Asymmetry Of Unit Cell Research Articles

A blueprint for truncation resonance placement in elastic diatomic lattices with unit cell asymmetrya).

Elastic periodic lattices act as mechanical filters of incident vibrations. By and large, they forbid wave propagation within bandgaps and resonate outside them. However, they often encounter "truncation resonances" (TRs) inside bandgaps when certain conditions are met. In this study, we show that the extent of unit cell asymmetry, its mass and stiffness contrasts, and the boundary conditions all play a role in the TR location and wave profile. The work is experimentally supported via two examples that validate the methodology, and a set of design charts is provided as a blueprint for selective TR placement in diatomic lattices.

Propagating plasmon in periodical graphene structure

The transformation of electromagnetic wave into propagating plasmon in periodical dual-grating-gated graphene with asymmetric unit cell structure is theoretically investigated. The conditions of cancelling the transmission of the electromagnetic wave through the graphene structure and Fabry-Perot blocking the reflection are applied. We found out optimal unit cell asymmetry for maximum transformation incident wave into plasmon modes with the wavevectors multiplied the inverse length of the structure period.

Resonant peak splitting in finite periodic superlattices with an unit cell of two barriers and two wells on monolayer graphene

The general expressions for transmission probability and resonant peaks in one-dimensional N-periods graphene superlattice with unit cell of two barriers and two wells are analytically derived, and two types of resonant peaks are obtained: (1) the periodicity induced resonant peaks splitting of (N − 1)-fold as N increases; and (2) the resonant peak through a unit cell unchanged as N varies. As the two-barriers in unit cell become asymmetric, the resonance transmission probability of unit cell becomes imperfect (T1 < 1), which drops quickly with the unit asymmetry increases. Thus, the unit cell related resonant peak could only be observed in superlattices with less unit cell asymmetry of a few of period numbers. With the period increases, the unit related resonant peak disappears and only periodicity induced (N − 1)-fold splitting remains. The splitting rule is further confirmed by the conductance and noise versus the incident energy and the misunderstandings in publication domain is cleared up.

Giant effect of terahertz-radiation rectification in periodic graphene plasmonic structures

The rectification of terahertz radiation due to plasmonic nonlinearities in a periodic graphene structure with a dual-grating gate and an asymmetric unit cell is considered. The influence of unit-cell asymmetry on the plasmonic rectification of the terahertz radiation near higher plasmonic resonances is considered for the cases of electron and hole conduction in parts of the graphene unit cell. The current responsivity of the rectification of terahertz radiation due to the plasmonic effects of differential charge-carrier drag and electron–hole ratchet is calculated.

Broadside Dual-Channel Orthogonal-Polarization Radiation Using a Double-Asymmetric Periodic Leaky-Wave Antenna

The paper demonstrates that double unit-cell asymmetry in periodic leaky-wave antennas (P-LWAs), i.e. asymmetry with respect to both the longitudinal and transversal axes of the structure -- or longitudinal asymmetry (LA) and transversal asymmetry (TA) -- allows for the simultaneous broadside radiation of two orthogonal modes excited at the two ports of the antenna. This means that the antenna may simultaneously support two orthogonal channels, which represents an interesting polarization diversity characteristics for wireless communications. The double asymmetric (DA) unit cell combines a circularly polarized LA unit cell and a coupled mode TA unit cell, where the former provides equal radiation in the series and shunt modes while the latter separates these two modes in terms of their excitation ports. It is also shown that the degree of TA in the DA unit cell controls the cross-polarization discrimination level. The DA P-LWA concept is illustrated by two examples, a series-fed line-connected patch (SF-LCP) P-LWA and a series-fed capacitively-coupled patch (SF-CCP) P-LWA, via full-wave simulation and also experiment for the SF-LCP P-LWA case.

Transversal Asymmetry in Periodic Leaky-Wave Antennas for Bloch Impedance and Radiation Efficiency Equalization Through Broadside

This paper demonstrates that unit cell asymmetry with respect to the transversal axis -or transversal asymmetry- is an essential design parameter in periodic leaky-wave antennas (P-LWAs). Specifically, it shows that transversal asymmetry can be leveraged to fully and systematically solve the well-known radiation degradation of P-LWAs at broadside, where it provides both open-stopband closure and efficiency equalization. The problem is addressed via a generic equivalent circuit model composed of a series resonator, a shunt resonator and ideal transformers for modeling asymmetry by a single and simple parameter, namely the transformation ratio. Once the series and shunt frequencies have been balanced (frequency-balancing), equalization is ensured by adjusting the degree of asymmetry in the unit cell so to match the at-broadside Bloch impedance to the off-broadside Bloch impedance. This equalization condition is referred to as quality factor balancing ( Q-balancing) and it is related to the Heaviside condition (distortionless propagation) in homogeneous transmission lines. Based on this theory, optimization schemes for employing commercial fullwave eigenmode and drivenmode solvers are proposed to design unit cells with equalized efficiencies. Finally, two examples of P-LWAs are presented, a composite right/left-handed (CRLH) P-LWA and a series-fed coupled patch (SFCP) P-LWA, and verified to fully confirm the predictions of the theory obtained by circuit modeling.

