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The 4 × 4 magneto-optical matrix is theoretically established to investigate the group delays of circular polarization, which are generated by the linearly polarized wave crossing through a sandwich composition that consists of a multi-Weyl semimetal (mWSM) layer and two identical Ag layers. Results denote that the cross-polarized transmission of right hand circularly polarized (RCP) and left hand circularly polarized (LCP) waves experience sharp increases from zero transmission to total transmission at their respective resonant tunneling wavelengths because of the nonzero off-diagonal components of mWSM. Based on the steep change of cross-polarized transmissions, the giant enlarged group delays of several picoseconds can be obtained. Furthermore, the resonant tunneling wavelength of RCP and LCP waves exhibit the dependence on tilt degree of Weyl cones, Fermi energy, Weyl nodes separation and topological charge, which provide feasible strategies to achieve the regulable enhanced group delays. Our findings reveal effective methods to acquire the tunable enlarged group delays of circular polarization with mWSM.

We theoretically set up a 4 × 4 magneto-optical matrix to investigate the transmittances induced by the forward and backward left hand circularly polarized (LCP) waves impinging the 4 μm thickness Weyl semimetal (WSM) material at normal incidence. Because of the different effective permittivity of the two LCP waves in WSM induced by the non-zero off-diagonal term, the two waves experience blocking transmission, bidirectional transmission and unidirectional transmission with the increase of the wavelength, respectively. Based on the wide wavelength of the unidirectional transmission, the broadband optical isolator can be realized. The threshold wavelength of the optical isolator can be regulated by changing the wavelength where the parallel component of the diagonal element is identical to the off-diagonal term. It is also found that the Fermi energy, tilt degree of Weyl cones and Weyl node separation reveal simple and available approaches to achieve the regulation. We believe that the adjustable broadband compact optical isolator can perform a significant role in fiber-optic communication system.

In this paper, we theoretically study the optical bistability (OB) of reflected beams in a composite multilayer structure with three-dimensional Dirac semimetals (3D DSMs) and one-dimensional photonic crystal (1D PC) on both sides of the spacer layer. The optical Tamm states (OTSs) excited by the 3D DSMs-1D PC multilayer structure significantly enhances the local electric field near the nonlinear 3D DSMs, which provides a positive condition for the realization of low threshold OB. Through parameter optimization, we obtain a threshold electric field with an incident electric field of 106 V/m levels. Furthermore, the influences of the Fermi energy of 3D DSMs, the angle of the incident light and dispersion characteristics of the spacer layer on the hysteretic behavior and the threshold of OB are clarified. We believe above results will provide possible options for nonlinear optical bistable devices.

In order to enhance the photonic spin hall effect (PSHE) of the reflected light at terahertz frequencies, we propose a multilayer structure where the one-dimensional photonic crystal (1D PC) is coated with a piece of monolayer graphene separated by a spacer layer. Enhanced photonic spin hall effects are obtained by this structure based on the excitation of optical Tamm states (OTSs) on the interface between the monolayer graphene and 1D PC. According to the analytical results, the PSHE of the reflected light can be flexibly controlled by properly varying the Fermi energy and relaxation time of graphene. It is also proved that the spin behavior of the composite structure has a great sensitivity to the incidence angle and the dispersion characteristics of the spacer layer, thus making the proposed structure a competitive candidate for designs of new photonic devices on the basis of photonic SHE at terahertz band.

We demonstrate theoretically a wide-angle and tunable mid-infrared perfect absorber by coupling the graphene and black phosphorus plasmonics and utilizing the concept of critical coupling. With the black phosphorus nanoribbons alone, the localized surface plasmon resonances (LSPR) can be excited, while the absorption is small due to its single-layer thickness. A Fabry-Perot cavity is formed by adding an Au mirror as the substrate, which is separated from the black phosphorus by a dielectric spacer, hence the absorption is enhanced markedly due to critical coupling. In order to further enhance the absorption, a graphene grating is introduced on the top of black phosphorus and is separated by a dielectric slab. It is shown that the perfect absorption can be obtained due to the critical coupling of LSPR of black phosphorus and graphene. Based on these result, it is further demonstrated that the perfect absorption can work in large incident angle range, and the working wavelengths can be adjusted by the Fermi level of graphene. Our results provide a new avenue to achieve wide-angle and adjustable perfect absorption, and it may be beneficial for a variety of mid-infrared plasmonic applications.

In this paper, we have shown that tunable nonlinear group delay of reflected light beam at terahertz frequencies can be achieved by a modified Otto configuration with the insertion of monolayer graphene and nonlinear substrate. This large nonlinear reflected group delay originates from the excitation of surface plasmon resonance at the interface of two dielectrics with graphene. Graphene sheet with unique optical properties can enhance the surface plasmon resonance and modulate bistable behavior of the reflected group delay by suitably adjusting the Fermi energy of the graphene and the relaxation time. Moreover, the large negative and positive of reflected group delay is sensitive to the thicknesses of the air layer. We believe these intriguing phenomena are promising for some potential applications in graphene-based optical delay devices. Keywords: Group delay, Graphene, Nonlinear surface plasmon, Otto configuration

As a latest novel two-dimensional (2D) material with naturally occurring van der Waals heterostructure, franckeite has the unique advantages compared with the artificial stacking heterostructures. Herein, we propose a surface plasmon resonance (SPR) sensor with 2D franckeite nanosheets to improve the sensitivity. In this structure, franckeite is covered on the surface of 50 nm silver film, so that the SPR sensitivity is related to the thickness of the franckeite overlayer. The sensitivity as high as 188 °/RIU in theory has been obtained, which is a 62% increasing compared with the conventional SPR biosensor. Moreover, an additional film of graphene is considered for the virtue of bio-compatible, and the sensitivity of the franckeite-graphene heterostructures is furthermore improved. As an air-stable 2D material, we believe that 2D franckeite nanosheets could have potential applications in the field of chemical and biological sensors in the future. Keywords: Franckeite, van der Waals heterostructure, Sensor, Surface plasmon resonance