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In this paper, we have studied two different mechanisms of nonreciprocal and asymmetric transmission in the one-dimensional asymmetric optical system composed of parity-time (PT) and magneto-optical materials with different defect layers. It is shown that there are three pairs of nonreciprocal dispersive curves with the perfect transmission in the three different band gaps, when the defect layer is filled with normal material. When the defect layer is filled with magneto-optical material, the transmittivity of two nonreciprocal frequencies can be modulated by the magnitude and direction of the defect layer's external magnetic field and appears to be asymmetric nonreciprocal transmission. One-way frequency corresponding to one direction has extraordinary transmission, and the other one-way frequency corresponding to the opposite direction is suppressed. When the defect layer is filled with loss or gain material, the transmittivity of two nonreciprocal frequencies can be amplificated or attenuated simultaneously, respectively. The nonreciprocal propagation is originated from the resonant modes in the system due to the defect layer, and the nonreciprocal and asymmetric transmission is determined by the broken PT system due to magneto-optical and gain/loss material in the defect layer. Such controllable and asymmetric nonreciprocal propagation in the composite system may have broad potential applications in nonreciprocal communication and integration devices. [ABSTRACT FROM AUTHOR]

Previous studies of the gold-nanoparticles-covered U-type medical optical fiber sensor with millimeter size were mainly confined to the experimental aspect, while the corresponding theoretical studies were only for bare fibers based on geometrical optics or those for micron level photonic crystal fibers based on wave optics. Combining wave and geometrical optics, the gold-nanoparticles-covered U-type optical fiber sensor was simulated with millimeter size. The localized surface plasmon resonance absorption peak near 540 nm is obtained in the simulation, very close to that (≈560 nm) of the experimental value for the gold nanoparticles of 37 nm size. Compared with the refractive index (RI) sensitivity (≈7.10/RIU) for the plain, U-type optical fiber (≈43.50/RIU) exhibits more than 610% enhancement in the gold-nanoparticles-covered sample. Present studies would be helpful to the further simulation and design for various noble metal nanoparticles covered optical fiber sensors with different shapes. [ABSTRACT FROM AUTHOR]

Transition metal disulfides (TMDCs) have attracted extensive attention in recent years for their novel physical and chemical properties. Based on the first-principles calculations together with semi-classical Boltzmann transport theory, we explored the electronic structures and transport properties of van der Waals WSe2/WTe2 heterostructure. WSe2/WTe2 heterostructure has distinctive hexagon structure and isotropic thermal transport properties. To prove the accuracy of band structure, both Perdew–Burke–Eruzerhof (PBE) and Heyd–Scuseria–Ernzerhof (HSE06) have been used to calculate the band structures. We simulated the band structures under uniaxial and biaxial strains from −8% to +8% and found that all band gaps calculated by HSE06 are larger than results calculated by PBE. More importantly, it was found that when the biaxial strain reaches ±8%, it undergone semiconductor to metal and the dynamic stabilities of WSe2/WTe2 heterostructure have been predicted at the same time. We calculated the mobilities of electrons and holes and found that the mobility of holes is larger than that of electrons. The obtained lattice thermal conductivity (LTC) of WSe2/WTe2 heterostructure at room temperature (70.694 W/mK) is significantly higher than other transition metal tellurium and transition metal selenium, such as PdSe2 (2.91 W/mK) and PdTe2 (1.42 W/mK) monolayers. Our works further enrich studies on the strain dependence of electronic structures and predicted high LTC of WSe2/WTe2 heterostructure, which provide the theoretical basis for experiments in the future. [ABSTRACT FROM AUTHOR]

A novel Michelson interferometric hydrogen sulphide sensor coated with copper/reduced graphene oxide (Cu/rGO) composite membrane is proposed and fabricated. A section of endlessly photonic crystal fibre (EPCF) was sandwiched in two single-mode fibres (SMFs). One SMF was spliced and tapered with EPCF; the other SMF was connected with the Faraday rotator mirror to construct the Michelson structure. The cladding of the EPCF was coated by the Cu/rGO-sensing membrane, which was prepared by the dip-coating method. The obtained Cu/rGO-sensing film has a length of 25.0 mm. The fabricated sensing membrane is characterised by the scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and so on. Experimental results demonstrated that the Cu/rGO-sensing film has a 24.56-nm thickness with a compact and uniform appearance. The XPS and Raman spectra indicate that there are three elements (C, O, and Cu), which are consistent with the expected compositions of the Cu/rGO membrane. With the increase of concentration of hydrogen sulphide, the interference spectra appear red-shifted. The linearity of 0.97662 and the sensitivity of 13.23 pm/ppm are achieved. In addition, the dynamic response and recovery time of the sensor are approximately 70 and 88 s, respectively. The surface adsorption energies of the film are calculated by the density functional theory. The theoretical results are in good agreement with the experimental findings. This sensor has some key advantages of small size, simple structure, easy fabrication, and great applicability for detecting the trace hydrogen sulphide. [ABSTRACT FROM AUTHOR]

The non-reciprocal transmission properties of transverse electric (TE) mode waves are investigated for two asymmetric, complex photonic crystals composed of magnetic gyrotropic and normal media. Three pairs of non-reciprocal transmission peaks in three different bandgaps for forward and backward waves occur because of optical Tamm states (OTSs) at the interface between the photonic crystals. Numerical results indicate that the non-reciprocal transmission properties are influenced by the incident angles, gyrotropic permeability, and number of layers of the photonic crystals. The asymmetric structure with multiple non-reciprocal transmission peaks at a certain incident angle can provide promising opportunities for the design of multichannel isolators. [ABSTRACT FROM AUTHOR]

In this paper, the light scattering properties for multiple silver-coated dielectric nanocylinders with the symmetrical distribution were investigated. Based on the transfer matrix method, we derive the general transmission and reflection coefficient matrices for multiple dielectric nanocylinders. When the incident light frequencies are less than the plasma frequencies, the surface plasmons (SPs) appear in the interface between the silver and dielectrics. Numerical simulations show that there are three peaks of absorption cross-section (ACS) in the relationship between the ACS and the frequencies of the incident light, when the distance between the silver-coated dielectric nanocylinders is chosen properly. These SPs resonance peaks are characterised as resonances intrinsic to the cylindrically periodic system corresponding to different inner cavity structures. These multi-resonant cavities may have potential applications in integrated devices, optical sensors and optical storage devices. [ABSTRACT FROM AUTHOR]