Multi-period Structure Research Articles

Multiplexing complex two-dimensional photonic superlattices

We introduce a universal method to optically induce multiperiodic photonic complex superstructures bearing two-dimensional (2D) refractive index modulations over several centimeters of elongation. These superstructures result from the accomplished superposition of 2D fundamental periodic structures. To find the specific sets of fundamentals, we combine particular spatial frequencies of the respective Fourier series expansions, which enables us to use nondiffracting beams in the experiment showing periodic 2D intensity modulation in order to successively develop the desired multiperiodic structures. We present the generation of 2D photonic staircase, hexagonal wire mesh and ratchet structures, whose succeeded generation is confirmed by phase resolving methods using digital-holographic techniques to detect the induced refractive index pattern.

The improvement of the Morlet wavelet for multi-period analysis of climate data

The multi-level dynamics of an atmosphere system exhibits temporal structures in different types of climate data. This article addresses two issues in multi-period analysis of climate data. Firstly, the advantages of the modified Morlet wavelet transform (MMWT) for analyzing multi-period structure of time series over Morlet wavelet transform (MWT) are emphasized. Secondly, the multi-period issues of temperature data are studied with MMWT through four steps: the four dominant periods of 60 year temperature data are determined with the wavelet variance; by analyzing the real part of MMWT, the warm and cold stages of the temperature data at different scales are determined, and the time intervals of the warm and cold interchange are singled out; the amplitude of each periodic component is quantitatively characterized by the amplitude of wavelet coefficients; the most intensive oscillation time intervals are computed by the squared modulus of the MMWT (MMPS).

Multiperiodic structure for unipolar fountain laser

The parameters providing suppression of the intersubband nonradiative relaxation between laser levels have been optimised based on the calculations of the potential profile and relaxation times for the active element of unipolar laser with strongly asymmetric (in height) barriers. The photoluminescence, photoluminescence excitation, and IR absorption spectra of a structure with a multiply repeated optimised active element have been investigated. The experimental data are found to be in good agreement with the calculation results.

Frequency band structure and absorption predictions for multi-periodic acoustic composites

This article introduces a computational framework for studying frequency band structure and absorption behavior in multi-periodic acoustic composite structures. Herein, multi-periodic acoustic composite structures are defined as periodically-layered acoustic media wherein each layer is composed of periodically-repeated fluid unit cells, especially those arising from the study of porous materials. Hence, at least two periodic scales (microscopic and mesoscopic, respectively) comprise the macroscopic acoustic composite media. Exploitation of the multi-periodicity allows for efficient generation of dispersion and absorption curves via the conventional multi-scale asymptotic method (for homogenizing the mesoscale) coupled to the acoustic transfer matrix methods (for the macroscale). The combined computational framework results in a single analysis procedure for evaluating complex dispersion relationships and acoustic absorption. The dispersion curves can be used to reveal frequency stop bands wherein the wave vector is highly imaginary—i.e., plane waves experience rapid attenuation. They can also be used to reinterpret classical absorption curves. The framework is applied to four multi-periodic acoustic composite structures in order to demonstrate the framework's utility and to reveal novel properties, particularly those which can be influenced by design of the mesoscopic unit cell.

Geological characteristics of low rank coalbed methane, China

Abstract Low rank coalbed methane is a very important and potential field of coalbed methane exploration and exploitation in China. The geological structure backgrounds of coal-bearing basins are complicated, and many coalbeds are reconstructed by multiperiod and multiproperty structures and combinations, as well as stress-strain. The accumulation and enrichment of coalbed methane have the following characteristics: big thickness, multiple beds, low gas content, preferable permeability, great resource, and abundance. The thick and widely distributed coalbeds and huge coal resources offset the shortcoming of low gas content, probably leading to the good prospect of exploration and exploitation. Matrix shrinkage effect induced by desorption causes high permeability in exploitation of low rank coalbed methane reservoirs, which is good for coalbed methane exploitation, and prone to form industrial gas flow. Low rank coalbed methane reservoirs have a simple forming process, including one deposition and one adjustment. If the main controlling factors, such as structure, coal-forming environment, and hydrologic geology, are matched well, the enriched coalbed methane zone with high production would be formed.

TIMES-EU: a Pan-European model integrating LCA and external costs

This paper deals with an experience of cooperative research carried out in the context of EU Sixth Framework Programme’s Integrated Project New Energy Externalities Developments for Sustainability (NEEDS, 2004–2008), and in particular in the research stream 2a ‘Modelling internalisation strategies including scenario building’, co-ordinated by the IMAA, CNR. The main objective of this work is to analyse scenarios with generated partial equilibrium technology rich energy models of 29 countries (EU-25 States plus Iceland, Norway, Romania, Switzerland), integrated into a Pan-European model, capable of analysing the impacts of different policies and price mechanisms, and also evaluating them at technology level both at the country level and from an EU-wide perspective. All these models are characterised by a multi-period structure (base-year: 2000, last milestone year: 2050) and will include, in the Pan-European model, the most important life-cycle emissions, materials, and costs analysed by other NEEDS research streams (life cycle assessment and ExternE). This paper describes the current status of the work carried out in research stream 2a, including the basic methodological assumptions and scenario generation.

