An ultranarrow photonic nanojet formed by an engineered two-layer microcylinder of high refractive-index materials

A photonic Ramjet (PNJ) is a tightly focused beam that emerges from the shadow surface of nucroparticles. Due to its high peak intensity and subwavelength beam waist, the PNJ has increasingly attracted attention, with potential applications in optical imaging. nanolithography, and nanoparticle sensing. A variety of ways have been demonstrated to further shrink the beam waist of PNJs, such as engineering the microparticle geometry and optimizing a multilayer structure. In this simulation work, we report the realization of an ultraliarrow PNJ, which is formed by an engineered two-layer microcylinder of high refractive-index materials. Finite element analysis shows that under 632.8 rim illumination, the full width at half maximum of the beam waist can reach 87 nm (-lambda/7.3). As far as we know, this is the narrowest PNJ ever reported. Using the backscattering intensity as a contrast mechanism, we also demonstrated the imaging resolution and capability of the ultranarrow PNJ through numerical simulations. We anticipate that this ultranarrow PNJ will open new possibilities in a variety of research areas, including nanoparticle detection, biomedical imaging, and nanolithography. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement