Single Period Structures Research Articles

Ultra-broadband selective tailoring of infrared spectral features for the application of compatible infrared and laser stealth

With the rapid development of detection technology, aircraft face growing threats from various detectors covering visible to infrared light. Therefore, aircraft must be equipped with the ability of compatible stealth. The emergency of the metasurface provides a promising method to realize this purpose. On this basis, in this work, a composite periodic metasurface is proposed by employing the type of metal-insulator-metal (MIM) structure for compatible infrared and laser stealth. In one unit cell, different dimensions of patterns are used to give rise to magnetic polaritons (MPs) at specific wavelengths. Through overall optimization of parameters of single periodic structures and the internal mechanisms in composite structures, appropriate parameters and arrangements are selected to obtain the required spectral features from laser to mid-infrared wavelengths. The simulated results demonstrate that the absorption at 1.06 μm can be over 70% and the absorption peaks in the region of 5∼8 μm are over 74%. High absorption at 1.06 μm is suitable for laser stealth, and the ability of high-performance infrared stealth comes with the advantages of low emissivity at the atmosphere windows and radiative cooling at the non-atmosphere window. This design is easy to fabricate and has a promising perspective for various compatible stealth and many other applications.

Antenna-enhanced high-resistance photovoltaic infrared detectors based on quantum ratchet architecture

We demonstrate a quantum ratchet detector, which is a high-resistance photovoltaic mid-infrared detector based on an engineered spatial arrangement of subbands. In photovoltaic quantum-well photodetectors, in which unidirectional photocurrent is generated by asymmetric quantum-well structures, maximization of device resistance by suppressing undesired electron transports is crucial for minimizing noise. A semi-quantitative guideline suggests the significance of spatial separation between wavefunctions for reducing the conductance from the ground state. Here, we employ a step quantum well made of a shallow floor and a deep well. Photoexcited electrons are quickly transferred to a separated location from the ground state through fast resonant tunneling and phonon scattering, and then they are allowed to flow in only one direction. This architecture is made possible by the use of a GaAs/AlGaAs material system, and it achieves a resistance as high as 6.0 × 104 Ωcm2 with a single-period structure. Combined with optical patch antennas for responsivity enhancement, we demonstrate a maximum background-limited specific detectivity of 6.8 × 1010 cmHz1/2/W at 6.4 μm, 77 K for normal incidence, and a background-limited-infrared-photodetector temperature of 98 K.

Vectorial far-field method for subwavelength grating based VCSELs

We proposed in detail a procedure for the vectorial far-field analysis of high-contrast subwavelength gratings (HCGs) for vertical cavity surface emitting lasers (VCSELs). The vectorial far-field analysis method (VFM) is based on the vectorial angular spectrum method. The transmission and reflection coefficients of the HCG are used to modulate the polarized far-field components, and the modulated far-field components are vectorially composited to obtain the final transmitted and reflected far-field distribution. The reflectance and transmittance of the HCG to a two-dimensional finite-size beam can be solved by the integral of the output beam. The near field of the transmitted and reflected beam can be solved by an inverse Fourier transform of the far-field distribution. The VFM can be effectively applied in the transmit and reflect far-field beam shaping design and polarization analysis of HCG-VCSELs. The near field reconstructed by VFM can be applied to the calculation of coupling efficiency and lens design. The results of VFM are verified by a three-dimensional finite-difference time domain based on the licensed RSoft FullWAVE tool. VFM only needs to calculate a single-period structure, and the response of HCG to various near-field distributions can be obtained by one calculation, which provides an efficient and accurate method for the design of polarized HCG-VCSELs. The VFM reveals the underlying physics of the shaping effect of subwavelength grating on polarized beams and can be easily extended to the far-field analysis of arbitrary periodical meta-surfaces.

