Highly tunable nanoscale metal-insulator-metal split ring core ring resonators (SRCRRs)

Nanophotonic structure inverse design for switching application using deep learning

Switching functionality is pivotal in advancing communication systems, serving as a paramount mechanism. Despite numerous innovations in this field, optical switch design, fabrication, and characterization have traditionally followed an iterative approach. Within this paradigm, the designer formulates an informed conjecture regarding the switch's structural configuration and subsequently resolves Maxwell's equations to ascertain its performance. Conversely, the inverse problem, which entails deriving a switch geometry to achieve a targeted electromagnetic response, continues to pose formidable challenges and necessitates substantial time and effort, particularly under the constraints of specific assumptions. In this work, we propose a deep neural network-based method to approximate the spectral transmittance of all-optical switches. The findings substantiate the efficacy of deep learning in the design of all-optical plasmonic switches, which are renowned as the fastest switches at the nanoscale. The nonlinear Kerr effect in square resonators is leveraged to demonstrate the switching performance. Juxtaposed with conventional simulations, the proposed model showcases a remarkable improvement in computational efficiency. Furthermore, deep learning can resolve nanophotonic inverse design problems without reliance on trial-and-error or empirical strategies. Compared to simulations, the mean squared error for both forward and inverse models is meager, with values of around 0.03 and 0.02, respectively. The deep learning-proposed switches exhibit excellent suitability for integration into photonic integrated circuits, substantially influencing the progression of all-optical signal processing.

High-Property Refractive Index and Bio-Sensing Dual-Purpose Sensor Based on SPPs

A high-property plasma resonance-sensor structure consisting of two metal-insulator-metal (MIM) waveguides coupled with a transverse ladder-shaped nano-cavity (TLSNC) is designed based on surface plasmon polaritons. Its transmission characteristics are analyzed using multimode interference coupling mode theory (MICMT), and are simulated using finite element analysis (FEA). Meanwhile, the influence of different structural arguments on the performance of the structure is investigated. This study shows that the system presents four high-quality formants in the transmission spectrum. The highest sensitivity is 3000 nm/RIU with a high FOM^* of 9.7 × 10⁵. In addition, the proposed structure could act as a biosensor to detect the concentrations of sodium ions (Na⁺), potassium ions (K⁺), and the glucose solution with maximum sensitivities of 0.45, 0.625 and 5.5 nm/mgdL⁻¹, respectively. Compared with other structures, the designed system has the advantages of a simple construction, a wide working band range, high reliability and easy nano-scale integration, providing a high-performance cavity choice for refractive index sensing and biosensing devices based on surface plasmons.

Directional coupling of surface plasmon polaritons at complementary split-ring resonators

We propose a complementary split-ring resonator (CSRR) for a directional coupling of surface plasmon polaritons. An air-slot split-ring in a gold film is investigated using the finite-difference time-domain method. The normally incident light couples to either a monopole or a dipole SPP depending on the polarization of light. Adjusting the angle of the linear polarization of the incident light enables a one-way propagation of SPPs on the gold film. Theoretical analysis based on the propagation of cylindrical waves from the SPP point source is provided with Hankel function. The propagated power in one direction is obtained to be 30 times higher than the opposite direction with a coupling efficiency of 18.2% from the simulation for an array of the CSRRs. This approach to the directional coupling of SPPs will be advantageous for miniaturizing photonic and plasmonic circuits and devices.

Peak modulation in multicavity-coupled graphene-based waveguide system

Plasmonically induced transparency (PIT) in a multicavity-coupled graphene-based waveguide system is investigated theoretically and numerically. By using the finite element method (FEM), the multiple mode effect can be achieved, and blue shift is exhibited by tunable altering the chemical potential of the monolayer graphene. We find that the increasing number of the graphene rectangle cavity (GRC) achieves the multiple PIT peaks. In addition, we find that the PIT peaks reduce to just one when the distance between the third cavity and the second one is 100 nm. Easily to be experimentally fabricated, this graphene-based waveguide system has many potential applications for the advancement of 3D ultra-compact, high-performance, and dynamical modulation plasmonic devices.

Theoretical analysis and applications in inverse T-shape structure

An inverse T-shape structure, consisting of a bus waveguide coupled with two perpendicular rectangular cavities, has been investigated numerically and theoretically. The position of the transparency window can be manipulated by adjusting the lateral displacement between the two perpendicular cavities. The effects of changing different structural parameters on the transmission features are investigated in detail. The results indicate that the length of two cavities play important roles in optimizing optical response. Finally, two simple applications based on the inverse T-shape structure are briefly discussed. The findings demonstrate that the first- and second-order modes can be separated without interference, and the sensitivity of the inverse T-shape is as high as 1750 nm per refractive index unit (RIU); the corresponding figure of merit (FOM) reaches up to 77.1  RIU^-1, which is higher than in previous reports. The plasmonic configuration possesses the advantages of easy fabrication, compactness, and higher sensitivity as well as higher FOM, which will greatly benefit the compact plasmonic filter and high-sensitivity nanosensor in highly integrated optical devices.

A novel transmission model for plasmon-induced transparency in plasmonic waveguide system with a single resonator

A novel transmission model with two branches is proposed to investigate the plasmon-induced transparency in a plasmonic waveguide system.

Plasmon-induced transparency in a single multimode stub resonator

We investigate electromagnetically induced transparency (EIT)-like effect in a metal-dielectric-metal (MDM) waveguide coupled to a single multimode stub resonator. Adjusting the geometrical parameters of the stub resonator, we can realize single or double plasmon-induced transparency (PIT) windows in the plasmonic structure. Moreover, the consistency between analytical results and finite difference time domain (FDTD) simulations reveals that the PIT results from the destructive interference between resonance modes in the stub resonator. Compared with previous EIT-like scheme based on MDM waveguide, the plasmonic system takes the advantages of easy fabrication and compactness. The results may open up avenues for the control of light in highly integrated optical circuits.

Single/multiple-mode-selection optical nanofilters based on end-coupled split-ring resonators

A plasmonic mode-selection filter based on end-coupled circular split-ring resonators is proposed and demonstrated. In contrast to regular ring resonators, which can support only the integer modes, extra noninteger resonance modes will be excited by placing metallic nanowalls in the proposed circular ring resonators. Since all the modes are highly sensitive to the position of the nanowall, the associated modes are effectively excited or suppressed by manipulating the position. This proposed scheme offers great flexibility to design the transmission spectrum with expected modes. Moreover, each integer or noninteger resonance mode with high transmittance can be selected individually by cascading two proposed split-ring resonators that share an intersection of transmission peaks. The corresponding spectra and the propagation characteristics are numerically investigated by using the finite-difference time-domain method.

Tunable band notch filters by manipulating couplings of split ring resonators

The couplings between single/dual split ring resonators (SRRs) and their mirror images in a rectangular waveguide are systematically investigated through theoretical analysis and experimental measurements. Such couplings can be manipulated mechanically by rotating the SRRs along a dielectric rod and/or shifting the SRRs up/down along the sidewall of the rectangular waveguide, resulting in shifts of the resonant frequencies and modulations of the resonant magnitudes. These controllable properties of SRRs pave the routers toward designing tunable band notch filters. In particular, it is experimentally demonstrated that the designed filters possess 7.5% tuning range in the X-band.