We investigate topological features of a one-dimensional photonic quasicrystal within the context of PT symmetry. Via the scattering characteristics, we analyze various properties of a particular mirrored structure, which supports topological edge modes in its band gaps. These interface modes display a nontrivial dependence on the quasiperiodic geometry, even in a passive system. Subsequently, the tailored addition of gain and loss generates curious PT -like features. For example, the quasicrystal high density of modes leads to complicated mode-merging behaviors between edge and band modes, such as the symmetry recovery phenomenon. Furthermore, anisotropic transmission resonances (connected with unidirectional invisibility) are also present, but they display richer patterns in comparison to previously studied periodic structures. Additionally, we examine lasing effects in detail, with numerics and a simple Fabry-Pérot model. The large variety of mode-merging behaviors opens the way to laser resonance engineering.
Typical parity-time (PT) symmetric structures switch from the unbroken to the broken phase when gain increases through an exceptional point. In contrast, we report on systems with the unusual, reverse behavior, where the symmetric phase is recovered after a broken phase. We study this phenomenon analytically and numerically in the simplest possible system, consisting of four coupled modes, and we present potential dielectric and plasmonic implementations. The complex mode merging scheme, with two distinct unbroken PT phases encompassing a broken one, appears for a specific proportion range of the coupling constants. This regime with "inverse" exceptional points is interesting for the design of novel PT devices.
We analyze the scattering properties of a parity-time (PT)-symmetric structure made of a waveguide and a finite chain of side-coupled resonators. Typical one-dimensional PT structures exhibit unidirectional invisibility (also called anisotropic transmission resonances), meaning unity transmission and zero reflection for incidence from one direction. The side-coupled nature of our structure provides these features as well, but with different characteristics than the traditional tight-binding chain. We explore these properties in detail with numerical and analytical approaches for various chain lengths and geometries. As an interesting feature, we can achieve a broadband unidirectional invisibility with only two resonators. Furthermore, we observe rich dispersions for these anisotropic transmission resonances with four resonators, which can be carefully tuned.
Low-emissivity glasses
rely on multistacked architectures with
a thin silver layer sandwiched between oxide layers. The mechanical
stability of the silver/oxide interfaces is a critical parameter that
must be maximized. Here, we demonstrate by means of quantum-chemical
calculations that a low work of adhesion at interfaces can be significantly
increased via doping and by introducing vacancies
in the oxide layer. For the sake of illustration, we focus on the
ZrO2(111)/Ag(111) interface exhibiting a poor adhesion
in the pristine state and quantify the impact of introducing n-type
dopants or p-type dopants in ZrO2 and vacancies in oxygen
atoms (nVO; with n =
1, 2, 4, 8, 10, 16), zirconium atoms (mVZr; with m = 1, 2, 4, 8), or both (nVO + mVZr; with m/n = 1:2, 1:4, 2:2, 2:4). In the case of doping,
interfacial electron transfer promotes an increase in the work of
adhesion, from initially 0.16 to ∼0.8 J m–2 (n-type) and ∼2.0 J m–2 (p-type) at 10%
doping. A similar increase in the work of adhesion is obtained by
introducing vacancies, e.g., VO [VZr] in the oxide layer yields a work of adhesion of ∼1.5–2.0
J m–2 at 10% vacancies. An increase is also observed
when mixing VO and VZr vacancies in a nonstoichiometric
ratio (nVO + mVZr; with 2n ≠ m), while a
stoichiometric ratio of VO and VZr has no impact
on the interfacial properties.
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