Optical vortex generation with a “fork” hologram under conditions of high-angle diffraction

Spatial characteristics of the optical-vortex (OV) beams created during the Gaussian beam diffraction by a grating with groove bifurcation are analyzed theoretically and numerically. In contrast to previous works, condition of small-angle diffraction is no longer required and the diffracted beam can be strongly deformed. This causes the intensity profile rotation and the high-order OV decomposition into a set of secondary single-charged OVs. These effects are studied quantitatively and confronted with similar properties of a Laguerre-Gaussian beam that undergoes astigmatic telescopic transformation. In contrast to the latter case, the secondary OVs do not lie on a single straight line within the beam cross section, and morphology parameters of the individual secondary OVs carried by the same beam are, in general, different. Conditions for maximum relative separation of the secondary OVs with respect to the beam transverse size are specified. The results can be used for practical generation of OV beams and OV arrays with prescribed properties.     

Propagation properties of an optical vortex carried by an Airy beam: experimental implementation

In this Letter, we experimentally studied the propagation dynamics of Airy beams (AiBs) carrying phase singularity or an optical vortex (OV). A 3/2 phase mask encoded with phase singularity was used to generate AiBs and OVs enjoying precise position alignment and compact optical configuration simultaneously. Experimental results showed that the OV deflection velocity was faster than that of the main lobe of the AiB, agreeing with the analytical prediction. Numerical simulation results are also in agreement with the experimental results. Furthermore, OVs with larger topological charges were also studied experimentally with the same approach.     

Optical vortices in self-focusing Kerr nonlinear media

In this work we numerically compare the interaction of optical vortices (OVs) in self-defocusing and self-focusing Kerr nonlinear media. We find that the basic scenarios (attraction/repulsion, translation/rotation vs. background) in the interaction of two and three vortices with equal and alternative topological charges (TCs) are the same in both media. However, the vortex dynamics under self-focusing conditions is influenced by the reshaping of the surrounding part of the background. Square structure of OVs with alternating TCs is found to be stable with respect to the vortex positions in self-focusing media. This elementary cell is successfully generalized in a large square array of OVs with alternative TCs which brings ordering in the multiple filamentation of the background beam in self-focusing conditions.     

Pseudo angular momentum and topological charge conservation for nonlinear acoustical vortices

We study acoustical vortices (AVs) and their connection with optical vortices (OVs). We show that AVs and OVs have the same properties if the concept of pseudomomentum is used. Radiation stress and conservation of topological charge are obtained with the pseudomomentum rule. In a weak nonlinear regime, the conservation of pseudo angular momentum imposes a linear increase of the topological charge with the harmonic order. This last result is experimentally verified.     

Composite optical vortices in two noncollinear cosh-Gaussian vortex beams and their propagation properties in free space

The composite optical vortices (OVs) formed by superimposing two parallel, noncollinear cosh-Gaussian (ChG) vortex beams at the waist plane and their propagation in free space are studied. The dependence of the number, position and net topological charge of composite OVs on the control parameter, such as the relative phase, amplitude ratio, waist-width ratio, relative off-axis distance and decentered parameter, and on the propagation distance is illustrated numerically. A comparison with the previous work is also made.     

Oxygen-vacancy-induced diffusive scattering in Fe/MgO/Fe magnetic tunnel junctions

By first principles analysis, we systematically investigate effects of oxygen vacancies (OV) in the MgO barrier of Fe/MgO/Fe magnetic tunnel junctions. The interchannel diffusive scattering by disordered OVs located at or near the Fe/MgO interface drastically reduces the tunnel magnetoresistance ratio (TMR) from the ideal theoretical limit to the presently observed much smaller experimental range. Interior OVs are far less important in influencing TMR, but they significantly increase the junction resistance. Filling OV with nitrogen atoms restores TMR to near the ideal theoretical limit.     

离轴平顶高斯涡旋光束形成的合成光涡旋及其在自由空间中的传输

对两束拓扑电荷m_1,2=±1的平行、离轴平顶高斯涡旋光束在束腰面叠加形成的合成光涡旋及其在自由空间的传输做了研究.详细的数值计算和分析表明,合光涡旋的位置、数目和净拓扑电荷与光束的控制参数,包括相对位相,振幅比,束腰宽度比,相对离轴参数,光束阶数,以及传输距离有关,但拓扑电荷不总是守恒. 关键词： 奇点光学 合成光涡旋 平顶高斯涡旋光束 拓扑电荷     

Propagation dynamics of an optical vortex imposed on an Airy beam

In this Letter, we demonstrate the general propagation dynamics of an Airy beam (AiB) carrying unit phase singularity, i.e., optical vortices (OVs). For the OV with a unit charge, theoretical analysis indicates that the OV carried by the AiB will propagate along the parabolic trajectory with an acceleration velocity twice as fast as conventional AiBs before a critical position. Thereafter, the AiB main lobe destroyed by OV will be reconstructed and the phase singularity will reappear in the middle of the AiB profile.     

Photoluminescence properties of silica-based mesoporous materials similar to those of nanoscale silicon

Photoluminescence (PL) from composites of 7- and 15-nm sized silica nanoparticles (SNs) and mesoporous silicas (MSs) induced by 266- (4.66-) and 532-nm (2.33-eV) laser light has been studied at room temperature. The multiband PL from MSs in the range of 1.0-2.1 eV is evidenced to originate from isolated bulk and surface non-bridging oxygens (NBOs) and from NBOs combined with variously placed 1-nm sized pore wall oxygen vacancies (OVs). The nature and diversity of NBO light-emitters are confirmed by ab initio calculations. The PL from SNs exhibits only a short wavelength part of the bands (1.5-2.1 eV) originated from isolated bulk and surface NBOs. This fact indicates that the highly OV-bearing structures occur only in extremely thin (∼ 1 nm) silica layers. The similarity of spectroscopic properties of silica-based nanoscale materials to those of surface-oxidized silicon nanocrystals and porous silicon, containing silica-passivating layers of the same width, is discussed. Received 20 November 2000     

Electrical analysis of inter-growth structured Bi_4Ti_3O_(12)-Na_(0.5)Bi_(4.5)Ti_4O_(15) ceramics

Inter-growth bismuth layer-structured ferroelectrics(BLSFs), Bi_4Ti_3O_(12)-Na_(0.5)Bi_(4.5)Ti_4O_(15)(BIT-NBT), were successfully synthesized using the traditional solid-state reaction method. X-ray diffraction(XRD) Rietveld refinements were conducted using GSAS software. Good agreement and low residual are obtained. The XRD diffraction peaks can be well indexed into I2 cm space group. The inter-growth structure was further observed in the high-resolution TEM image. Dielectric and impedance properties were measured and systematically analyzed. At the temperature range 763-923 K(below T_c), doubly ionized oxygen vacancies(OVs) are localized and the short-range hopping leads to the relaxation processes with an activation energy of 0.79-1.01 eV. Above T_c, the doubly charged OVs are delocalized and become free ones, which contribute to the long-range dc conduction. The reduction in relaxation species gives rise to a higher relaxation activation energy ~ 1.6 eV.