Ling-Ling Wang

Van der Waals (vdW) heterostructures provide an effective strategy for exploring and expanding the potential applications of two-dimensional materials. In this study, we employ first-principles density functional theory (DFT) to investigate the geometric, electronic, and optical properties of MoGe₂N₄/AlN and MoSiGeN₄/AlN vdW heterostructures. The stable MoGe₂N₄/AlN heterostructure exhibits an indirect band gap semiconductor with a type-I band gap arrangement, making it suitable for optoelectronic devices. Conversely, the stable MoSiGeN₄/AlN heterostructure demonstrates various band gap arrangements depending on stacking modes, rendering it suitable for photocatalysis applications. Additionally, we analyze the effects of mechanical strain and vertical electric field on the electronic properties of these heterostructures. Our results indicate that both mechanical strain and vertical electric field can adjust the band gap. Notably, application of an electric field or mechanical strain leads to the transformation of the MoGe₂N₄/AlN heterostructure from a type-I to a type-II band alignment and from an indirect to a direct band transfer, while MoSiGeN₄/AlN can transition from a type-II to a type-I band alignment. Type-II band alignment is considered a feasible scheme for photocatalysis, photocells, and photovoltaics. The discovery of these characteristics suggests that MoGe₂N₄/AlN and MoSiGeN₄/AlN vdW heterostructures, despite their high lattice mismatch, hold promise as tunable optoelectronic materials with excellent performance in optoelectronic devices and photocatalysis.

A long-range surface plasmon polariton (SPP) waveguide, composed of double-layer graphene, can be pivotal in transferring and handling mid-infrared electromagnetic waves. However, one of the key challenges for this type of waveguide is how to excite the SPP modes through an incident light beam. In this study, our proposed design of a novel grating, consisting of a graphene-based cylindrical long-range SPP waveguide array, successfully addresses this issue using finite-difference time-domain simulations. The results show that two types of symmetric coupling modes (SCMs) are excited through a normal incident light. The transmission characteristics of the two SCMs can be manipulated by changing the interaction of the double-layer gratings of graphene as well as by varying various parameters of the device. Similarly, four SCMs can be excited and controlled by an oblique incident light because this light source is equivalent to two orthogonal beams of light. Furthermore, this grating can be utilized in the fabrication of mid-infrared optical devices, such as filters and refractive index sensors. This grating, with double-layer graphene arrays, has the potential to excite and manipulate the mid-infrared electromagnetic waves in future photonic integrated circuits.

In this paper, we present an easy-to-implement metamaterial absorber based on bulk Dirac semimetal (BDS). The proposed device not only obtains an ultrahigh quality factor (Q-factor) of 4133 and dynamic adjustability at high absorption, but also exhibits an excellent sensing performance with a figure of merit (FOM) of 4125. These outstanding properties are explained by the surface lattice resonance, which allows us to improve the quality factor significantly and control resonance wavelength precisely by tuning the unit cell periods, Fermi energy of the BDS, and structural parameters. Our findings can provide high-performance applications in terahertz filtering, detection, and biochemical sensing.

A dynamically tunable anisotropic narrowband absorber based on monolayer black phosphorous (BP) is proposed in the terahertz (THz) band. The proposed absorber consists of a monolayer BP and a silicon (Si) grating, which is placed on a silica (SiO₂) isolation layer and a gold (Au) substrate. The benefit from the critical coupling mechanism with guided resonance is the efficiency of the absorption can reach 99.9% in the armchair (AC) direction and the natural anisotropy of BP makes it only 87.2% in the zigzag (ZZ) direction. Numerical and theoretical studies show that the absorption efficiency of the structure is operatively controlled by critical coupling conditions, including the geometric parameters of the Si grating, the electron doping of BP and the angle of incident light, etc. More importantly, in the absence of plasmon response, this structure greatly enhances the interaction between light and matter in monolayer BP. In particular, there are several advantages in this structure, such as extremely high-efficiency absorption, excellent tunability, outstanding intrinsic anisotropy and easy manufacturing, which will show unusual and promising potential applications in the design of BP-based tunable high-performance devices.

Due to the weak absorption of (CH₃NH₃)PbI₃ in the communication regime, which restricts its optoelectronic applications, we design a adjustable dual-band perfect absorber based on the (CH₃NH₃)PbI₃ to significantly enhance its absorption capability. Since the localized plasmon (LP) mode and surface plasmon (SP) mode are excited in the structure, which can both greatly enhance light absorption of the (CH₃NH₃)PbI₃ layer, dual-band perfect absorption peaks are formed in the communication regime, and the absorption of (CH₃NH₃)PbI₃ layer is increased to 43.1% and 64.2% at the dual-band absorption peaks by using finite-difference time-domain (FDTD) methods, respectively. By varying some key structural parameters, the dual-band absorption peaks of (CH₃NH₃)PbI₃ can be separately shifted in a wide wavelength region. Moreover, the designed absorber can keep good performance under wide angles of incidence and manifested polarization correlation. Furthermore, not just for (CH₃NH₃)PbI₃, the physical mechanism in this absorber can also be utilized to strengthen the absorption of other halide perovskites.

The out-of-plane metal-insulator-graphene (MIG) heterostructure separated from metal substrates by a silica spacer has been intensively investigated. Both numerical simulations and theoretical calculations exhibit that due to the dominate effect of adjacent metal layers, multiple strongly confined Fabry-Pérot (F-P) resonances are simultaneously excited on the ingenious out-of-plane MIG heterostructure; and thus, are conductive to the realization of a nine-band plasmonic perfect absorber at mid-infrared wavelengths. The spectral positions of absorption peaks are handily tuned by changing the structural parameters of MIG heterostructures and the electrically controlled chemical potential of graphene. More interestingly, the good absorption stability at varying incident angles is observed. Numerical simulations are in excellent agreement with theoretical results. Such tunable angle-independent nine-band perfect absorber undoubtedly finds utility in mid-infrared optical filters and modulators. The proposed out-of-plane MIG heterostructure allows possibilities for multispectral enhancement of light-graphene interactions.

The constructive interference of bright and dark plasmonic modes results in plasmon-induced absorption (PIA) effect. Here, we theoretically investigate PIA effect, which is realized by the constructive interference between a Fabry-Perot (F-P) resonance mode and a graphene quasi-guided mode. Numerical simulation reveals at least three advantages of our structure over previous ones. First, the extinction ratio can reach ~ 99.999%, resulting in the ultra-high figure of merit* (FOM*) as high as 106. Second, the intensity of this pronounced PIA effect can be optimized by adjusting the coupling distance. Third, the resonance frequency can be easily tunable by tuning the graphene Fermi level. This system may have potential applications in dynamically optical switching and biochemical sensing.

Plasmonic metasurfaces made from coupled nanostrips with stubs demonstrate electromagnetically induced transparency (EIT) resonances. We exploit novel approaches of presenting the phenomenon by using the near-field coupling between two specific radiative resonances to generate two distinct EIT windows within a relative short wavelength range. Furthermore, the nanostructures provide a tunable control of plasmonic spectral response through modulating the related parameters. The multi-channel selectable EIT windows based on these coupling effects are realized in the compact metasurfaces, which may have potential applications in highly composite circuits.