Theoretical Study on Surface Mode in Photonic Crystal Fishbone Nanocavity

We propose and theoretically investigate a novel 1-D photonic crystal fishbone (FB) that can sustain surface waves. By designing a nanocavity in an FB, the confined surface mode with a high quality factor (∼ 10⁵) and extremely concentrated field near the FB surface (small mode volume, ∼ 2.3 × 10^-2 (λ/2)³) cause strong interactions between light and the surrounding medium for optical sensing and manipulation. In simulation, as an optical sensor, the proposed design achieved a high index sensitivity of 650 nm/RIU and minimum detectable index variation of 2 × 10^-5. As optical tweezers, a simulated injected optical threshold power of only 80 μW is needed for stably trapping a polystyrene sphere (PS) 100 nm in diameter. In addition, a method of selectively trapping a PS of specific size is theoretically proposed via our design. We believe that our proposed FB nanocavity with a surface mode would provide enhanced features for on-chip optical sensors and tweezers.