Superscattering-enhanced narrow band forward scattering antenna

We present a narrow band forward scattering optical antenna which is based on the excitation of distinctive whispering gallery modes(WGMs). The antenna is composed of three coaxial cylinder layers: a dielectric layer is sandwiched between a metallic core and cladding. Owing to the destructive interference between the scattering of the outer metallic cladding and the WGM in the backward direction, the power flow in the forward direction is increased. Simulation and analysis show that in proper geometry conditions, the cavity can be tuned into a superscattering state. At this state, both the zeroth and the first order of WGM are excited and contribute to the total scattering. It is shown that the power ratio(power towards backward divided by power towards forward) can be enhanced to about 27 times larger than that for a non-resonant position by the superscattering. Owing to the confinement of the cladding to WGMs, the wavelength range of effective forward scattering is considerably narrow(about 15 nm).     

Comparison of photonic nanojets key parameters produced by nonspherical microparticles

Photonic nanojet (PNJ) phenomenon arising near transparent dielectric microparticles subject to plane wave illumination in the visible is considered. The near-field light scattering patterns produced by shaped wavelength-sized particles (hexahedron, cuboid, sphere, hemisphere, axicon, assembled particles) are numerically simulated and key PNJ parameters are analyzed. Particle shape influence on the peak intensity and spatial resolution of produced PNJ is investigated. We demonstrate that due to the reciprocal action of spherical-type and conical-type focusing of the special type of composite particles constituted of a hemisphere and an axicon can produce highly localized PNJ with peak intensity considerable higher than that for isolated regular particle (sphere, microaxicon, hemisphere).     

Analysis of an encapsulated whispering gallery mode micro-optical sensor

Whispering gallery mode (WGM) based micro-optical sensors are known to have higher sensitivity than fiber Bragg grating, Fabry–Perot, and microbend sensors. WGM sensors are created by optical coupling of a dielectric microparticle with an optical fiber. The combination of a microparticle and an optical-fiber to create the sensor requires encapsulating them in a suitable material so that the sensor can be used in practical applications. The sensitivity of the encapsulated sensors needs to be calibrated before they can be used. The present study conducts a parametric study to understand the effect of variables such as particle size and particle–fiber distance on the sensitivity of the encapsulated WGM sensors. Solid and hollow microparticle based sensors are studied. In the case of hollow particles, their wall thickness effects are also characterized. Results show that despite small strain, change in the index of refraction of the particle material due to the applied force contributes significantly in determining the sensitivity of these sensors.     

A wide-band Whispering Gallery Mode dielectric resonator having multiple high-Q cavities for high-field ESR spectroscopy

The behaviour of a disc-shaped dielectric resonator made of sintered alumina and working in the Whispering Gallery Mode (WGM) has been studied in the frequency interval from about 8 up to about 26 GHz. Many equidistant resonances have been detected in this frequency interval each of which being characterized by a satisfactoryQ-value due to the very good self-confinement of the microwave field which is typical of WGM propagation. Particular care has been devoted to the optimization of the coupling between the resonating disc and the microwave source. Examples of ESR spectra obtained on each of the detected resonances are reported which show the variation of the lineshape with the increase of the frequency. The obtained results allow one to foresee that the same dielectric disc can be used for ESR spectroscopy also at frequencies much higher than those up to now exploited.     

Uniform plasmonic near-field nanopatterning by backward irradiation of femtosecond laser

We present localized optical field distribution properties in the vicinity of gold particles on a silicon substrate by backward and forward irradiation. It is technically difficult to fabricate nanostructures on the surface by a conventional forward laser incident to the substrate because gold nanoparticles easily aggregate to form double-layered particle arrays. We calculated enhanced optical field properties in order to pattern the substrate surface only with a template of the bottom-layered particle arrays in the case that the backward irradiation of a femtosecond laser is used in the system of aggregated double-layered gold nanoparticle arrays. With the backward irradiation, the optical field intensity in the substrate for the double-layered hexagonal arrays is found to be only 30% lower than the mono-layered system. Moreover, a near field cannot be generated with the forward irradiation. As a result, only the backward irradiation scheme is found to be effective for uniform surface nanopatterning at enhanced plasmonic near-field zones.     

