Engineering an ultra-narrow localized optical beam with a hybrid flat-ended microcylinder

Recently, an ultra-narrow photonic nanojet (PNJ) supported by a high-index flat-ended microcylinder has been presented. Herein, we investigate a hybrid flat-ended microcylinder for a further compressed PNJ. The hybrid flat-ended cylinder is composed of two round heads and a flat body. Three kinds of PNJs can be produced by the hybrid flat-ended cylinder. Our simulation shows that under 638 nm illumination, the full width at half maximum (FWHM) of the beam waist can reach 75 nm (~ λ/8.5), when the refractive index of the hybrid flat-ended cylinder is chosen as nR = 2.5 for the two round heads and nB = 3.5 for the flat body. It is found that the beam waist (~ λ/8.5) is beyond the solid immersion limit (~ λ/7). Considering the practice, a hybrid flat-ended cylinder composed of a silicon body and two silica round heads achieving nearly 56 nm (~ λ/11.4) beam is designed. Our results are of interest for various applications in particle manipulation and nanolithography.