Short- and long-range screening of optical phase singularities and C points

Screening of signed (charged) singularities-phase vortices in scalar fields, C points in vector fields, is discussed for paraxial optical fields with short- and long-range correlations. A circular region of radius R is assumed. Short-range screening is exemplified by a Gaussian field correlator, long-range screening by a J₀ Bessel function. The short-range screening length is obtained analytically; this is found to be in substantial agreement with recent experiments. For long-range screening, an accurate asymptotic formula suitable for quantitative comparison with data (numerical or laboratory) is derived for the variance of the net charge. A J₀ correlation function is not attainable in practice, but it is shown how to generate a pseudo-long-range optical field whose correlation function closely approximates this form; screening in such a field is well described by our theoretical results for J₀. The charge variance can be measured by three different methods: by counting positive and negative singularities inside the region of interest, by counting signed zero crossings on the perimeter of this region; or by measuring phase derivatives along the perimeter. For the first method, the charge variance is calculated by integration over the charge correlation function, for the second (third) by integration over the zero crossing (phase derivative) correlation function. It is proven explicitly that, as expected, all three calculations yield the same result. It is also shown analytically that for short-range screening the zero crossings can be counted along a straight line whose length is 2πR, but that for long-range screening this useful simplification no longer holds; for this case another formula is given that is suitable for data correction. The effects of boundary smoothing are discussed, and a class of generalized exponential smoothing functions is introduced. Analytical (numerical) results are given for the large R limit of the charge variance for the short (long) range case. Finally, it is shown that for realizable optical fields, both for the short and pseudo-long-range cases, for sufficiently small R the charge variance grows as R², whereas for sufficiently large R it grows as R.     

Experimental umbilic diabolos in random optical fields

The intensity of a random optical field consists of bright speckle spots (maxima) separated from dark areas (minima and optical vortices) by saddle points. We show that hidden in this complicated landscape are umbilic points--singular points at which the eigenvalues Lambda (+/-) of the Hessian matrix that measure the curvature of the landscape become degenerate. Although not observed previously in random optical fields, umbilic points are the most numerous of all special points, outnumbering maxima, minima, saddle points, and vortices. We show experimentally that the directions of principal curvature, the eigenvectors Psi (+/-), rotate about intensity umbilic points with positive or negative half-integer winding number, in accord with theory, and that Lambda (+) and Lambda (-) generate a double cone known as a diabolo. At optical vortices the curvature of the amplitude is singular, and we show from both theory and experiment that for this landscape Psi (+/-) rotate about vortex centers with a positive integer winding number. Diabolos can be classified as elliptic or hyperbolic, and we present initial results for the measured fractions of these two different types of umbilic diabolos.     

Experimental measurements of topological singularity screening in random paraxial scalar and vector optical fields

There exists a substantial body of theory that predicts mutual screening of signed topological singularities (topological charges) in random optical fields (speckle patterns). Such screening appears to be rather mysterious because there are neither energetic nor entropic reasons for its existence. We present the first experimental confirmation of mutual screening by the stationary points of the intensity, the canonical optical scalar field, and of mutual screening by C points in elliptically polarized light, the generic optical vector field. We also elucidate specific aspects of the geometry and topology of these fields that we argue give rise to screening.     

半绝缘GaAs光电导开关产生太赫兹波电场屏蔽效应的二维Monte Carlo模拟

利用Ensemble-Monte Carlo模拟方法，对不同实验条件下半绝缘GaAs(SI-GaAs)光电导开关作为偶极辐射天线在辐射太赫兹电磁波(太赫兹波)中体内电场的分布以及空间电荷屏蔽效应对太赫兹波辐射的影响进行了模拟.载流子的时域空间电场分布表明:用高能量激光脉冲触发低压偏置的GaAs开关，空间电荷屏蔽是限制太赫兹波辐射功率的一个重要因素，并且空间电荷屏蔽能够引起太赫兹波呈现双极性.当高能量飞秒激光脉冲以全电极间隙触发大孔径光电导天线时，空间电荷电场屏蔽效应对太赫兹波的影响不大. 关键词： 光电导开关 Ensemble-Monte Carlo模拟 辐射场屏蔽 空间电荷屏蔽     