Effect of the direction of biasing field on magnetostatic volume wave delays

An investigation of the effect of the direction of a dc biasing field on the magnetostatic volume wave delays in YIG sandwiched between two ground planes has been made. Specifically, the magnetic field has been assumed to be arbitrary in three planes: (a) forward volume wave to backward volume plane; (b) forward volume wave to surface wave plane; (c) backward volume wave to surface wave plane. A general dispersion relation has been derived. The numerical results indicate that delays can be controlled by the direction of the dc magnetic field. The effect of the thickness of the dielectric on the delay characteristics of magnetostatic backward volume waves has also been studied. The present study shows that a suitable filter can be designed, whose bandwidth may be varied by rotating the direction of the magnetic field.     

Frequency conversion and field pattern rotation in WGM resonator with transient inclusion

An investigation of the transformation of whispering gallery modes (WGM) in circular dielectric resonators due to non-uniform temporal changes of the permittivity is presented. An analytical solution is obtained for the case of an instantaneous refractive index change within a circular inclusion in the resonator. Analytic-numerical analysis of the obtained solution shows conservation of the spatial field distribution and a frequency shift. The possibility of field pattern rotation, dependent on the size and the position of the inclusion, is discussed.     

ESR spectroscopy at 75 GHz by using a whispering gallery mode dielectric resonator and an electromagnet: Part I

Disc-shaped Whispering Gallery Mode (WGM) dielectric resonators were used to obtain ESR spectra of powder samples at 75 GHz. A mechanically and electronically controlled Gunn diode was used while the static magnetic fieldB ₀ was obtained by modifying a Bruker electromagnet. WGM resonators having a diameter of 30 mm were prevalently used. The samples were contained in polyethylene holders suitably designed so that the resonator could retain its original circular symmetry and the sample could stay as close as possible to the curved outer rim of the resonator.Q-factors of the order of 10⁴ were obtained at room temperature when single crystal quartz was used for the disc resonator. Anyway, the value ofQ could be easily controlled by using different materials for the WGM disc. The gained experience allows one to foresee that WGM resonators can be used with similar results also at frequencies up to 150 and 300 GHz. ESR spectra of many organic and inorganic samples will be reported in Part II of this paper. Here, an evaluation is made of the intensity of the electromagnetic field on a selected sample. Suitable holders for liquid samples are under study.     

包覆微米碳颗粒的光散射特性

利用分层介质球光散射理论公式,计算分析了包覆水和黄铜介质层对碳微粒的微分散射特性的影响,比较了包覆前后散射的极化特性。在计算中为保持收敛性,对Bessel函数采用了递推求解,并对求和项数进行了限定。结果表明,在包覆前后,单粒子的微分散射截面发生了变化,而包覆层的影响与其厚度和光在其中的趋肤深度有关;厚度小于趋肤深度的包覆层对粒子的前后向散射的极化特性没有影响,而在其它方向上却影响较大。     

A X-band ESR apparatus using a Whispering Gallery Mode dielectric resonator

The behaviour of a disc-shaped dielectric resonator (DR) working in the Whispering-Gallery Modes (WGM) when used for X-band ESR experiments has been tested. It has been verified that the WGM in the resonator can be excited by using the microwave bridge of a Bruker spectrometer. The microwave field is well confined in the DR also in the absence of metal shieldings as demonstrated by the high Q values of the detected resonances. A large number of such resonances has been detected from eight up to twelve gigahertz making the DR a very broadband device. ESR spectra of various powder and aqueous samples have been obtained and compared with those of the same samples recorded by using a metal cavity. The relative amplitude of the spectra demonstrated that for aqueous samples the sensitivity of DR is similar to that of the metallic cavity. Further investigations are in progress to enhance the overall performance.     

A Photonic Nanojet as Tunable and Polarization‐Sensitive Optical Tweezers

The ability to manipulate small objects with focused laser beams has opened a venue for investigating dynamical phenomena relevant to both fundamental and applied sciences. However, manipulating nano‐sized objects requires subwavelength field localization, provided by auxiliary nano‐ and microstructures. Particularly, dielectric microparticles can be used to confine light to an intense beam with a subwavelength waist, called a photonic nanojet (PNJ), which can provide sufficient field gradients for trapping nano‐objects. Herein, the scheme for wavelength‐tunable and nanoscale‐precise optical trapping is elaborated, and the possibility of lateral nanoparticle movement using the PNJ's side lobes is shown for the first time. In addition, the possibility of subwavelength positioning using polarization switching is shown. The estimated stability with respect to Brownian motion is higher compared to conventional optical trapping schemes.     

