Topological corner state localized bound states in continuum in photonic crystals.

In the field of optics, bound states in the continuum (BICs) are of significant practical importance as they can trap electromagnetic waves spatially, even though their frequency lies within the continuous spectrum. Previous research, however, has shown that BICs localized in optical cavities are highly sensitive to geometric and environmental changes. This sensitivity implies that slight variations can lead to the loss of BICs, necessitating extreme precision in manufacturing, which poses a challenge for practical implementation. To overcome this issue, this study employs topological photonic crystals (PhCs) to engineer topological corner states (TCS) within PhCs. By doing so, it establishes a method for creating topological BICs that are inherently robust against disturbances, thereby enhancing their suitability for real-world applications.

Combining sensitivity and robustness: EIT-like characteristic in a 2D topological photonic crystal.

The study of topological photonics has gained significant attention due to its potential application for robust and efficient light manipulation. In this work, we theoretically investigate a two-dimensional photonics crystal that exhibits a topological edge state (TES) and a topological corner state (TCS). Furthermore, we also achieve a coupling between a topological corner state and a trivial cavity (TC), resulting in a phenomenon similar to the electromagnetically induced transparency (EIT) effect. To verify the stability of the EIT-like effect, disorders around TES and TCS are introduced, and the theoretical results show that this structure is immune to the disorders. The achievement of the coupling between topological states can have potential applications in the areas of waveguiding, sensing, and logic gates. It is hoped that this work will contribute to the ongoing efforts in the exploration and utilization of topological photonics.

Slow-light effects based on the tunable Fano resonance in a Tamm state coupled graphene surface plasmon system.

We theoretically realize the tunable Fano resonance in a hybrid structure that allows the coupling between Tamm plasmon-polaritons (TPPs) and graphene surface plasmon-polaritons (SPPs). In this coupling system, a distributed Bragg reflector (DBR)/Ag structure is designed to generate the TPP with a narrow resonance, and the graphene SPP is excited by grating coupling with a broad resonance. The overlap of these two kinds of resonances results in the Fano resonance with a high-quality factor close to 1500. The behaviors of the Fano resonance are discussed carefully, and the results show that both the graphene Fermi level and the incidence angle can actively tune the profile of the Fano resonance. Owing to the ultrasharp spectrum of the tunable Fano resonance, our design may offer an alternative strategy for developing various optoelectronic devices such as filters, sensors, and nonlinear and slow-light devices. Finally, as an example of the potential applications, we apply the tunable Fano resonance to the slow-light effect, a high performance slow-light effect can be achieved, and the group delay can reach up to 52 ps.

Generation of symmetry-protected bound states in the continuum in a graphene plasmonic waveguide system for optical switching.

A graphene-involved plasmonic lossy system that allows coupling between surface plasmon polaritons and waveguide modes is proposed. The physical mechanism behind the hybrid resonance modes is investigated carefully through finite element method (FEM) simulations and rigorous coupled wave theory (RCWA). We demonstrate that by introducing an incident angle to break the symmetry of the structure, the bound states in the continuum (BIC) evolve to an observable quasi-BIC with new resonance dips, and the generated signals possess a very high Q-factor. Such transformation is investigated carefully by calculating the band structure of the system and the corresponding Q-factors. The results showed that the calculated results from the band structure are consistent with the simulations. In addition, the hybrid plasmonic system allows for switching modulation due to the tunability of graphene, and the max modulation depth of nearly 100% is reached. The outstanding Q-factor and dynamic tunability of this easy-to-fabricate hybrid structure may be helpful in engineering various plasmonic devices, including tunable optical switches, absorbers, sensors, etc.

Investigation of bound states in the continuum in dual-band perfect absorbers.

Enhancing the light-matter interaction of two-dimensional materials in the visible and near-infrared regions is highly required in optical devices. In this paper, the optical bound states in the continuum (BICs) that can enhance the interaction between light and matter are observed in the grating-graphene-Bragg mirror structure. The system can generate a dual-band perfect absorption spectrum contributed by guided-mode resonance (GMR) and Tamm plasmon polarition (TPP) modes. The optical switch can also be obtained by switching the TE-TM wave. The dual-band absorption response is analyzed by numerical simulation and coupled-mode theory (CMT), with the dates of each approach displaying consistency. Research shows that the GMR mode can be turned into the Fabry-Pérot BICs through the transverse resonance principle (TRP). The band structures and field distributions of the proposed loss system can further explain the BIC mechanism. Both static (grating pitch P) and dynamic parameters (incident angle θ) can be modulated to generate the Fabry-Pérot BICs. Moreover, we explained the reason why the strong coupling between the GMR and TPPs modes does not produce the Friedrich-Wintgen BIC. Taken together, the proposed structure can not only be applied to dual-band perfect absorbers and optical switches but also provides guidance for the realization of Fabry-Pérot BICs in lossy systems.