Two-period model for calculation of level populations in subbands of multi-period quantum-cascade superlattice structures

For two periods of quantum-cascade laser structures, we propose a system of closed balance equations that make it possible to calculate the occupancy of the energy levels, the quasi-Fermi levels, and also the injection current density taking into account different charge carrier scattering mechanisms.

Genetic algorithm using independent component analysis in x-ray reflectivity curve fitting of periodic layer structures

A novel genetic algorithm (GA) utilizing independent component analysis (ICA) was developed for x-ray reflectivity (XRR) curve fitting. EFICA was used to reduce mutual information, or interparameter dependences, during the combinatorial phase. The performance of the new algorithm was studied by fitting trial XRR curves to target curves which were computed using realistic multilayer models. The median convergence properties of conventional GA, GA using principal component analysis and the novel GA were compared. GA using ICA was found to outperform the other methods with problems having 41 parameters or more to be fitted without additional XRR curve calculations. The computational complexity of the conventional methods was linear but the novel method had a quadratic computational complexity due to the applied ICA method which sets a practical limit for the dimensionality of the problem to be solved. However, the novel algorithm had the best capability to extend the fitting analysis based on Parratt's formalism to multiperiodic layer structures.

Integrating market and credit risk: A simulation and optimisation perspective

We introduce a modelling paradigm which integrates credit risk and market risk in describing the random dynamical behaviour of the underlying fixed income assets. We then consider an asset and liability management (ALM) problem and develop a multistage stochastic programming model which focuses on optimum risk decisions. These models exploit the dynamical multiperiod structure of credit risk and provide insight into the corrective recourse decisions whereby issues such as the timing risk of default is appropriately taken into consideration. We also present an index tracking model in which risk is measured (and optimised) by the CVaR of the tracking portfolio in relation to the index. In-sample as well as out-of-sample (backtesting) experiments are undertaken to validate our approach. The main benefits of backtesting, that is, ex-post analysis are that (a) we gain insight into asset allocation decisions, and (b) we are able to demonstrate the feasibility and flexibility of the chosen framework.

A sandwiched multiperiod EBG structure for microstrip patch antennas

A microstrip patch antenna with a 2D multiperiod electromagnetic band gap (EBG) sandwiched between the patch and the ground plane is proposed and studied in this paper. The proposed models are assessed using finite-difference time-domain (FDTD) simulations and experimental measurements. The simulated and measured results show that, with the sandwiched multiperiod EBG structure, the microstrip patch antenna can achieve wider impedance bandwidth and higher gain than the sandwiched single-period EBG structure. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 437–440, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21010

Prediction of fracture-cavity system in carbonate reservoir: A case study in the Tahe oilfield

The carbonate rocks in Tahe oilfield, which suffered from multi-period polycycle karstification and structure deformation, are heterogeneous reservoirs that are rich in pores, cavities, and fractures. The reservoirs are diversified in scale, space configuration, and complex in filling. For this kind of reservoir, a suite of seismic prestack or poststack prediction techniques has been developed based on the separation of seismic wavefields. Through cross-verification of the estimated results, a detailed description of palaeogeomorphology and structural features such as pores, cavities, and fractures in unaka has been achieved, the understanding of the spatial distribution of reservoir improved.

Mid-infrared electroluminescence from device with changeable electron–hole distance

A report is presented on the observation of mid-infrared electroluminescence from a multi-period bilayer type-II structure in which the effective interlayer separation is controlled by bias. The emission with powers in the microwatt range is characterised by a linear dependence of the photon energy on the bias.

Multiperiod EBG structure for wide stopband circuits

A multiperiod electromagnetic band-gap (EBG) structure is proposed and studied. The stopbands of single-period and multiperiod EBG structures are assessed using finite-difference time-domain (FDTD) simulation and experimental measurements. Simulated and measured results show that the multiperiodic EBG structure can achieve a wider stopband compared to a single period EBG structure.

Engineering of a dual-periodic optical superlattice used in a coupled optical parametric interaction

Quasi-phase matching facilitates the phase matching of any parametric interaction by setting an appropriately modulated period. We propose a dual-periodic structure that is used in the construction of a frequency-conversion device. For example, we have designed and fabricated such a dual-periodic structure in a LiTaO3 crystal. Using a fundamental source at 1.064 µm, we obtained ultraviolet radiation at 355 nm and green radiation at 532 nm simultaneously by frequency tripling and frequency doubling the fundamental source. Theoretically, this idea can be further extended to the design of a multiperiodic structure for achievement of more quasi-phase-matched processes in a single optical superlattice.