Interfacial delamination-induced unidirectional propagation of guided waves in multilayered media

Unidirectional nonreciprocal wave propagation is an unprecedented phenomenon, which has attracted much research interest. Connecting a phononic crystal with an asymmetric structure to break the spatial inversion symmetry is a popular manner to realize this phenomenon using the wave mode transformation. In this paper, a new model is proposed based on a single periodic structure. The arrays of asymmetric and symmetric interfacial delaminations are intentionally introduced into the top and the bottom part of a stack of periodic elastic layers, respectively. So, the structural spatial inversion symmetry can be broken and the guided waves can pass through the whole structure only from the top side with the changed mode generated by the array of asymmetric interfacial delaminations. Thus, it is indispensable for the part of phononic crystal that the partial band-gaps of symmetric and antisymmetric guided waves have to be separated, which is the reason why we introduce the array of symmetric central or side interfacial delaminations into the stack of periodic elastic layers. The transmission spectra of the guided waves and the dispersion curves for the unit cell imposed by the Bloch–Floquet boundary condition are both calculated by the spectral element method. Then, the interfacial delamination-induced unidirectional propagation of guided waves in the finite stack of periodic elastic layers is numerically confirmed. This paper provides a new concept to control the waves propagating in phononic crystals via the insertion of some interfacial delaminations or cracks.

Study on the nonreciprocal absorption properties of cylindrical photonic crystals embedded in graphene cascaded by periodic and Rudin–Shapiro sequences at large incident angles

Using the transfer matrix method, the absorption, reflection, and nonreciprocity of the cylindrical photonic crystals (CPCs) consisting of graphene and two layers of ordinary medium cascaded by a periodic sequence and a Rudin–Shapiro quasiperiodic sequence are investigated under a large incidence angle of electromagnetic wave. By comparing the cascade of two periodic structures and the case of a single periodic structure, it is concluded that the structure proposed in this paper has better nonreciprocal phenomena and wider relative absorption bandwidth at a large incidence angle, reaching 162.2%, which is also much higher than the general planar photonic crystals. The absorption performance of this structure in TE and TM modes is compared at different angles and it is found that TM mode has a wider absorption bandwidth and has an ideal bandwidth in a large range of incident angle from 20° to 80°. Meanwhile, the optimum parameters of chemical potential and medium thickness are discussed, which can meet the requirements of large absorption bandwidth and significant nonreciprocity at a large incident angle. The CPCs embedded in graphene adopted in this paper are structures that have never been studied before. The electrical conductivity of graphene can be adjusted by the chemical potential, which can more conveniently realize many optical phenomena and provide reference and application values for optical sensing, optical filtering, and optical detection.

An improved X-band relativistic triaxial klystron amplifier with active suppression of asymmetric TM mode self-excitation

The self-excitation of asymmetric modes is one of the main factors limiting the long-pulse operation of the relativistic triaxial klystron amplifier (TKA). Research studies have revealed that the asymmetric TM modes with negative beam loading conductance (Ge/G0) in the bunching cavities are the main culprits of self-excitation. The existing studies focus on passively destroying the growth of asymmetric TM modes, but we attempt to actively suppress their self-excitation in this paper. Investigation results demonstrate that the employment of single-gap bunching cavities can significantly reduce the risk of self-excitation of asymmetric TM modes, as the Ge/G0 of asymmetric TMn11 modes in single period structures are positive. The current modulation capacity of a single-gap bunching cavity, however, is not sufficient, so a cascade structure consisting of two groups of bunching cavities is employed to compensate for this deficiency. Based on the above conclusions, an X-band TKA device with four single-gap beam–wave interaction cavities is proposed and investigated in this paper. The three-dimensional particle-in-cell simulation results demonstrate that there is no self-excitation of asymmetric modes in the proposed X-band TKA device.