Electro-optic sampling of surface space-charge fields on III–V compounds

Time-resolved reflective electro-optic sampling (REOS) of ultrafast longitudinal surface fields on III–V compounds is accomplished with 50 fs optical pulses at 2 eV. Ultrafast carrier transport and screening effects of the photoexcited carriers initiate electric field transients short enough to impulsively launch longitudinal-optical phonons, in phase. A high-sensitivity measurement system allows the time-resolved investigation of the coherent phonon modes and carrier transport. In a special example the mapping of process-induced changes in the surface space charge region is demonstrated.     

Isotropic or anisotropic screening in black phosphorous: Can doping tip the balance?

Black phosphorus (BP), a layered van der Waals (vdW) crystal, has unique in-plane band anisotropy and many resulting anisotropy properties such as the effective mass, electron mobility, optical absorption, thermal conductivity and plasmonic dispersion. However, whether anisotropic or isotropic charge screening exist in BP remains a controversial issue. Based on first-principles calculations, we study the screening properties in both of single-layer and bulk BP, especially concerning the role of doping. Without charge doping, the single-layer and bulk-phase BP show slight anisotropic screening. Electron and hole doping can increase the charge screening of BP and significantly change the relative static dielectric tensor elements along two different in-plane directions. We further study the charge density change induced by potassium (K) adatom near the BP surface, under different levels of charge doping. The calculated two-dimensional (2D) charge redistribution patterns also confirm that doping can greatly affect the screening feature and tip the balance between isotropic and anisotropic screening. We corroborate that screening in BP exhibits slight intrinsic anisotropy and doping has significant influence on its screening property.     

The new method of topological charge determination of optical vortices in the interference field of the optical vortex interferometer

In this paper, the new method of determination of the topological charge of vortex points in the interference field obtained by three plane waves interference is presented. Such optical fields are used in the optical vortex interferometer (OVI) and the determination of vortex points’ topological charge allows of unique determination of the relative phase between interfering waves (phase unwrapping problem). The new method uses additional plane wave, which produce a characteristic fork-like fringe structure in the neighbourhood of vortex points. By analysing the orientation of these fork-like patterns one can read the sign of the topological charge of the given vortex point. The method is simple and can be used for OVI calibration performed before the measurements.     

Boundary-driven phase transitions in open driven systems with an umbilic point

Different phases in open driven systems are governed by either shocks or rarefaction waves. A presence of an isolated umbilic point in bidirectional systems of interacting particles stabilizes an unusual large scale excitation, an umbilic shock (U-shock). We show that in open systems the U-shock governs a large portion of phase space, and drives a new discontinuous transition between the two rarefaction-controlled phases. This is in contrast to strictly hyperbolic case where such a transition is always continuous. Also, we describe another robust phase which takes place at the phase governed by the U-shock, if the umbilic point is not isolated.     

Partially Annealed Disorder and Collapse of Like-Charged Macroions

Charged systems with partially annealed charge disorder are investigated using field-theoretic and replica methods. Charge disorder is assumed to be confined to macroion surfaces surrounded by a cloud of mobile neutralizing counterions in an aqueous solvent. A general formalism is developed by assuming that the disorder is partially annealed (with purely annealed and purely quenched disorder included as special cases), i.e., we assume in general that the disorder undergoes a slow dynamics relative to fast-relaxing counterions making it possible thus to study the stationary-state properties of the system using methods similar to those available in equilibrium statistical mechanics. By focusing on the specific case of two planar surfaces of equal mean surface charge and disorder variance, it is shown that partial annealing of the quenched disorder leads to renormalization of the mean surface charge density and thus a reduction of the inter-plate repulsion on the mean-field or weak-coupling level. In the strong-coupling limit, charge disorder induces a long-range attraction resulting in a continuous disorder-driven collapse transition for the two surfaces as the disorder variance exceeds a threshold value. Disorder annealing further enhances the attraction and, in the limit of low screening, leads to a global attractive instability in the system.     

