Topological materials are considered as a novel quantum state of matter, which can be characterized by symmetry-protected Dirac interfacial states, and exhibit an exotic phenomenon when combined with the other phases. The topological phase in the perovskite structures is important since it can provide various heterostructure interfaces with multifunctional properties. Alpha-(α-) phase cesium-based halide perovskites CsSnX3 (X = I, Br, Cl) can be considered as a promising candidate for topological semiconductors under hydrostatic pressures. The narrow bandgap of these compounds (≤1.83 eV) has made them interesting materials for the electronic, optoelectronic, and photovoltaic applications. In the current research, we systematically carry out first-principles density functional theory (DFT) to study the effects of hydrostatic pressure on the electronic structure of CsSnX3 (X = I, Br, Cl) compounds. The topological phase of these compositions is investigated using the Fu–Kane and Wilson loop methods in order to identify the Z2 topological invariants for each structure. The topological surface states (TSSs) of the (001) plane of these compounds are investigated using the semi-infinite Green's function. These TSSs guarantee the nontrivial nature of CsSnX3 compounds under pressure. With respect to the engineering applications, three important mechanical properties of these compounds including elastic anisotropy, ductility, and hardness are also investigated.

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The field of mid-infrared (MIR) plasmonics has shown great potential applications in spectroscopic sensing, infrared light sources and detectors. MIR plasmonic materials that are compatible with common fabrication processes may enable cost-effective and reliable plasmonic device platforms. In this work, we examined aluminium metal (Al), gold-tin (AuSn) and titanium-tungsten (TiW) alloys regarding their usability for surface plasmon polariton (SPP) excitation in the MIR regime using a grating configuration. The angle dependence and the influence of varying depths of gratings were numerically and experimentally studied for the chosen materials. The structures were fabricated on eight-inch silicon (Si) substrates and characterized with a free-beam reflection measurement setup in the MIR regime. The fabricated gratings show narrow resonance dips, which are in good agreement with the simulations, revealing that Al, AuSn and TiW alloys are reliable plasmonic materials for MIR plasmonic devices.

Plasmonic slot waveguides have attracted much attention due to the possibility of high light confinement, although they suffer from relatively high propagation loss originating from the presence of a metal. Although the tightly confined light in a small gap leads to a high confinement factor, which is crucial for sensing applications, the use of plasmonic guiding at the same time results in a low propagation length. Therefore, the consideration of a trade-off between the confinement factor and the propagation length is essential to optimize the waveguide geometries. Using silicon nitride as a platform as one of the most common material systems, we have investigated free-standing and asymmetric gold-based plasmonic slot waveguides designed for sensing applications. A new figure of merit (FOM) is introduced to optimize the waveguide geometries for a wavelength of 4.26 µm corresponding to the absorption peak of CO2, aiming at the enhancement of the confinement factor and propagation length simultaneously. For the free-standing structure, the achieved FOM is 274.6 corresponding to approximately 42% and 868 µm for confinement factor and propagation length, respectively. The FOM for the asymmetric structure shows a value of 70.1 which corresponds to 36% and 264 µm for confinement factor and propagation length, respectively.

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