High second-order NLO responses of dehydrogenated hydrogen cyanide borane(1) oligomers

Second-order non-linear optical (NLO) properties of dehydrogenated hydrogen cyanide borane(1) oligomers (up to dodecamers) are investigated by the Hartree–Fock, hybrid DFT (including long-range corrected functionals) and MP2 approaches. Due to one-dimensional extension of the systems the longitudinal component of the properties is addressed with focus on the electronic contributions. Direct and indirect electron correlation effects are evaluated at the MP2 level paying particular attention to the basis set effects. Unit-cell asymmetry and delocalization effects in the NLO responses are discussed in terms of the evolution of the bond-length alternation parameter, dipole moment and polarizability (reduced per unit cell) with the increasing chain length. It was found that suitable combination of the unit-cell asymmetry and electron delocalization leads to extremely large NLO response (MP2/6-311++G(d,p) value for the dodecamer is 43.7 × 10 3 a.u. per unit cell) what makes the conjugated BCN oligomers (if one takes into account their thermodynamic stability) a promising material for NLO applications.

Enhancement of the second-order NLO responses of boron–nitrogen oligomers by copolymerization with polyyne

The enhancement of the static electronic second-order NLO properties of the linear boron–nitrogen chains by copolymerization with polyyne that is related mainly to the changes in the bond length alternation (BLA) parameter has been investigated at the HF and MP2 levels using the 6-311G(d,p) and 6-311+G(d,p) basis sets. The non-zero BLA parameter combined with ununiform electron distribution along the chain yields a unit cell asymmetry necessary for large second-order NLO responses. Alternance of the single and triple bonds brings an efficient electron delocalization resulting in large polymeric electronic dipole polarizability (186 ± 1 a.u. per unit cell). The unit cell asymmetry and large delocalization lead to sizeable longitudinal first hyperpolarizability (5850 ± 100 a.u. per unit cell).

Ab initio static polarizability and first hyperpolarizability of model polymethineimine chains. II. Effects of conformation and of substitution by donor/acceptor end groups

An ab initio coupled Hartree–Fock study has been carried out on polymethineimine (PMI) oligomers with an increasing number of unit cells (up to N=20) to determine the effect of conformational differences, and of end-capping by a strong donor-acceptor pair, on the static electronic longitudinal polarizability (αL) and first hyperpolarizability (βL). For the unsubstituted polymer the differences in βL per unit cell among the four conformations examined—trans-transoı̈d (TT), cis-transoı̈d (CT), trans-cisoı̈d (TC) and helical (HEL)—are large. βL/N is positive for the CT polymer, in contrast with the other conformers and with μL/N, which is negative in all cases. The TC polymer has the largest |βL|/N despite having a relatively small αL/N. Substitution of a strong acceptor (NO2) at the carbon end of an oligomer and a strong donor (NH2) at the nitrogen end results in a large increase of βL/N for small/medium N. In the case of the TT and, particularly, the CT conformer this leads to a maximum in |βL|/N vs N at N=4–6, which is substantially greater in magnitude than the one at N=∞. The latter is unaffected by substitution. We compare the [βL(N)/N]max obtained at intermediate N for substituted PMI chains with those calculated for linear polyenes and other π-conjugated oligomers. The shape of all the βL/N vs N curves obtained is fit to a function which yields parameters characterizing the delocalization along the backbone, the unit cell asymmetry, and the asymmetry of the chain ends.

Atomic and electronic contributions to Si(111)-(7 x 7) scanning-tunneling-microscopy images.

Bias-dependent scanning tunneling microscopy (STM), used in conjunction with atomic-charge superposition calculations, is shown to provide information on both atomic and electronic surface structure. Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) STM images measured with different bias voltages are compared with a number of recent structural models. Striking agreement is found with the stacking-fault-adatom model of Takayanagi. The unit-cell asymmetry found at negative sample bias is attributed to a stacking fault in one half of the unit cell, locally modifying the surface electronic structure.

Vibrational studies of one-dimensional conductors: K2Pt(CN)4Br0.3⋅3H2O and K2Pt(CN)4Cl0.3⋅3H2O

Raman and infrared spectra of K2Pt(CN)4⋅Br0.3⋅3H2O, KCP (Br); K2Pt(CN)4⋅Cl0.3⋅3H2O, KCP(Cl), and K2Pt(CN)4⋅3H2O were obtained at ambient and nonambient conditions of temperature and pressure. The systems KCP(Br) and KCP(Cl) represent very interesting and unusual materials for ir–Raman spectroscopic studies because while neutron and x-ray diffraction measurements confirm the assignment of a noncentrosymmetric space group, the observable effects of the unit cell asymmetry are so slight, with respect to the vibrations of the Pt(CN)2−4 moiety, that the group may be considered as being present in a symmetric D2h environment (vide infra).