Diffraction-free beams and their caustics

We discuss the feasibility of generating focal areas in different diffraction fields and prove that the term ‘diffraction-free’ beams is used to designate a spatially limited portion of the toroidal light wave. The light line, or axial caustic, is a result of interference of intersecting areas of this wave. It is shown that the ‘diffraction-free’ beams are generated by such optical elements whose phase transmission function includes linear terms with respect to the radial coordinate. A comparison of optical characteristics of caustics in the near- and far-diffraction fields is made. Synthesis of a new diffraction-modulated axicon that forms two focal areas of different types is reported. The results of experimental studies of its optical characteristics are discussed.     

Phase Modulation Characteristics Analysis of Optically-Addressed Parallel-Aligned Nematic Liquid Crystal Phase-Only Spatial Light Modulator Combined with a Liquid Crystal Display

An optically addressed parallel-aligned nematic-liquid-crystal spatial light modulator (PAL-SLM) has been studied as a dynamic phase-only light modulation device. The phase modulation characteristics of the PAL-SLM using a liquid crystal display (LCD) as an addressing mask were investigated by analyzing diffraction efficiencies resulting from binary gratings projected from the LCD. A more than 2π phase-only modulation depth was achieved. The highest first-order diffraction efficiency of approximately 38% was also obtained; this is close to the theoretical limit. The experimental results of diffraction efficiencies depending on the phase modulation depth are in good agreement with the simulation for the system operation.     

Free electron laser based on the Smith–Purcell radiation

A Smith–Purcell (S–P) free electron laser (FEL) composed of a metallic diffraction flat grating, an open cylindrical mirror cavity and a relativistic sheet electron beam with moderate energy, is presented. The characteristics of this device are studied by theoretical analysis, experimental measurements and particle-in-cell (PIC) simulation method. Results indicate that coherent radiation with output peak power up to 50 MW at millimeter wavelengths can be generated by using relativistic electron beam of moderate energy.     

Simulation of the diffraction characteristics of a one-dimensional dual-periodicity superlattice

The diffraction characteristics of a dual-periodicity superlattice—a diffraction grating whose unit cell contains two different regions, one homogeneous and the other periodically nonhomogeneous—were numerically simulated. After choosing the second region to have sinusoidally varying permittivity, the widely used rigorous coupled-wave approach was applied to compute the reflection and transmission efficiency spectrums. An extensive parametric study revealed that the dual-periodicity superlattice exhibits anomalies in the long-wavelength regime which can be considered to be resonance anomalies (RAs), which are distinct from the Rayleigh–Wood anomalies. Also, double-periodicity superlattices exhibit RAs at higher wavelengths than single-periodicity superlattices.     

Calculation of Diffraction Characteristics of Sub wavelength Conducting Gratings Using a High Accuracy Nonstandard Finite-Difference Time-Domain Method

Gratings with subwavelength groove depth and period are frequently used in optics for various purposes. The polarization dependent diffraction characteristics of subwavelength (high frequency) gratings can only be calculated by solving Maxwell’s equations of electromagnetism. In this paper, we calculate the classical diffraction characteristics of subwavelength conducting gratings numerically, using a new high accuracy version of nonstandard finite-difference time-domain (NS-FDTD) algorithm. For the purpose of analysis, we employ a gold sinusoidal grating with light incident at a large angle. We have compared high accuracy NS-FDTD simulation results with those obtained from standard finite-difference time-domain (S-FDTD), and the finite element method (FEM) simulations.     

Photodiffraction in InGaAs/InGaAsP multiple quantum wells enclosed in a microcavity

We report new results on the diffraction properties of photoinduced gratings in InGaAs/InGaAsP MQW structures. The original feature of this device is that the QWs are enclosed in an asymmetric Fabry–Perot microcavity in order to increase the diffraction efficiency. We observe oscillations in the diffraction efficiency due to resonant effects in the microcavity. The experimental spectra are compared with theory. Diffraction efficiency at 1.55 μm attains a maximum value of 2.7% at a write beam fluence of 260 μ J cm⁻², and then decreases at higher fluences. We explain this phenomenon by an absorption saturation at high excitation.     

Phenomenological representation of deep and high contrast lamellar gratings by means of the modal method

Some remarkable effects are analyzed at the light of the ‘vertical mode’ representation of diffraction on a deep lamellar grating such as total diffraction in transmission and resonant total reflection. The general methodology of this phenomenological representation is given together with rigorous numerical examples.     

