Broadband On-Chip Directional Coupler with Oblique Nanoslits

Directional coupling of light at the nanoscale plays a significant role in both fundamental research and practical applications, which are crucial for the development of on-chip photonic devices. In this work, we propose a broadband directional coupler for surface plasmon polaritons (SPPs) utilizing a pair of obliquely perforated nanoslits. We demonstrate that tilting the slits significantly enhances the sensitivity of plasmonic coupling phase variation to the wavelength of the incident light, enabling precise wavelength-dependent control over SPP propagation. By optimizing the width and tilting angle of each nanoslit, we achieve an extinction ratio exceeding 10 dB with a bandwidth exceeding 400 nm and a maximum unidirectional transmission of up to 30 dB. This broadband directional SPP coupler presents a promising platform for the design and fabrication of integrated plasmonic circuits and high-performance optical devices and sensors.