperature, but for L < Lc ? 0.97 ∗ R there is an intermediate phase which is deconfined with broken chiral symmetry, and the chiral symmetry is restored at TχSB ? 0.154/L. All of these phase transitions are of first order. 相似文献
9.
I.S. Osad'ko 《Optics Communications》2010,283(23):4671-4675
A single two-level molecule driven by CW-laser field and a photomultiplier tube (PMT) are considered as two parts of the united dynamical system connected with each other by photons of molecular fluorescence. Each PMT is characterized by a rate α of photo-effect and by a rate β of PMT recovery. A theory for the photon distribution function wN(t) and for the photoelectric pulse distribution function fn(t) for such a system is built up. If times 1/ α and 1/ β characterizing PMT are much shorter as compared to the average time interval 1/ k between two successively emitted photons of fluorescence, the photon and the photoelectron distribution functions coincide with each other, i.e. fn(t) ≅ wN(t). A relation between wN(t) and fn(t) is studied in detail for the case in which PMT works slower as compared to the rate k of photon emission, i.e. at 1/ α, 1/ β ≥ 1/ k. 相似文献
10.
Mitsutomo Hirota Takeshi Yokomori Yoshitaka Nagai Goro Masuya 《Proceedings of the Combustion Institute》2007,31(1):893-899
We investigated the local flame speed of a two-dimensional, methane-air triple flame in a rectangular burner. The velocity fields and the concentration profiles were measured with particle image velocimetry and the Rayleigh scattering method, respectively. There was a requisite combination of initial velocity and initial concentration gradient for consistency of the local concentration gradient at the leading edge of the flame. In these cases, the flame curvatures were also consistent. Accordingly, the burning velocity, defined as local flow velocity at the triple point, was determined by the flame curvature. The burning velocity increased with increasing flame curvature, when the curvature was near zero. After that, the burning velocity decreased with increasing curvature. The peak value thus exceeded the adiabatic one-dimensional laminar burning velocity. Comparing the effects of the measured flame stretch rate on the flow strain κs and flame curvature κc, κs is larger and increases more rapidly than κc for flame curvatures satisfying 1/Rf < 250 m−1 and then becomes constant while κc still increases for 250 m−1 < 1/Rf, so that κc becomes much larger than κs. There is also a peak in burning velocity at roughly the transition in flame curvature specified above. Therefore, the burning velocity for a low concentration gradient correlates with the flame stretch rate. 相似文献
11.
The present analysis investigates the (axial) acoustic radiation force induced by a quasi-Gaussian beam centered on an elastic and a viscoelastic (polymer-type) sphere in a nonviscous fluid. The quasi-Gaussian beam is an exact solution of the source free Helmholtz wave equation and is characterized by an arbitrary waist w0 and a diffraction convergence length known as the Rayleigh range zR. Examples are found where the radiation force unexpectedly approaches closely to zero at some of the elastic sphere’s resonance frequencies for kw0 ? 1 (where this range is of particular interest in describing strongly focused or divergent beams), which may produce particle immobilization along the axial direction. Moreover, the (quasi)vanishing behavior of the radiation force is found to be correlated with conditions giving extinction of the backscattering by the quasi-Gaussian beam. Furthermore, the mechanism for the quasi-zero force is studied theoretically by analyzing the contributions of the kinetic, potential and momentum flux energy densities and their density functions. It is found that all the components vanish simultaneously at the selected ka values for the nulls. However, for a viscoelastic sphere, acoustic absorption degrades the quasi-zero radiation force. 相似文献
12.
The concept of partially coherent vectorial nonparaxial cosh-Gaussian (ChG) beams is introduced, and their analytical propagation expressions for the cross-spectral density matrix in free space are derived by using the generalized vectorial Rayleigh diffraction integrals. Some interesting cases, in particular, the vectorial nonparaxial Gaussian-Schell-model (GSM) beams are discussed and treated as special cases of our general expressions. It is shown that the f and fσ parameters play a crucial role in determining the vectorial property and nonparaxiality of partially coherent ChG beams, but the decentered parameter additionally affects their behavior. 相似文献
13.
14.
研究了两光束的合成方式(相干和非相干合成)对俘获金属瑞利粒子的辐射力和稳定性的影响,着重研究了辐射力与合成方式、离轴距离、相干参数和粒子半径的关系.结果表明,不同合成方式下,离轴距离和相干参数都分别存在临界值dc和αc,在0dc或α>αc时,焦面处光强呈中心凹陷分布,此时横向梯度力不能作为回复力俘获金属瑞利粒子.在0<d≤dc时,与非相干合成光束比较,相干合成光束在焦面处光强、辐射力、俘获刚性和纵向俘获范围更大.因此,适当选择合成方式,较小离轴距离和较低相干参数可有利于合成光束对金属瑞利粒子的俘获. 相似文献
15.