Dynamically tunable bound states in the continuum supported by asymmetric Fabry-Pérot resonance.

The dynamic regulation of quasi-bound states in the continuum (quasi-BIC) is a research hotspot, such as incident angle, polarization angle, temperature, a medium refractive index, and medium position regulation. In this paper, a dual-band ultra-high absorber composed of upper asymmetric graphene strips and lower graphene nanoribbons can generate a symmetry-protected quasi-BIC and Fabry-Pérot resonance (FPR) mode. The band structure further demonstrates the symmetry-protected BIC. Research shows that the absorption system can withstand a relatively wide range of incidence and polarization angles. Interestingly, the quasi-BIC and FPR modes can be modulated by the Fermi levels of the graphene1 and graphene2, respectively, realizing a multifunctional switch with high modulation depth (MD > 94%), low insertion loss (IL < 0.23 dB), and large dephasing time (DT > 4.35 ps). This work provides a new approach for the dynamic regulation of quasi-BIC and stimulates the development of multifunctional switches in the absorber.

Dynamically controllable multi-switch and slow light based on a pyramid-shaped monolayer graphene metamaterial.

Graphene, a new two-dimensional (2D) material, has attracted considerable attention in recent years because of the metallic characteristics at terahertz frequencies. The phase coupling of multilayer graphene-coupled grating structures is normally used to realize multiple plasmon-induced transparency (PIT) spectral responses. However, the device becomes more complicated with the increase in the number of graphene layers. In this work, we propose a five-step-coupled pyramid-shaped monolayer graphene metamaterial and predict a dynamically controllable PIT with four transparency peaks for the first time in the monolayer graphene metamaterial. A tunable multi-switch and good slow light effect is predicted over the wide PIT window, and the maximum modulation depth is high up to 16.89 dB, which corresponds to 97.95%, while the time delay of the induced transparent window is as high as 0.488 ps, where the corresponding group refractive index is 586. The electric field distributions and quantum level theory are used to explain the physical mechanism of the PIT with four transparency peaks. The coupled mode theory (CMT) is employed to establish the mathematical model of the PIT with four transparency peaks, and the consistency between the simulated and the calculated results is nearly perfect. We believe that the pyramid-shaped monolayer graphene metamaterial could be useful in efficient filters, switches, and slow light devices.

Slow-light analysis based on tunable plasmon-induced transparency in patterned black phosphorus metamaterial.

In this paper, a tunable plasmon-induced transparency (PIT) structure based on a monolayer black phosphorus metamaterial is designed. In the structure, destructive interference between the bright and dark modes produces a significant PIT in the midinfrared band. Numerical simulation and theoretical calculation methods are utilized to analyze the tunable PIT effect of black phosphorus (BP). Finite-difference-time-domain simulations are consistent with theoretical calculations by coupled mode theory in the terahertz frequency band. We explored the anisotropy of a BP-based metasurface structure. By varying the geometrical parameters and carrier concentration of the monolayer BP, the interaction between the bright and dark modes in the structure can be effectively adjusted, and the active adjustment of the PIT effect is achieved. Further, the structure's group index can be as high as 139, which provides excellent slow-light performance. This study offers a new possibility for the practical applications of BP in micro-nano slow-light devices.

Terahertz multifunction switch and optical storage based on triple plasmon-induced transparency on a single-layer patterned graphene metasurface.

A terahertz metasurface consisting of a graphene ribbon and three graphene strips, which can generate a significant triple plasmon-induced transparency (triple-PIT), is proposed to realize a multifunction switch and optical storage. Numerical simulation triple-PIT which is the result of destructive interference between three bright modes and a dark mode can be fitted by coupled mode theory (CMT). The penta-frequency asynchronous and quatary-frequency synchronous switches can be achieved by modulating the graphene Fermi levels. And the switch performance including modulation depth (83.5% < MD < 93.5%) and insertion loss (0.10 dB < IL < 0.26 dB) is great excellent. In addition, the group index of the triple-PIT can be as high as 935, meaning an excellent optical storage is achieved. Thus, the work provides a new method for designing terahertz multi-function switches and optical storages.

Fano resonance in double waveguides with graphene for ultrasensitive biosensor.