Preparation and photoluminescence of Sc-doped ZnO nanowires

We demonstrate bulk synthesis of single-crystal Sc-doped ZnO nanowires by using (Sc+Zn) powders at . These mass nanowires are characterized through X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy (TEM), selected area electron diffraction, and high-resolution TEM, which have uniform diameters of about 40 nm and microns of several decades in length. The growth of ZnScO nanowires is suggested for self-catalyzed vapor–liquid–solid. In particular, PL spectra of these nanowires show emission peaks that intensely shift to long wavelength with increasing Sc and the doping quantity is found responsible for the different characteristics, in which PL mechanism is explained in detail.     

Low-magnification particle positioning for 3D velocimetry applications

Three-dimensional (3D) position and velocity information can be extracted by directly analysing the scattering patterns in velocimetry imaging of seeding particles using real-time CCD cameras. A Fraunhöfer diffraction simplification of generalised Lorenz–Mie theory is shown to yield a representative model of particle position, such that particle position can be approximately deduced from typical experimental particle images. Data are obtained by pattern-matching theoretical to experimental images using a Nelder–Mead algorithm, subject to digitisation considerations and the concept of “locales”. In this way, information about the characteristics of positional error as a function of magnification, pixel size, intensity resolution, and spatial resolution can be derived. This work shows that an optimum magnification exists, beneath which error begins to increase drastically. A practical application is demonstrated. The theory, simulations and experimental verification of this basic problem are discussed.     

Electromagnetic-wave diffraction by slot in faster-than-light flow of nondispersive medium

Plane-wave diffraction by a slot in an ideally conducting shield in a moving nondispersive medium is examined. The main diffraction characteristics are determined for the case of faster-than-light motion of the medium. The particular cases of narrow and wide slots are analyzed.St. Petersburg State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 35, No. 8, pp. 678–687, August, 1992.     

Producing binary diffraction gratings in the double-diffraction system

In this paper, a method — based on the diffraction of light — for producing binary diffraction gratings is presented. It is shown that using two master gratings of the Ronchi type and utilizing the results of the double diffraction system it is possible to produce binary gratings with an arbitrary opening ratio. The results have been extended for a two-dimensional case and it is shown that producing binary grid gratings with arbitrary opening ratios in both x and y directions is also possible.     

Icosahedral-Cluster-Reinforced Structures in Irradiated Metallic Materials

A nonequilibrium state has been discovered which is induced by ion irradiation in metallic materials (solid solutions of Fe–Ni, Fe–Cr–Ni, Ni–Cr, Cu–Ni, Fe–Cr, and V–Ti–Cr systems and in pure metals Zr and Ti) at high levels of radiation damage, and the features of this state are considered. In the region of existence of this state, both the ion and the electron subsystems of the metal show highly anomalous properties. Moreover, the occurrence of this state is accompanied by substantial diffraction effects – X-ray line splitting – and, as indicated by electron microscopy, by the formation of a cluster structure. Simulation by the methods of molecular dynamics suggests that the clusters observed are atomic groups of icosahedral (quintuple) symmetry formed in the neighborhood of radiation vacancies. These clusters reinforce the matrix, and this should result in substantial changes in strength and electronic properties of the material. The results of the computer simulation agree with the observed diffraction effects.     

Study of the polymorphic hexagonal-face-centered-cubic transformation in Cu2S

The results of a dynamic high-temperature x-ray diffraction investigation into the special characteristics of the hexagonal-fcc polymorphic transformation in Cu₂S are presented. Close to the phase-transformation temperature, two groups of superstructural reflections appear on the x-ray diffraction patterns, some of these being indexed in the 12H hexagonal lattice (Ramsdel) and others in the 14H lattice. The mechanism of the hexagonal — fcc phase transition in Cu₂S is considered as passing through the intermediate 12H and 14H polytypic structures by way of the successive accumulation of periodically disposed stacking faults. The effect of thermal cycling within the phase-transformation temperature range on the course of the actual transformation is verified.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 100–103, May, 1976.     