高功率微波输出窗内侧击穿动力学的PIC/MCC模拟研究

高功率微波在受控热核聚变加热、微波高梯度加速器、高功率雷达、定向能武器、超级干扰机及冲击雷达等方面有着重要的应用.本文针对高功率微波输出窗内侧氩气放电击穿过程,建立了二次电子倍增和气体电离的一维空间分布、三维速度分布（1D3V）模型,并开发了相应的PIC/MC程序代码.研究了气压、微波频率、微波振幅对放电击穿的影响.结果表明：在真空情况下,介质窗放电击穿只存在二次电子倍增过程;在低气压和稍高气压时,二次电子倍增和气体电离共存;在极高气压时,气体电离占主导.给出了不同气压下电子、离子的密度和静电场的空间分布.此外还观察到,在500 mTorr时,随着微波振幅或微波频率的变化,气体电离出现的时刻和电离产生的等离子体峰值位置有较大差异,尤其是当微波频率（GHz）在数值上是微波振幅（MV/m）的2倍时,气体电离出现的较早.     

400 AMeV 12C诱发乳胶核反应重电离粒子前后关联研究

对400 AMeV 12C诱发乳胶核反应重电离粒子的前后关联进行了研究。重电离粒子来源于靶核碎片,分为灰径迹粒子和黑径迹粒子两种。实验结果很好地验证了核-核碰撞几何模型。灰径迹粒子的发射在前、后半球是各向异性的,而黑径迹粒子的发射基本是一致的。向前、向后发射的灰径迹粒子、黑径迹粒子平均多重数对重电离粒子数n_h呈线性依赖,黑径迹粒子、灰径迹粒子的向前发射随n_h的增加而增加的幅度分别大于其向后发射随n_h的增加而增加的幅度,但核内级联效应受靶核大小的影响,向后半球内发射的灰径迹粒子平均多重数随n_h的增加有饱和现象,这些多重数关联特性基本上可以利用基于碰撞几何的旁观体-反应体模型及级联蒸发模型来解释。The forward-backward multiplicity correlation of heavily ionized particles produced in 12C-emulsion interactions at 400 AMeV is investigated. The heavy ionized particles, come from the target fragments, are divided into grey track particle and black track particle. The experimental results can be well explained by the geometry model of the nucleus-nucleus collisions. The emission of grey track particles in forward and backward hemisphere is not isotropic, but the emission of black track particle is almost isotropic. The averaged multiplicity of grey track particles and black track particles in forward and backward hemisphere linearly depend on the number of heavily ionized particle n_h, the correlation strength in forward hemisphere is greater than that in backward hemisphere, but the dependence of grey track particle in backward hemisphere on the number of heavily ionized particle n_h shows the saturation because of the intranuclar cascade effect is influenced by target size. The characteristics of multiplicity correlations can be well explained by the participant-spectator model based on the colliding geometrical picture and the cascade evaporation model of high energy nucleus-nucleus collisions.     

The nonlinear interaction of waves generated by the stimulated Raman scattering with plasma in the PALS experiment

This paper is devoted to the study of the nonlinear interaction of the waves generated by stimulated Raman scattering in plasma. The influence of nonlinear interaction of plasma wave with plasma electrons on the sum of action densities of waves generated by the laser wave is solved. The electron acceleration in the forward and backward wave fields is described. The change of the electric field of the quasimode of forward and backward plasma waves of Raman scattering given by trapping of plasma particles is calculated. Numerical results are calculated for typical parameters of the PALS experiment.     