Fano resonance is realized in the multilayer structure consisting of two planar waveguides (PWGs) and few layer graphene, and the coupling mechanism between the two PWG modes with graphene is analyzed in detail. It is revealed that the Fano resonance originates from the different quality factors due to the different intrinsic losses of the graphene in the two waveguides, and the electric field distributions in the multilayer structure confirms our results. Fano resonance in our proposed structures can be applied in the ultrasensitive biosensor, and a significantly improved figure of merit (FOM) of 9340 RIU-1 has been obtained by optimizing the structure parameters, which has a 2~3 orders of magnitude enhancement compared to the traditional surface plasmon polaritons (SPR) sensor. Especially, it is found that both transverse magnetic (TM)-polarization and transverse electric (TE)-polarization can support the Fano resonance, and hence it can work as ultrasensitive biosensor for both polarizations.

Fano Resonance in Waveguide Coupled Surface Exciton Polaritons: Theory and Application in Biosensor.

Surface exciton polaritons (SEPs) are one of the three major elementary excitations: Phonons, plasmons and excitons. They propagate along the interface of the crystal and dielectric medium. Surface exciton polaritons hold a significant position in the aspect of novel sensor and optical devices. In this article, we have realized a sharp Fano resonance (FR) by coupling the planar waveguide mode (WGM) and SEP mode with Cytop (perfluoro (1-butenyl vinyl ether)) and J-aggregate cyanine dye. After analyzing the coupling mechanism and the localized field enhancement, we then applied our structure to the imaging biosensor. It was shown that the maximum imaging sensitivity of this sensor could be as high as 5858 RIU-1, which is more than three times as much as classical FR based on metal. A biosensor with ultra-high sensitivity, simple manufacturing technique and lower cost with J-aggregate cyanine dye provides us with the most appropriate substitute for the surface plasmon resonance sensors with the noble metals and paves the way for applications in new sensing technology and biological studies.

Improving the Performance of an SPR Biosensor Using Long-Range Surface Plasmon of Ga-Doped Zinc Oxide.

Transparent conducting oxides (TCOs) have appeared in the past few years as potential plasmonic materials for the development of optical devices in the near infrared regime (NIR). However, the performance of biosensors with TCOs has been limited in sensitivity and figure of merit (FOM). To improve the performance of the biosensors with TCOs, a biosensor based on long-range surface plasmon with Ga-doped zinc oxide (GZO) is proposed. It is shown that a larger FOM with a 2~7 times enhancement compared to the traditional surface plasmon polaritons (SPPs) sensor and higher detection accuracy (DA) can be realized in our proposed sensor compared with the surface plasmon resonance (SPR) sensor with GZO. Therefore, this sensor can be used to detect biological activity or chemical reactions in the near infrared region.

Ultrasensitive Terahertz Biosensors Based on Fano Resonance of a Graphene/Waveguide Hybrid Structure.

Graphene terahertz (THz) surface plasmons provide hope for developing functional devices in the THz frequency. By coupling graphene surface plasmon polaritons (SPPs) and a planar waveguide (PWG) mode, Fano resonances are demonstrated to realize an ultrasensitive terahertz biosensor. By analyzing the dispersion relation of graphene SPPs and PWG, the tunable Fano resonances in the terahertz frequency are discussed. It is found that the asymmetric lineshape of Fano resonances can be manipulated by changing the Fermi level of graphene, and the influence of the thickness of coupling layer and air layer in sandwich structure on the Fano resonances is also discussed in detail. We then apply the proposed Fano resonance to realize the ultrasensitive terahertz biosensors, it is shown that the highest sensitivities of 3260 RIU-1 are realized. Our result is two orders of a conventional surface plasmon resonance sensor. Furthermore, we find that when sensing medium is in the vicinity of water in THz, the sensitivity increases with increasing refractive index of the sensing medium.

Self-Referenced Refractive Index Biosensing with Graphene Fano Resonance Modes.

A hybrid structure composed of periodic monolayer graphene nanoribbons and a dielectric multilayer structure was designed to generate a Fano resonance (FR). The strong interaction between the surface plasmon resonance of graphene and the dielectric waveguide mode results in the FR. The finite element method is utilized to investigate the behaviors of the FR, and it matches well with the theoretical calculations using rigorous coupled wave theory. The results demonstrate that the profile of the FR can be passively tuned by the period of the graphene nanoribbons and actively tuned by the Fermi level of the graphene. The decoupled nature of the FR gives it potential applications as a self-calibrated refractive index biosensor, and the sensitivity can reach as high as 4.615 µm/RIU. Thus, this work provides a new idea for an excellent self-referencing refractive index biosensor.