Calculations of phase reflection diffraction gratings

Intensities of principal diffraction maxima for a phase reflection diffraction grating with a rectangular groove profile were calculated by the Fresnel method, using a special algorithm. The procedure under consideration enables us to perform high-efficiency calculations of diffracted-beam characteristics and can be used extensively to optimize diffraction-grating parameters.__________This revised version was published online in May 2005 with a correction on affiliation.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 38–41, November 2004.     

Low-energy electron diffraction on the planes non parallel with the surface

Some problems of the interpretation of the geometry of low-energy electron diffraction patterns obtained on metal samples with a general crystallographic orientation near that of some densely occupied atomic plane are discussed. It appears that for the interpretation of these diffraction patterns the commonly used classical model of the diffraction on the surface plane cannot be used. The new geometrical model — model of the diffraction on densely occupied planes non parallel with the surface — in constructed. Using this model the calculation of the diffraciton patterns is made for a surface with an orientation close to the densely occupied (100) plane. The calculated diffraction patterns are compared with those obtained on monocrystalline niobium with known surface orientation near that of the (100) plane. Experimental results confirm the mentioned model to be rightful. This diffraction model can be used for determination of the polycrystalline grain orientation by means of the selected area low-energy electron diffraction. The experiments were made in the emission electron microscope adapted for low-energy electron diffraction.     

Reflecting phase volume holographic grating (PVHG) in LiNbO3

The spectral-angular selectivity of reflecting PVHG in LiNbO 3 and electric control of light diffraction are investigated. It is shown that a PVHG with approximate 7000 lines/mmand thickness spatial frequency 3.7mmhas a spectral selectivity 0.023 nm and angular selectivity that varies in the range from 2 to 50 ang. min, depending on the angle between the beams that record the grating. Application of an 850 V electric field decreases the diffraction effectiveness by one-half for an extraordinary wave and a transverse scheme (the transverse dimension of the PVHG is 5 mm). It is established that the spectral-angular characteristics of reflecting PVHG and electric control of light diffraction can be described within the framework of the theory of coupled wave and the electrooptical effect.Translated from Lazernye Sistemy, pp. 112–120, 1982.     

Invertible dark-center diffraction of the metal film induced by femtosecond laser

We found reversible dark-center diffraction of the transmitted probe beam passing through the chromium film, which is induced by the pump femtosecond laser. The dark-center diffraction of the transmitted probe beam appears and disappears with and without the pump beam. A view of diffractive optics with binary phase plate is put forward, which explains the reversible dark-center diffractive optical phenomenon. The pre-ablated hole on the metal film can be regarded as a uniform light filed without phase modulation, the surrounding circular part around the pre-ablated hole can be regarded as “phase modulated”. Therefore, this diffraction optic view might be helpful for us to understand the phase change of the metal film introduced by the femtosecond laser pulse.     

Investigation of the spectral characteristics of pulses of diffraction radiation

The physical phenomena which lead to distortions of the spectrum of diffraction radiation pulses are analyzed by means of theoretical estimates and experimental investigations. The experimental investigations were carried out for pulses of high-frequency oscillations with a duration of 0.15–10sec, excited in a diffraction radiation generator in the millimeter-wavelength range. The effect of steepness of the electron frequency retuning, the magnitude of the focusing magnetic field, and the pressure of the residual gases on the spectral characteristics of the instrument is established. The range of variation of the generator parameters is determined, where parasitic effects leading to distortions of the spectrum are expressed only slightly.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 21, No. 12, pp. 1853–1861, December, 1978.     

Analysis of x-ray diffraction patterns for crystals subjected to certain kinds of external fields

We derive the dependence of the x-ray structure factor of a crystal on the characteristics of an external force field acting on the sample. Relations are obtained which enable a detailed analysis of the positions, heights, and peak shapes in the resulting diffraction patterns. Specific examples are presented of the diffraction patterns formed in the presence of certain types of external fields.S. Ordzhonikidza State Geologic Exploration Institute, Moscow. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 3–7, May, 1995.     

Field of a Point Source Within Perfectly Conducting Parallel-Plates in a Homogeneous Biisotropic Medium

The spherical wave scattering response by a perfectly conducting open–ended waveguide in a biisotropic medium is obtained. Interestingly, the vector diffraction problem is reduced to the scattering of a single scalar field, this scalar field being the normal component of either a left–handed or a right–handed Beltrami field. Here, we explicitly consider the scattering of the left–handed field component, that of the other scalar field being analogously tractable. The solution is constructed with the aid of the Wiener–Hopf technique.