W. J. Hsueh

Features of the perfect transmission in Thue–Morse dielectric multilayers

We proposed a novel method to determine the perfect transmission (PT) of photonics through the Thue–Morse dielectric multilayers for arbitrary basis thickness and generation orders based on the band edge map diagram. As the order of the system increases, the density of resonance peaks increases exponentially. However, the PTs of the resonance peaks are kept even if the peaks become denser for higher-order systems. We present two iterative rules to determine the frequency of the resonance peaks for an arbitrary order of the multilayers. In addition, simple equations are proposed to approximate the repeated band edges and the PT for the arbitrary incident angle of the system with arbitrary order and basis thickness.

Enhancement of tunnel magnetoresistance in magnetic tunnel junction by a superlattice barrier

Tunnel magnetoresistance of magnetic tunnel junction improved by a superlattice barrier composed of alternate layers of a nonmagnetic metal and an insulator is proposed. The forbidden band of the superlattice is used to predict the low transmission range in the superlattice barrier. By forbidding electron transport in the anti-parallel configuration, the tunnel magnetoresistance is enhanced in the superlattice junction. The results show that the tunnel magnetoresistance ratio for a superlattice magnetic tunnel junction is greater than that for traditional single or double barrier junctions.

Simple expressions for the maximum omnidirectional bandgap of bilayer photonic crystals

We propose three dimensionless approximate expressions to predict the thickness filling factor, gap center, and gap width of the maximum omnidirectional gap (MODG) for various refractive indices in one-dimensional photonic crystals. These expressions are simple and do not include trigonometric or inverse trigonometric functions. It is easy to obtain the MODG from given refractive indices but also to estimate the refractive indices from the MODG based on the results.

Stable and accurate method for modal analysis of multilayer waveguides using a graph approach

A method for calculating the propagation constants of allowed guided and leaky modes in multilayer planar waveguides is presented. We develop a two-way graph model to describe the tangential fields propagating in the waveguides. According to the special structure of the graph model, it is convenient to employ a topology scheme to derive analytical and closed-form dispersion equations for TE and TM modes. By comparing the dispersion equations formulated by series-expansion methods, approximation methods, and transfer-matrix methods, we find that the use of these equations for finding the eigenmodes has some benefits. First, this method can be easily employed to solve eigenmodes accurately in numerical computation without using series truncation. Second, the dispersion equations are exact. Moreover, all the eigenmodes can be determined according to the formulas without losing roots or causing numerical instability even for a waveguide with thick layers.

Effective Bragg conditions in a one-dimensional quasicrystal.

We present occurrence of the effective Bragg conditions with wide gapwidth and high reflectance in a Fibonacci superlattice, which is a typical one-dimensional quasicrystal. In the Fibonacci material, the number of effective Bragg conditions is two rather than one which appears in traditional periodic structures. Based on the effective Bragg conditions, this study proposes existence of omnidirectional, wideband and high reflectance in the quasiperiodic materials analogous to that in traditional materials.

Characterization of omnidirectional bandgaps in multiple frequency ranges of one-dimensional photonic crystals

Characterization of omnidirectional bandgaps in the multiple frequency ranges of one-dimensional photonic crystals based on gap map diagrams is proposed. In the gap map, there is one maximum range of the omnidirectional gap in each region, which is divided by the half-wave lines. The occurrence of the maximum range of the omnidirectional gap can be obtained by the characteristics of these gap map diagrams, including the zero-gap points, the occurrence of midgaps, and the frequency ranges of midgaps, for normal and grazing incidences with transverse-electric and transverse-magnetic polarizations. Moreover, concise empirical schemes are proposed to approximately determine the center and gap width of the maximum omnidirectional gaps in multiple frequency ranges.

Localized modes in one-dimensional symmetric Thue-Morse quasicrystals

We present occurrence of the strongly localized modes with high transmission in one dimensional symmetric Thue-Morse quasicrystals. This quasicrystal has some interesting properties, including (i) there are strongly localized modes in separated regions which are around odd semi-quarter-wave thickness of the system, (ii) both the frequency of localized mode and the thicknesses of the space layer to appear localized modes are variant for different generation orders of the system, and (iii) the sharpness of the resonant peaks in the transmission spectra increases as the generation order of the system increases.

Strong localization of photonics in symmetric Fibonacci superlattices

Strong localization from the Fabry-Perot-like resonances that occur in symmetric Fibonacci superlattices is presented in this study. When compared with traditional Fabry–Perot resonators, in symmetric Fibonacci superlattices, the middle space is a variant rather than an invariant half-wavelength thickness for each resonance with different orders. In addition, the electric fields of the resonances may be located on both sides of the space layer in the superlattice, which is in contrast to those in a traditional Fabry–Perot resonator. The electric field of the resonances is strongly localized as the generation order increases. Moreover, the group delays of these peaks increase with generation order. More strongly localized modes can be found from the symmetric Fibonacci superlattices than from the traditional Fabry–Perot resonators, which makes the proposed structure an attractive alternative to a wide variety of optoelectronic devices.

Ultrahigh tunnel magnetoresistance using an artificial superlattice barrier with copper and aluminum oxide

This paper proposes an ultrahigh tunnel magnetoresistance that is achieved by a magnetic tunnel junction with an artificial superlattice barrier that is composed of alternate layers of copper and aluminium oxide. By designing proper thickness filling factor of the superlattice barrier, ultrahigh magnetoresistance can be achieved. The tunnel magnetoresistance increases as the number of cells in the superlattice barrier increases. This ultrahigh magnetoresistance effect is attributed to the crystalline property of superlattices, similarly to the high magnetoresistance effect achieved by traditional crystalline MgO. There are more adjustable parameters, such as the lattice constant and the barrier height, in the artificial superlattice barrier than in a traditional crystalline-MgO barrier. This ultrahigh magnetoresistance effect may be used to design spintronic devices.

Strong photoluminescence emission from resonant Fibonacci quantum wells

Strong photoluminescence (PL) emission from a resonant Fibonacci quantum well (FQW) is demonstrated. The maximum PL intensity in the FQW is significantly stronger than that in a periodic QW under the Bragg or anti-Bragg conditions. Moreover, the peaks of the squared electric field in the FQW are located very near each of the QWs. The optimal PL spectrum in the FQW has an asymmetrical form rather than the symmetrical one in the periodic case. The maximum PL intensity and the corresponding thickness filling factor in the FQW become greater with increasing generation order.