A wave finite element approach for the analysis of periodic structures with cyclic symmetry in dynamic substructuring

A numerical approach is proposed to compute the dynamic response of periodic structures with cyclic symmetry, and assemblies made up of these periodic structures. The wave finite element (WFE) method is used to describe the wave modes which occur around the circumferential direction of these periodic structures. Emphasis is placed on assessing the dynamic flexibility modes of a periodic structure by considering unit forces which are successively applied to the boundary degrees of freedom. It is shown that the matrices of dynamic flexibility modes can be quickly computed. This yields an efficient dynamic substructuring technique to analyze the dynamic behavior of assemblies made up of several periodic structures. Numerical experiments are carried out which concern the analysis of a single periodic structure as well as assemblies made up of two and three structures.

Contactless Air-Filled Substrate Integrated Waveguide

The contactless version of the air-filled substrate integrated waveguide (AF-SIW) is introduced for the first time. The conventional AF-SIW configuration requires a pure and flawless connection of the covering layers to the intermediate substrate. To operate efficiently at high frequencies, this requires a costly fabrication process. In the proposed configuration, the boundary condition on both sides around the AF guiding medium is modified to obtain artificial magnetic conductor (AMC) boundary conditions. The AMC surfaces on both sides of the waveguide substrate are realized by a single-periodic structure with the new type of unit cells. The PEC–AMC parallel plates prevent the leakage of the AF guiding region. The proposed contactless AF-SIW shows low-loss performance in comparison with the conventional AF-SIW at millimeter-wave frequencies when the layers of both waveguides are connected poorly.

Dissociation of the 60° basal dislocation in wurtzite GaN

AbstractWe have carried out atomistic calculations, using a self‐consistent tight‐binding method based on density functional theory, to determine formation energies of different core configurations of the perfect 60° and 90°, 30° Shockley partial dislocations in wurtzite GaN. Our analysis of different combinations of core configurations concluded that a 60° dislocation with a Ga polarity dissociates into a 90° partial dislocation with a single period and a 30° partial dislocation with a double period structure. Otherwise, a N polarity 60° dislocation dissociates into a 90° partial dislocation with a double period and a 30° partial dislocation with a single period structure (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis of enlargement of the omnidirectional total reflection band in a special kind of photonic crystal based on the incident angle domain

A special kind of one-dimensional (1D) magnetic photonic crystal (MPC) is investigated. The refractive indices of the alternating layers in such a MPC are equal. As a result, the stop-band edges of TE mode coincide with those of TM mode, and the stop-bands for both modes expand as the incident angle increases. Based on the incident angle domain method, the omnidirectional total reflection (ODTR) band determined by the wave impedance ratio between the composites can be enlarged substantially using a heterostructure in which the stop-band edges of the sub-PCs can be completely adjacent at any incident angle. Moreover, the requirement of ODTR of each sub-PC is unnecessary, which extends the range of materials; that is, the wave impedance ratio between the composites and the refractive index ratio between the composites and the incident media can be much lower than that required in a single periodic structure.

Criterion of omnidirectional reflection in a one-dimensional photonic heterostructure

We systematically investigate the criterion of omnidirectional reflection in one-dimensional photonic heterostructures and show that the photonic heterostructures can extend not only the frequency range of omnidirectional reflection but also the range of materials; that is, the refractive-index contrast between the composites can be much lower than those required in single periodic structures. This idea is of immense practical importance and can be extended to other structures whose underlying physics are the same as those of a photonic heterostructure, such as a staggered structure and some types of quasi-period structure.

Planar millimetre wave structures

Periodic structures in the millimetre wave range, with high precision requirements such as those needed for travelling wave tubes or for particle acceleration, must be planar structures for fabricational reasons. This paper treats single and double periodic devices semi-analytically with the mode matching technique. The method is explained for a side-coupled structure where Floquet’s theorem was generalized for point-symmetric arrangements. With only slight modifications, the results can be adjusted to the case of on-axis coupled double-periodic structures. A special case is the standard single-periodic structure. Results show dispersion diagrams, field plots and beam coupling impedances.

