Interference Maxima Research Articles

Diffraction assisted rough ground effect: Models and data

The destructive interferences observed in Excess Attenuation (EA) spectra over periodically and randomly spaced roughness elements with different cross-sectional profiles (semicylindrical, rectangular and wedge-shaped strips) have been investigated. If the roughness is spaced periodically, then two or three destructive interference maxima are observed in the same frequency range as the one or two observed with randomly distributed roughness. Roughness-induced surface waves are investigated also. Their amplitudes and the frequencies at which they occur are found to depend on the roughness height, mean center-to-center spacing and the extent to which the roughness is periodic. A semianalytical Multiple Scattering Theory and a numerical method (the Boundary Element Method) have been used to make predictions of the EA spectra which are compared with measurements. In addition it is found that the effective surface impedance spectra deduced from complex EA measurements over rough surfaces exhibit resonances similar to those observed for a hard-backed porous layer. On this basis a heuristic effective impedance model for rough hard surfaces is developed and the corresponding predictions of EA spectra are compared with data.

The Regularities of the formation of acoustic fields receiving cylindrical piezoceramic transducer located in the annular layer

The problem on the reception of sound in a circular cylindrical converter layer with electroelastic properties of piezoelectric membrane and the transition layer in the "through" setting is considered. The interference patterns of the pressure distribution to the circular layer in the illuminated area with acoustically hard, acoustically soft elastic material and an intermediate layer for different layer thicknesses, as well as the angular distributions of the total fields at different distances from the transducer surface is investigated. The dependence of the interference maxima and minima in the near field of the degree of stiffness (elasticity) of the wave layer is showing. The influence of the choice of materials and the size of the wave layer on the size and location of the interference maxima and minima of the pressure in the illuminated region, as well as the angular characteristics of the total field is showing. Reference 8, figures 8.

Anomalous isotopic effect on electron-directed reactivity by a 3-μm midinfrared pulse

We have theoretically studied the effect of nuclear mass on electron localization in dissociating H₂⁺ and its isotopes subjected to a few-cycle 3-μm pulse. Our results reveal an anomalous isotopic effect in which the degree of electron-directed reactivity can be even higher for heavier isotopes in the intense midinfrared field. We show, for the first time, the pronounced electron localization can be established through the interferences among the multi-photon coupling channels. Due to the relative enhancement of higher-order coupling channels with growing mass, the interference maxima at different kinetic energy of the spectra gradually become in phase, ultimately resulting in the larger dissociation asymmetries of heavier isotopes.

Performance Comparison of Conventional and Inverted Organic Bulk Heterojunction Solar Cells From Optical and Electrical Aspects

The conventional and inverted organic solar cells (OSC and IOSC) based on the bulk heterojunction structure are investigated from both optical and electrical aspects. When the optical aspect is considered only, with the increase of the active layer thickness, the number of photons absorbed in the active layer and the external quantum efficiency tend to increase with the obvious interference behavior for both OSC and IOSC. However, compared to OSC, IOSC shows a better performance except for the thicknesses around which the interference maxima of OSC are obtained. When the electrical aspect is also considered, an effective area in the active layer will be induced by the charge drift length (L), and only the photons absorbed in this effective area have contribution to the photocurrent. By considering optical and electrical aspects together, OSC and IOSC show different behaviors. Compared to IOSC, OSC performs better for relatively thick active layers. Simultaneously, the optical modulation effect is also investigated by introducing an optical spacer layer. It is found that the optical spacer layer can notably enhance the performance of OSC with thin and thick active layers, while it could only degrade the performance of IOSC with relatively thick active layers.

The possibility of using the equivalent plane wave model to increase the efficiency of taking bearings of low-frequency signals in shallow water

The possibility of approximating the sound field in the region of interference maxima using the equivalent plane wave model with the actual amplitude and the average “effective” phase velocity calculated or measured by the phase gradient at the array aperture is discussed. The method is substantiated by studying the mode, interference, and phase structures of the low-frequency sound field along with the spatial responses of an extended linear array. For bottom-moored or towed geophysical arrays whose sizes are large compared to the wavelength, both the necessity and the possibility of reducing the error in taking the bearing of a sound source in a waveguide are justified. The use of the proposed model is recommended for approximate matching of the array to the transfer function of the waveguide to reduce the bearing error.