Anomalous optical constants of thin films

The explanation of anomalous optical constants in thin chemically distinct layers on substrates offered by Plumb is re-examined and extended. The model invokes the concept of the space charged boundary layer and treats the charge carrier population as a free-electron gas to derive the optical behaviour of thin surface films. The implication of the space charge means that the optical constants of a dielectric film on a metal will vary over a distance directly proportional to the dielectric constant of the film and inversly proportional to the concentration of the electrons at the metal/film interface. Similarly as the temperature increases this space charge region should extend to larger distances from the interface.     

Phonon screening in high-temperature superconductors

In good conductors optical phonons are usually screened, and therefore not observed. However, sharp features due to infrared-active modes in the copper-oxygen planes are observed in the optical conductivity of Pr1.85Ce0.15CuO4 and YBa2Cu3O6.95. Oscillator strengths indicate that the screening of these modes is poor or totally absent. These materials are compared with eta-Mo4O11, in which lattice modes appear suddenly below the charge-density wave transition. It is proposed that poor screening in the cuprates originates from fluctuating charge inhomogeneities in the copper-oxygen planes.     

Heating and cooling of a two-dimensional electron gas by terahertz radiation

The absorption of terahertz radiation by free charge carriers in n-type semiconductor quantum wells accompanied by the interaction of electrons with acoustic and optical phonons is studied. It is shown that intrasubband optical transitions can cause both heating and cooling of the electron gas. The cooling of charge carriers occurs in a certain temperature and radiation frequency region where light is most efficiently absorbed due to intrasubband transitions with emission of optical phonons. In GaAs quantum wells, the optical cooling of electrons occurs most efficiently at liquid nitrogen temperatures, while cooling is possible even at room temperature in GaN heterostructures.     

Duality between phase vortices and Argand-plane caustics

Singularities in the mappings of complex two-dimensional vortical wavefields to Argand space are used here to investigate the duality between vortices in the physical field and caustic-like structures in the Argand plane. Such map singularities are exemplified via a method of optical vortex generation based on low-pass filtering of two-dimensional noise. The presence of Argand-plane fold caustics crossing the Argand-plane origin is tied to changes in vortex numbers in the physical wavefield. Increasing the low-pass filter cutoff leads to higher-order Argand-plane caustic singularities, such as the hyperbolic umbilic catastrophe. This work presents a duality between vortices and caustics, explores the Argand plane as a tool in analysing optical wavefields, and provides a new environment for studying catastrophes.     

孔径平均对自由空间光通信误码率的影响

研究了接收端孔径平均效应对自由空间光通信误码率的影响。对数辐射方差是反映接收孔径上某一点闪烁起伏的参量。如果自由空间光通信的误码率仅仅是和对数辐射方差有关,那么得到的就是点接收器的结果。对数辐射的协方差是与接收平面上某两点间距离有关的闪烁起伏参量。孔径平均效应意味着要将误码率和对数辐射的协方差联系起来。文中共考虑了两种模型:球面波和平面波,并假设接收孔径直径等于第一菲涅耳区尺度。分析的结果显示,孔径平均后的误码率有明显的减小。     

The self-consistent response of the electron density of a crystal copper film to an external electrostatic field

The response of the electrons of an fcc copper (001) film to an external electrostatic field is calculated. In order to determine the distribution of the screening charge, the equations of the electron density-functional theory are solved self-consistently by an original method. The position of the “image plane,” which is involved in a correct asymptotic expression for the exchange-correlation potential in the vacuum region, is first determined when performing a quantum-mechanical calculation for an anisotropic crystal film. The nonlinear electron response is characterized by the evolution of the “center of gravity” of the induced charge, which is also investigated. The calculations take into account the crystal structure of the film, and the results differ essentially from the predictions of the “jelly” model.     