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.

Finite element modeling of single periodic curved gratings

The analysis of the scattering of a plane acoustic wave by an in-plane periodic, active, or passive structure has been previously performed with the help of the finite element method [A. C. Hladky-Hennion etal., J. Acoust. Soc. Am. 94, 621–635 (1993)]. In that case, the compliant tubes grating is a single periodic structure that is periodic in one direction and infinite in the other one. The method previously described has been extended to take into account the effect of the curvature of the grating, periodic along the symmetry axis. Within this approach, only a bidimensional mesh is needed because the structure is axisymmetrical. Moreover, only one unit cell of the periodic structure, including a small part of the surrounding fluid domain, has to be meshed, due to the use of a classical Block-type relation between the displacement components of points that are separated by the grating spacing. Finally, the effect of the external fluid domain is accounted for by matching the pressure field in the finite element domain to plane waves expressed in terms of a series of Bessel or Hankel functions. This paper describes the general mathematical model and then provides the results obtained for the scattering of a plane wave form simple axisymmetrical structures, this validating the method. Next, the method is applied to the case of curved compliant tubes gratings, thus showing the influence of the curvature.

Scanning Tunneling Microscopy of the ReO3 Surface

The surface of a ReO3 single crystal was studied using the scanning tunneling microscope (STM) with a tungsten tip. Atomic images were taken and they revealed that the surface consists of {100} planes, and that there are many drains whose depth and width are integer multiples of the unit cell length. There are biperiodic structures on the surface coexisting with a single-period structure. The biperiodic structure may originate in the biperiodic arrangements of the top oxygen ions.

Analysis of the scattering of a plane acoustic wave by a doubly periodic structure using the finite element method: Application to Alberich anechoic coatings

The finite element approach has been previously used, with the help of the ATILA code, to model the scattering of acoustic waves by single periodic structures, such as compliant tube gratings [A. C. Hennion et al., J. Acoust. Soc. Am. 87, 1861–1870 (1990)]. In this paper, the same approach is extended to doubly periodic structures, such as Alberich anechoic coatings. To do this, only the unit cell of the periodic structure, including a small part of the surrounding fluid domain, has to be meshed, due to the use of classical Bloch type relations. Then, the effects of the remaining fluid domain are accounted for by matching the pressure field in the finite element mesh with simple plane-wave expansions of the ingoing and outgoing waves. After an outline of the method, the paper describes the results obtained for the scattering of a plane wave by different periodic structures. Internal losses are taken into account and the incident plane wave impinges at normal or oblique incidence. Numerical results obtained for Alberich anechoic coatings are first analyzed to check the convergence and then compared to previous numerical results or to experimental results, demonstrating that the finite element approach is accurate and well suited to predict the behavior of these gratings. Moreover, careful attention is devoted to the analysis of the inclusion vibrations, to identify the origin of the resonance mechanisms.

The importance of examining cohort uniqueness in the formulation of human investment policies

It is shown in this paper that the same single-period structure in relationships among schooling, experience, and earnings can have sharply contrasting meanings for life-cycle patterns. One of the observations may refer to a static economy in which expanding education at the higher levels brings diminishing marginal effects on national income, declining internal rates of return to investments in schooling and little learning in the post-school years. Yet exactly the same single-year age crosssection structure could reflect quite the opposite situation, with rising marginal social returns over successive cohorts, rising internal rates of return to the investments in higher education, and rising contributions of experience to productivity. This is the scenario of a dynamic economy in which investments in post-school human resource formation and in other inputs complementary with the services of well-educated people keep pace with the expansion in schooling. The contrast is not one of poor versus rich societies but of static versus dynamic ones. Policy implications in the two situations would be very different despite their identity in a conventional benefit-cost accounting.