Electroabsorption and refractive index modulation induced by intersubband transitions in GaN/AlN multiple quantum wells

The aim of the present paper was to determine the index variation in the GaN/AlN heterostructures related to the population/depletion of the quantum well fundamental state leading to the absorption variation in the spectral domain around 1.5 µm. The variation of the refractive index was deduced from the shift of the position of the beating interference maxima of different order modes in a guided wave configuration. The obtained index variation with bias from complete depletion to full population of the quantum wells is around -5 × 10(-3). This value is similar to the typical index variation achieved in InP and is an order of magnitude higher than the index variation obtained in silicon.

Rapid and scalable method for direct and indirect microstructuring of vertical aligned carbon nanotubes

Multi-walled carbon nanotube (MWCNT) films are grown by chemical vapor deposition (CVD) on silicon substrates using an alumina buffer and a Fe/Co layer as catalyst with different Fe/Co ratios. For both coatings, a scalable wet-chemical technique is applied. The highest CNT forests (100–125μm) are obtained with Fe content between 60 and 80wt.%. After deposition of the films, direct laser interference patterning (DLIP) method is used for fabricating micro patterns of the CNTs, using a frequency tripled Nd:YAG laser emitting 10ns pulses. Two different approaches for fabricating periodic MWCNT arrays are presented. The first approach is the direct patterning of the CNT layer itself (CNT-DLIP) obtaining well defined line-like structures with 10μm spatial period. By adjusting the laser fluence (from 150 to 250mJcm−2) and the number of laser pulses (from 1 to 20), the morphology (structure depth and line width) of the fabricated arrays can be varied. Thermal simulations of the CNT ablation process validate the experimental observations. The second approach involves indirect patterning of the CNTs, by fabricating line-like structures on the iron/cobalt catalyst layer (CAT-DLIP). In the last case, moderate energy densities (100mJcm−2) permitted to remove the catalyst layer locally at the interference maxima positions. By altering the number of laser pulses from 1 to 20 the line width can be tuned. The CNT forests are subsequently grown on the patterned catalyst.Using Raman spectroscopy, we demonstrate that in the case of direct patterning of the CNTs (CNT-DLIP) the chemical structure is preserved if a low number of laser pulses as well as moderate laser fluences are utilized. In the case of CAT-DLIP, the chemical structure of the grown CNTs on the patterned CAT-layer show a similar quality compared to the CNTs grown on the non-patterned CAT layer.

The use of effective phase velocity to decrease the direction finding error for a low-frequency signal in a waveguide

The mode, interference, and phase structures of the low-frequency sound field and the spatial responses of the extended linear array in a waveguide are studied. The wide-angle parabolic approximation and the normal mode method are used to perform calculations. A possibility of approximating the field in the zone of interference maxima by the equivalent plane wave model with the real amplitude and averaged effective phase velocity (EPV) calculated or measured from the phase gradient at the array aperture is investigated. The use of EPV in the zone of interference maxima is shown to decrease substantially the direction finding error. The conclusion is drawn that the array and the transfer function of the waveguide can be approximately matched if the speed of sound in water is substituted with the EPV in the bearing algorithm.

Method of reducing the influence of speckles when processing diffraction patterns from moving woven materials

A program has been developed that makes it possible to reduce the noise level determined by the presence of speckles when woven materials are being monitored by laser diffraction. It is shown that, when one accumulates (adds up) the diffraction patterns obtained from metallic, synthetic, or carbon woven fabrics and considers the ratio of the intensity of the principal interference maxima in the cumulative diffraction pattern to the intensity of the speckles observed in them, one observes a growth of this ratio proportional to the number of additions. These results are explained on the assumption that the appearance of speckles in the diffraction pattern is caused by diffraction from randomly located inhomogeneities in the structure of the fabric that are always present inside the repeats of the weave in the illuminated section of the fabric.

Nanoscale cavitation instability of the surface melt along the grooves of one-dimensional nanorelief gratings on an aluminum surface

Femtosecond laser nanostructuring at low fluences produces a onedimensional quasiperiodic grating of grooves on an aluminum surface with a period (≈0.5 μm) that is determined by the length of a surface elec� tromagnetic wave. The structure of the grooves of the surface nanograting is formed by regular nanopeaks fol� lowing with a period of about 200 nm. Some nanopeaks manifest craters at their tops. It is suggested that nanopeaks are formed due to the frozen nanoscale spallative ablation of a nanolayer of an aluminum melt in quasiperiodic regions corresponding to interference maxima of the laser radiation with the surface electro� magnetic wave. The periodicity of the appearance of nanopeaks along grooves is due to the previously pre� dicted mechanism of cavitation deformation of the melt surface in the process of macroscopic spallation ablation. However, in this case, cavitation is coherent (similar to a nearcritical spinodal decay) rather than spontaneous. DOI: 10.1134/S0021364011160065

Frequency shifts of sound field maxima in oceanic waveguides

The frequency shifts of sound field maxima that are caused by the variability of the medium in an oceanic waveguide are described. The condition under which the time spectrum of the frequency shifts of interference maxima is proportional to the time spectrum of fluctuations in the dispersion characteristic of the waveguide is determined.