The concept of partially coherent nonparaxial Hermite-Gaussian (HG) beams is proposed. By using the generalized Rayleigh-Sommerfeld diffraction integral, the closed-form propagation equations of partially coherent nonparaxial HG beams in free space are derived. The on-axis intensity, far-field and paraxial expressions, and specifically, the free-space propagation equations of partially coherent nonparaxial Gaussian Schell-model beams are given, and treated as special cases of our results. It is found that the f and fσ parameters play a crucial role in determining the nonparaxiality of partially coherent TEMmn-mode HG beams. Only if the f and fσ parameters are small enough, the paraxial approximation is applicable. 相似文献
16.
The intensity distribution of the J0-correlated Schell-model (JSM) vortex beams focused by a lens and propagation in weak-to-strong turbulent atmosphere are investigated. It is shown that the beam spreading increases with the increase in topological charge n, the source coherent length α−1, turbulent outer scale L0 and propagation distance z. The center hollow depth of intensity distribution of the J0-correlated Schell-model (JSM) vortex beams decrease with the increase of topological charge n, turbulent outer scale L0 and propagation distance z or the decrease of the source coherent length α−1. 相似文献
17.
Based on the beam coherent-polarization (BCP) matrix approach and propagation law of partially coherent beams, the focal shift and focal switch of partially polarized Gaussian Schell-model (PGSM) beams passing through a system with the aperture and spherically aberrated lens separated is studied in detail. Our main attention is focused on the effect of spherical aberration and partial coherence on the focal shift and focal switch of PGSM beams. It is shown that for polarizer-free case there is no focal switch of PGSM beams, the focal shift of PGSM beams is closely related with spherical aberration coefficient C4, auto-coherence length σa, truncation parameter δ and relative position s/f between the aperture and lens in general, and is independent of the cross-coherence length σc. After inserting a polarizer the focal switch can take place. Numerical calculation results are given to illustrate how the spherical aberration and partial coherence affect the focal shift and focal switch of PGSM beams. 相似文献
18.
Kim-Cuong T. Nguyen Lawrence H. Le Tho N.H.T. Tran Mauricio D. Sacchi Edmond H.M. Lou 《Ultrasonics》2014
Long bones are good waveguides to support the propagation of ultrasonic guided waves. The low-order guided waves have been consistently observed in quantitative ultrasound bone studies. Selective excitation of these low-order guided modes requires oblique incidence of the ultrasound beam using a transducer-wedge system. It is generally assumed that an angle of incidence, θi, generates a specific phase velocity of interest, co, via Snell’s law, θi = sin−1(vw/co) where vw is the velocity of the coupling medium. In this study, we investigated the excitation of guided waves within a 6.3-mm thick brass plate and a 6.5-mm thick bovine bone plate using an ultrasound phased array system with two 0.75-mm-pitch array probes. Arranging five elements as a group, the first group of a 16-element probe was used as a transmitter and a 64-element probe was a receiver array. The beam was steered for six angles (0°, 20°, 30°, 40°, 50°, and 60°) with a 1.6-MHz source signal. An adjoint Radon transform algorithm mapped the time-offset matrix into the frequency-phase velocity dispersion panels. The imaged Lamb plate modes were identified by the theoretical dispersion curves. The results show that the 0° excitation generated many modes with no modal discrimination and the oblique beam excited a spectrum of phase velocities spread asymmetrically about co. The width of the excitation region decreased as the steering angle increased, rendering modal selectivity at large angles. The phenomena were well predicted by the excitation function of the source influence theory. The low-order modes were better imaged at steering angle ?30° for both plates. The study has also demonstrated the feasibility of using the two-probe phased array system for future in vivo study. 相似文献
19.
The analytical propagation expression of partially coherent flat-topped vortex beams through a paraxial optical ABCD system is derived, and it then is used to investigate the coherence vortex properties of partially coherent flat-topped vortex
beams in the Fourier transform and fractional Fourier transform systems. It is shown that in the Fourier transform system
the coherence vortex depends on the flat-topped beam order N, spatial coherence parameter α and position (x
1,y
1) of the reference point, whereas in the fractional Fourier transform system the flat-topped beam order N does not affect the spectral degree of coherence. Furthermore, in both transform systems, depending on the choice of the
reference point, the zero value point of the spectral degree of coherence may be present or absent. In particular, if x
1=y
1=0 is selected, the phase at the zero value point of the spectral degree of coherence may be determinate; thus the coherence
vortex does not exist. 相似文献