Abrupt transitions in composition and doping profile in GaAs–Ga1−xAlxAs heterostructures by atmospheric pressure MOVPE

Ga1−xAlx As–GaAs quantum wells are grown by atmospheric pressure MOVPE. Wells as narrow as 25 Å are made. Interface quality in relation to alloy clustering and abruptness in change of Al content is characterized with the aid of luminescence spectrum measurements. A remarkable agreement is found between the experimental n=1 electron to heavy hole transition energies and those which are calculated with the simple rectangular well model. The results indicate that the Al content changes at the interfaces over less than one unit cell. Single and multiple period modulation-doped GaAs–Ga1−xAlxAs heterostructures have been grown. Single period structures exhibit electron mobilities of 6700, 80 000, and 90 000 cm2 V−1s−1 at 300, 77, and 4 K for sheet carrier concentrations of 7.4×1011, 8.3×1011, and 3.9×1011 cm−2, respectively. To our knowledge, these values represent the best mobilities reported in this structure grown by atmospheric pressure MOVPE. Preliminary results on field effect transistors show a threefold increase of transconductance as temperature decreases from ambient to 77 K. Furthermore, we report mobility enhancement in ‘‘inverted’’ structures in which the conducting accumulation layers are located at the interfaces formed by growth of GaAs on top of (Ga,Al)As.

Current Transport in Modulation-Doped AlxGa1-xAs/GaAs Heterojunction Structures at Moderate Field Strengths

Current vs. electric field characteristics of single and multiple period modulation-doped GaAs–AlxGa1-xAs heterostructures are reported for lattice temperatures in the range of 10–300K. Measurements were performed at electric fields up to 2 kV/cm which is below the field range where non-linear effects due to real-space transfer or κ-space transfer (Ridley-Watkins-Hilsum (RWH) mechanism) are expected to occur. The basic heterostructure included an undoped “intrinsic” layer of AlxGa1-xAs between each doped (∼1×1017 cm-3) AlxGa1-xAs layer and undoped GaAs layer. The low field mobilities were found to decrease sharply with increasing electric field at low temperatures, but remained nearly constant at higher temperatures. The low field mobilities of the single period structures showed significant improvement with increasing AlAs mole fraction in the range 0.16≤x≤0.36. The increased mobility enhancement occurs in these structures due to the virtual elimination of electron scattering by increasing the spacing between the donors and electrons with the undoped AlGaAs layers. The current-field characteristics showed this mobility enhancement to increase, reach a maximum and then decrease with increasing undoped layer thickness. At a field strength of 2 kV/cm, modulation-doped single period (x=0.36) structures showed an improvement in current of four times at 78 K and 1.6 times at 300 K, compared to bulk GaAs with identical electron concentration. If used for normally-off field effect transistors (FETs) a speed improvement of 60% at 300 K and 400 % at 78 K can be obtained as compared to conventional normally-off GaAs FETs.

Electron mobility in single and multiple period modulation-doped (Al,Ga)As/GaAs heterostructures

Single and multiple period modulation-doped (Al,Ga)As/GaAs heterostructures have been grown by molecular beam epitaxy (MBE) and characterized by Hall measurements. The basic structure consists of alternating layers of undoped GaAs and doped (Al,Ga)As separated by undoped ’’intrinsic’’ layers of (Al,Ga)As. The thickness of each of these layers was varied independently in a one, three, or nine period structure. Mobilities as high as 211 000, 95 500, and 8 360 cm2V−1 s−1 were obtained at 10 K, 78 K, and 300 K, respectively. The total sheet charge concentrations were about 2.25×1012 cm−2 in the highest mobility structures. Single period structures with an AlAs mole fraction of 0.33 typically had higher room-temperature mobilities while multiple period structures with an AlAs mole fraction of 0.2 gave better results at cryogenic temperatures. The enhanced mobilities are attributed to the separation of electrons and donors by the undoped (Al,Ga)As layers and the high quality interfaces obtainable by MBE. To our knowledge, these results on single and multiple period structures are comparable to the best values reported to date.