Interference pattern generation in evanescent electromagnetic waves for nanoscale lithography using waveguide diffraction gratings

The generation of interference patterns of evanescent electromagnetic waves with an essentially subwavelength period using dielectric waveguide diffraction gratings is considered. Using simulations within the framework of the electromagnetic theory, the possibility of obtaining high-quality interference patterns due to enhancement of evanescent diffraction orders under resonance conditions is demonstrated. The contrast of the interference patterns in the case of TE polarisation of the incident wave is close to unity. The field intensity in the near-field interference maxima exceeds the intensity of the incident wave by 25–100 times. The possibility of generation of the interference patterns of evanescent waves corresponding to higher diffraction orders is shown. The use of higher orders reduces the requirements to the fabrication technology and allows generation of interference patterns with a high spatial frequency, using diffraction gratings with a low spatial frequency. Examples of generating interference patters with periods six times smaller than those of the used diffraction gratings are presented.

Excitation two-center interference and the orbital geometry in laser-induced nonsequential double ionization of diatomic molecules

We address the influence of the molecular orbital geometry and of the molecular alignment with respect to the laser-field polarization on laser-induced nonsequential double ionization of diatomic molecules for different molecular species, namely ${\mathrm{N}}_{2}$ and ${\mathrm{Li}}_{2}$. We focus on the recollision excitation with subsequent tunneling ionization (RESI) mechanism, in which the first electron, upon return, promotes the second electron to an excited state, from where it subsequently tunnels. We assume that both electrons are initially in the highest occupied molecular orbital (HOMO) and that the second electron is excited to the lowest unoccupied molecular orbital (LUMO). We show that the electron-momentum distributions exhibit interference maxima and minima due to the electron emission at spatially separated centers. We provide generalized analytical expressions for such maxima or minima, which take into account $s$-$p$ mixing and the orbital geometry. The patterns caused by the two-center interference are sharpest for vanishing alignment angle and get washed out as this parameter increases. Apart from that, there exist features due to the geometry of the LUMO, which may be observed for a wide range of alignment angles. Such features manifest themselves as the suppression of probability density in specific momentum regions due to the shape of the LUMO wave function, or as an overall decrease in the RESI yield due to the presence of nodal planes.

Bragg Scattering of Light in Vacuum Structured by Strong Periodic Fields

Elastic scattering of laser radiation due to vacuum polarization by spatially modulated strong electromagnetic fields is considered. The Bragg interference arising at a specific impinging direction of the probe wave concentrates the scattered light in specular directions. The interference maxima are enhanced with respect to the usual vacuum polarization effect proportional to the square of the number of modulation periods within the interaction region. The Bragg scattering can be employed to detect the vacuum polarization effect in a setup of multiple crossed superstrong laser beams with parameters envisaged in the future Extreme Light Infrastructure.

Acoustic monitoring of background internal waves with the use of the correlation method for measuring the frequency shifts of interference maxima

A numerical experiment is carried out to demonstrate the reconstruction of the frequency spectrum of background internal waves with the use of the correlation method for measuring the frequency shifts of interference maxima. The method is based on monitoring the behavior of the frequency shift for the maximum of the cross-correlation function of signal spectra received at different instants of time. The noise immunity of the correlation method is analyzed in comparison with the direct method based on monitoring the frequency shift of a chosen maximum of the interference pattern.

Study of interference in a double-slit without walls by plasmon tomography techniques

We investigate the free propagation of two parallel surface plasmon polaritons (SPP) beams using plasmon tomography. In the Fourier-plane images, we observed interference features that are not in correspondence with the images of SPPs on the sample's surface. We clearly observed that the interference maxima and minima are distributed over an arc of a circle. We explain the characteristics of the observed interference patterns assuming that each SPP beam can be considered as a “slit without walls”. We discuss important implications of this work for SPP tomography and interferometric plasmonic sensors.

