Encaved optical fiber nano-antenna exciting morphology dependent shape resonance with focussed off-axis beam

This letter demonstrates morphology dependent shape resonance (MDSR) with the help of an encaved optical nano-antenna developed inside an optical fiber tip cavity. The nano-antenna generates a tightly focused beam with a spot-size of ~3 µm. A barium titanate microsphere is placed besides the optical axis inside the cavity. The focused beam remains off-axis of the microresonator and excites the MDSR. The off-axis excitation shows unique resonating properties depending on the location of the resonator. A resonant peak with quality factor as high as Q ∼7 × 10⁴ is achieved experimentally. Another design with a shorter cavity length for a bigger resonator is also demonstrated by embedding a bigger microsphere on the cleaved fiber tip surface. The optical probe holds great potential for photonic devices and is ideal for studying morphology-based scattering problems.