Whispering gallery mode biosensors in the low-Q limit

Whispering gallery modes (WGM) of dye-doped polystyrene beads with diameters from 20 down to 1.5 μm are studied with respect to their appearance and linewidth by excitation of the entire mode spectrum within the emission range of the dye. The lowest order (q=1) modes, which travel most closely to the inner particle surface, are assigned to their individual quantum numbers by means of a least-square-fit, resulting in a precise determination of particle radius and eccentricity. On this basis, the suitability of these microscopic cavities for applications in optical (bio-)sensing is explored. Due to the low quality (Q) factors of these small cavities, particles with diameters below 6 μm exhibit only q=1 modes, thereby causing a drastic simplification of the WGM spectrum. In such spectra, the shift in the WGM positions upon molecular adsorption can be easily monitored, as we demonstrate for the adsorption of bovine serum albumin as well as multiple layers of polyelectrolytes onto the surface of particles with 2 μm diameter. Mie simulations are used to confirm our findings. With a mass sensitivity limit of 3 fg, these microscopic sensors are highly competitive in the field of label-free detection techniques. Moreover, their small size and the simplified, dye-mediated excitation and detection scheme may pave the way to remote in-vitro biosensing in the future. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users. PACS 07.07.Df; 42.60.Da; 42.70.Jk     

Fractional Feynman-Kac Equation with Space-Dependent Anomalous Exponent

To describe the distribution of functionals for inhomogeneous subdiffusion in space- and time-dependent force field, we derive forward and backward time-fractional Feynman-Kac equations with space-dependent anomalous exponent based on the space-jump random walk model. In our examples, we get the statistic of occupation fraction and first passage time for anomalous infiltration of free particles in disordered systems by applying the backward version.     

Controlling the parameters of photon nanojets of composite microspheres

We consider the spatial and amplitude characteristics of PNJ forming in the neighborhood of the shadow surface of micron-sized composite particles consisting of a nucleus and a shell with various refraction indices when laser radiation scatters on them. We study the longitudinal and transverse dimensions of a photon flux and its peak intensity depending on the microparticle shell thickness. We show that a certain choice of the refraction index of the shell relative to the nucleus in two-layer composite spherical microparticles can significantly lengthen the forming PNJ or increase their peak intensity. The width of the photon flux during this changes insignificantly.     

Simulation of multiple light scattering in a bounded nematic

The multiple-scattering problem is formulated for a single-constant model of a nematic liquid crystal with allowance for the dependence on the orienting external magnetic field. Based on the Bethe-Salpeter equation, an original Monte Carlo procedure in a birefringent medium is developed with allowance for the long-range action of dielectric permittivity fluctuations and backward and forward scatterings are calculated. The dependence of multiple scattering in a nematic on the distance between the incident and scattered rays is quantitatively studied for the first time and a nonmonotonic dependence on the external magnetic field is discovered. We numerically have established that the light-diffusion anisotropy noticeably weakens as compared to the extinction length anisotropy; this is shown to be caused by the weak anisotropy of the transport length.     

Forward–backward emission of target evaporated fragments in high energy nucleus–nucleus collisions

The multiplicity distribution, multiplicity moment, scaled variance, entropy and reduced entropy of target evaporated fragments emitted in forward and backward hemispheres in 12 A Ge V4 He, 3.7 A Ge V16 O,60 A Ge V16 O, 1.7 A Ge V84 Kr and 10.7 A Ge V197Au-induced emulsion heavy target(Ag Br) interactions are investigated. It is found that the multiplicity distribution of target evaporated fragments emitted in both forward and backward hemispheres can be fitted by a Gaussian distribution. The multiplicity moments of target evaporated particles emitted in the forward and backward hemispheres increase with the order of the moment q, and the secondorder multiplicity moment is energy independent over the entire energy range for all the interactions in the forward and backward hemisphere. The scaled variance, a direct measure of multiplicity fluctuations, is close to one for all the interactions, which indicate a correlation among the produced particles. The entropy of target evaporated fragments emitted in both forward and backward hemispheres are the same within experimental errors.     

Dielectric tensor elements for the description of waves in rotating inhomogeneous magnetized plasma spheroids

The dielectric permittivity tensor elements of a rotating cold collisionless plasma spheroid in an external magnetic field with toroidal and axial components are obtained. The effects of inhomogeneity in the densities of charged particles and the initial toroidal velocity on the dielectric permittivity tensor and field equations are investigated. The field components in terms of their toroidal components are calculated and it is shown that the toroidal components of the electric and magnetic fields are coupled by two differential equations. The influence of thermal and collisional effects on the dielectric tensor and field equations in the rotating plasma spheroid are also investigated. In the limiting spherical case, the dielectric tensor of a stationary magnetized collisionless cold plasma sphere is presented.