Near-threshold femtosecond laser fabrication of one-dimensional subwavelength nanogratings on a graphite surface

Superimposed one-dimensional quasiperiodic gratings with multiple periods \ensuremath{\Lambda} \ensuremath{\approx} 110--800 nm well below or comparable to the pump laser wavelength of 744 nm, and ridge orientations perpendicular to the linear polarization of infrared femtosecond laser pulses, were fabricated after multiple near-threshold laser shots on a planar surface of quasimonocrystalline graphite in ambient air. The broad range of the grating periods corresponds to the large number of spatial Fourier harmonics of the final nanorelief (up to $m=7$th order, ${\ensuremath{\Lambda}}_{m}\ensuremath{\approx}800$ nm/$m=110--800$ nm), qualitatively representing the nonsinusoidal profile of the laser-induced intermediate surface relief (the set of periodic, broadly spaced narrow nanotrenches), which provides the corresponding multiangle diffraction of the incident femtosecond laser pulses. Experimental measurements and modeling of the transient optical constants of the photoexcited graphite justify the excitation, at the first stage, of the first-order (${\ensuremath{\Lambda}}_{1}\ensuremath{\approx}800$ nm) surface plasmon-polaritonic (SPP) wave on the photo-excited initial planar graphite surface becoming metallic via photo-generation of dense electron hole plasma ($\ensuremath{\sim}{10}^{21}$ cm${}^{\ensuremath{-}3}$). Such an SPP wave provides intermediate nanorelief in the form of the nonsinusoidal surface grating via its interference with the incident laser wave, resulting under near-threshold laser irradiation conditions in the highly localized surface ablation of the material in the interference maxima. During the next stage, the multiperiod subwavelength nanogratings develop through the multiangle diffraction of the multiple incident laser pulses on the intermediate nonsinusoidal surface grating.

PROPAGATION FACTOR AND PATH LOSS SIMULATION RESULTS FOR TWO ROUGH SURFACE REFLECTION COEFFICIENTS APPLIED TO THE MICROWAVE DUCTING PROPAGATION OVER THE SEA

The performance assessment of maritime microwave communications and radar systems requires accounting simultaneously for the non-homogeneous propagation medium over the sea and the rough sea surface scattering. The tropospheric ducting, speciflc for over water propagation, is one of the most di-cult to treat propagation mechanisms. The proposed work combines a recently published in the literature phase correction, responsible for the shadowing efiects, to the Ament rough surface re∞ection coe-cient and the Parabolic Equation method (as implemented in the Advanced Propagation Model) to simulate the microwave propagation over the sea under evaporation duct conditions. Propagation factor and path loss results calculated for phase-corrected Ament, non-phase-corrected Ament and the other widely used, Miller-Brown, rough surface re∞ection coe-cient are compared and discussed. The main efiects from the accounting of the shadowing result in the shift of the interference minima and maxima of the propagation factor, changes in the path loss pattern and destruction of the trapping property of the duct.

X-ray diffraction in a uniformly bent crystal in reflection geometry

The features of the intensity distribution over the exit surface of a uniformly bent crystal have been investigated in reflection geometry. Experiments have been performed using thin-layer heterostructures Si(1−x)Gex/Si. For heterosystems, internal stresses induced in the film and substrate lead to an elastic bending of the whole system. The section topographs exhibit deformation interference fringes. It is important that the contrast of this interference pattern is almost one order of magnitude higher than the contrast in the case of Bragg scattering in a perfect crystal. The observed interference pattern depends on the radius of bending of the crystal. As the bending radius increases, all maxima shift toward the basic Bragg peak. Correspondingly, all distances between the interference fringes decrease. It has been shown that the positions of intensity maxima do not depend on the sign of the crystal bending. For a negative sign of the radius of the crystal bending (positive strain gradient), the integrated intensity increases. The results of the numerical simulation of the diffraction images agree well with the experimental topographs. A comparison of the numerical simulation of the interference pattern with the experimental topographs makes it possible to exactly determine the radius of the crystal bending (4%). The formulas describing the positions of interference maxima as a function of the bending radius of the sample have been obtained using the results of the numerical simulation of the experiment.

Surface modification of thin tetrahedral amorphous carbon films by means of UV direct laser interference patterning

In this work, direct laser interference patterning of hydrogen-free tetrahedral amorphous carbon (ta-C) thin films using an ultraviolet (UV) nanosecond pulsed laser is investigated. Using this method, line-like arrays with submicrometer resolution were fabricated. In particular, a 180 nm grating period was successfully produced. In addition, it was found that depending on the laser energy density, the tetrahedral carbon film either graphitizes or crystallizes locally at the interference maxima positions. Furthermore, the crystallization is accompanied with a delamination of the film and even the formation of carbon fiber-like structures. The electrical properties of graphitized and delaminated ta-C were measured.