In modern photonics and optics, the development of high-performance optical devices, such as sensors, lasers, and filters with ultra-high quality factor (Q-factor), is crucial. Bound states in the continuum (BICs) are considered an effective approach for achieving this goal, as they can theoretically reach an infinite Q-factor under ideal conditions, showing great potential for various applications. BICs are generally classified into two types based on their decoupling from far-field radiation: symmetry-protected BICs (S-P BICs) and resonance-coupled BICs, also known as parameter-tuned BICs. While S-P BICs are appealing due to their straightforward predictability, their practical realization often encounters significant obstacles, including intricate structural designs, limited integration capabilities, and complex design challenges. Resonance-coupled BICs, with their simpler structure and ease of fabrication, have become the focus of research. However, the Q-factor and structural integration of resonance-coupled BICs reported in the literature still require improvement. To address this challenge, we propose a novel multilayer grating hybrid structure composed of alternating silica and titanium dioxide films. By precisely controlling the bandgap center wavelength, grating period, and material parameters of the multilayer structure, the localization of the optical field is further enhanced, resulting in a significant increase in the Q-factor. This enhancement aims to achieve breakthroughs in optical device performance and open up new possibilities for advanced photonic applications.

Based on temporal coupled mode theory, a new structure for forming resonance-coupled BICs has been proposed. Using the reflection principle of the distributed Bragg reflector (DBR) method, a multilayer film structure is designed to precisely control the propagation and reflection characteristics of light waves within the multilayers. Utilizing equivalent medium theory in the design of grating structures can effectively regulate the propagation path of light waves, achieving precise control over their propagation characteristics. Finally, by employing the control variable method, we adjust system parameters such as the incident light angle, grating period, and grating material in a five-layer film grating structure to investigate the principles governing the realization of BIC in the hybrid multilayer film grating structure.

A novel multilayer grating hybrid structure composed of alternating silica and titanium dioxide films is designed by combining multilayer and grating structures (Fig. 1). First, a thorough analysis of the potential modes in the structure is conducted. By comparing a simple TiO₂ grating with the proposed multilayer grating structure, guided-mode resonance (like-GMR) and Bloch surface wave (like-BSW) modes are identified. The introduction of a defect layer allows the two modes to satisfy phase-matching conditions, leading to destructive interference between the modes and forming a resonance-coupled BIC, significantly enhancing field localization. By carefully designing the period and central wavelength of the multilayer structure, further refinements are made to precisely control the spatial distribution and coupling strength between the modes. This optimization achieves a high Q-factor of ~10⁵. In addition, the effects of incident light angle, grating period (P), and the refractive index of the grating material on the position and Q-factor of the resonant BIC are systematically studied, establishing a regular tuning strategy for achieving the resonant BIC. The results show that the incident angle has little influence on the BIC. As the grating period is incrementally enlarged, the wavelength at the BIC is observed to increase linearly, exhibiting a direct proportionality of about 1.5 times the period length. Although the material’s refractive index has a subtle effect on the BIC’s position, an increase in the refractive index is found to enhance field localization and achieve a high Q-factor (Fig. 5).

In this paper, we propose a multilayer grating hybrid structure that effectively forms a resonance-coupled BIC. The results show that two optical modes, like-GMR and like-BSW, coexist in the hybrid structure. By adjusting the layer thickness, the resonant frequencies of the modes can be altered, leading to strong coupling between the two modes, reducing the resonant wavelength, and enhancing light localization. Introducing a defect layer of specific thickness on the surface of the multilayer system or altering the center wavelength of the multilayer bandgap structure can achieve phase matching between the two optical modes, forming a resonance-coupled BIC with a Q-factor up to 10⁵. In the multilayer grating structure, the wavelength of the resonance-coupled BIC increases linearly with the grating period while maintaining a high Q-factor. Furthermore, enhancing the refractive index of the multilayer materials can improve the Q-factor of the resonance-coupled BIC, further intensifying the localization of field energy. This approach provides effective theoretical guidance for designing high-Q resonances across different wavelength bands.