Nonequilibrium photocurrent modeling in resonant tunneling photodetectors

An efficient and versatile many-body nonequilibrium approach is formulated for computation of photocurrent and photoexcited properties of device structures where quantum effects dominate. This method, based on nonequilibrium Green’s function quantum transport equations, makes it possible to consider open systems of arbitrary dimensionality having complex potentials, complex geometries, and multiple terminals. In contrast to other approximate computational approaches, no a priori assumptions regarding the particular nature of the phototransitions are required (i.e., bound-to-bound, bound-to-continuum, or continuum-to-continuum). Furthermore, if desired, electron–phonon and electron–electron interactions can also be rigorously accounted for within the same formalism. In this article, the method is applied to two typical resonant-tunneling infrared detector heterostructures as examples: (1) a single-quantum-well structure, and (2) a multiperiod superlattice structure.

Long-term variations in the characteristics of the equatorial stratospheric zonal winds

Stratospheric equatorial zonal winds from 1951 onwards up to the present show considerable long-term variations, more so at higher levels, These are rarely monotonic and often show multi-periodic structures, including a sunspot cycle (II year variations), Stratospheric temperatures and geopotential heights also show multi-periodic variations, A periodicity near 20 years is encountered often.

Multiperiod piezoelectric-barrier all-optical light modulator

We present preliminary new results on multiperiod piezoelectric-barrier all-optical modulators. It is shown first on single period structures that an incident beam of a few W/cm2 is able to create a space-charge field of ∼20kV/cm in the structure, inducing a 10meV shift in the photoluminescence (PL) spectrum. We discuss then a multiperiod structure showing similar optically-driven PL shifts. Even though the multiperiod structure was not optimized for transmission modulation experiments, we observed a 12% transmission variation at 760nm induced by a 10W/cm2 optical beam.

Projective imaging of periodic objects: recognition and retrieval of lattice structures in real space

Projective illumination of periodic objects by a pointlike source gives rise to magnified self-focussed images in a denumerable set of focal planes without the use of lenses or mirrors. Using a HeNe laser with pinhole spatial filter and CCD camera, we demonstrate that periodic detail can be retrieved selectively from a multiperiodic structure and in the presence of random aperiodic detail. The selectivity of the process increases with the width of the Fourier spectrum of the object transmission function. With the development of appropriate sources, the principles tested here should prove especially useful in imaging and structure analysis by short-wavelength electromagnetic radiation and by matter waves for which suitable lenses either do not exist or give rise to aberrations difficult to correct.

Rapid Oscillations in Cataclysmic Variables. XIII. WZ Sagittae Revisited

ABSTRACTWe report new observations of the dwarf nova WZ Sagittae, showing the return of the 27.87 s oscillation in 1995/1996 high‐speed optical light curves. As was true in 1978, the main peak of the power spectrum generally occurred at 27.87 or 28.96 s, with the signals sometimes simultaneously present. Weak transient signals were also seen at 28.20 and 29.69 s. An ASCA X‐ray observation showed a significant peak at 27.86 ± 0.01 s in the 2–6 keV band, which appeared to establish the star's membership as a “DQ Herculis star,” in which accretion onto the white dwarf is channeled by a strong magnetic field. The X‐ray period is most likely the true white dwarf spin period. However, we certainly do not understand the distribution of power among these various signals of slightly different period. This puzzling multiperiodic structure remains an impediment to full acceptance of WZ Sge as a DQ Her star. The star's soft X‐ray orbital light curve shows a strong but transient energy‐dependent dip around orbital phase 0.7, which is probably due to absorption by an extended gas cloud in which the mass‐transfer stream strikes the disk.

Growth Transients During the Nucleation of GaInAs on InP by Organometallic Vapor Phase Epitaxy

AbstractBy using lattice matched short period (50 Å) GaInAs/InP superlattices, we are able to extensively characterize, ex-situ, nucleation of thin (20–30 Å) GaInAs layers on InP. Systematic variation of the interface formation technique allows us to understand how parameters such as growth temperature and growth rate can contribute to interfacial roughness, giving rise to growth transients near the interfaces that are unexpected from characterization of thick layers. Growth of multi-period structures such as those studied here enables the use of Raman scattering, photoluminescence, and double crystal x-ray diffraction to determine that Ga incorporation is favored over In incorporation near the InP to GaInAs interfaces. Furthermore, by examining the surfaces and structural properties of epitaxial layers (15 Å-1 μm) of both InP and GaInAs with atomic force microscopy and Raman scattering, respectively, we are able to correlate results on the superlattice structures with results from single layers. Finally, we show how understanding the nucleation of thin films through the process described here enables the growth of complicated, previously unrealizable, heterostructures for use in novel optoelectronic devices.