Detecting the topological charge of vortex beams using an annular triangle aperture

The Fraunhofer diffraction pattern of a vortex beam by an annular triangle aperture is analyzed theoretically and experimentally. It is found that the pattern of the far-field diffraction intensity distribution exhibits a triangular lattice array, which becomes much clearer with the increase of the ratio of the inner to the outer side of the annular triangle aperture. The number of spot points of any external side of the triangular lattice array minus one is just equal to the topological charge value of the measured optical vortex. For the vortex beam with negative topological charge, the triangular diffraction pattern after the annular triangle aperture will be rotated by 180° in relation to the case of the positive topological charge. Based on the above properties, we propose a simple and feasible method to determine the magnitude and sign of the topological charge of an optical vortex beam.     

Investigation on the electric-field-induced Pockels effect and optical rectification in near-intrinsic silicon samples

Based on the classical polarization theory, we studied and specified the physical mechanism of the electric-field-induced (EFI) Pockels effect and optical rectification in the space charge region of a near-intrinsic silicon sample with the planar capacitor structure. Especially, the effect of the applied DC bias on these EFI effects was investigated. The results show that the electro-optic signal from Pockels effect in silicon linearly increases with the applied DC voltage and the modulating voltage, and the signal of optical rectification is linearly enhanced by the DC bias as well, but the polarization characteristic of optical rectification does not vary. The enhancement of these EFI effects is mainly owed to the strengthening of the built-in field and the extension of the space charge region in the silicon sample. The Kerr effect of silicon was also detected and contrasted against the EFI Pockels effect, and it is verified that the EFI Pockels effect is much stronger than the Kerr effect in the silicon sample. These EFI effects are significant for the development of silicon photonics or silicon optoelectronics.     

Mechanisms for the generation of coherent longitudinal-optical phonons in GaAs /AlGaAs multiple quantum wells

We show that coherent optical phonons in GaAs multiple quantum wells are generated in a completely different way as compared to bulk GaAs. Unlike in bulk GaAs where the ultrafast screening of electric fields by photogenerated charge carriers is known to be dominant, three distinctive generation mechanisms contribute simultaneously in multiple quantum wells. The interplay between impulsive Raman scattering, forbidden Raman scattering, and screening of surface electric fields, whose relative strengths are determined by laser intensity, detuning from the exciton resonance, and the barrier width, generates a rich variety of new phenomena.     

The effect of Ag adsorption on the structural,electronic, and optical properties of the ZnO(101̅0) surface: A first-principles study

The effect of the Ag adsorption on the structural, electronic and optical properties of the clean ZnO$(10\bar{1}0)$ surface was investigated using the first principles method. The obtained results show that adsorbed Ag atoms transfer charge to the surface which results in a charge accumulation in near-surface region accompanied with a decrease of the work function. On the other hand, our results show that the adsorption of Ag atoms leads also to the new optical absorption peaks in the visible region which could improve ZnO photocatalytical properties.     

Photoinduced magnetization in the organic-based magnet Mn(TCNE)(x)* y(CH2Cl2)

Photoinduced magnetization in a magnet based on organic species is reported for the first time. Upon optical excitation in the blue region of spectrum, Mn(tetracyanoethylene)(x)*y(CH2Cl2) (x approximately 2, y is approximately 0.8) exhibits increased magnetic susceptibility at temperatures as high as 75 K, accompanied with photoinduced absorption in the visible and infrared spectral regions. These effects are partially reversible by lower energy visible light and fully reversible by thermal treatment. The results suggest trapping of the photoexcited charge in a metastable state with enhanced exchange interaction.