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

A photonic nanojet (PNJ) is a tightly focused beam that emerges from the shadow surface of microparticles. 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 ultranarrow PNJ, which is formed by an engineered two-layer microcylinder of high refractive-index materials. Finite element analysis shows that under 632.8 nm illumination, the full width at half maximum of the beam waist can reach 87 nm (~λ/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.

Optimization of photonic nanojets generated by multilayer microcylinders with a genetic algorithm.

We employ a genetic algorithm coupled with Mie theory to optimize the magnetic field intensity profile of photonic nanojets (PNJs) generated by multilayer microcylinders at visible wavelengths in free space. We first optimize five-layer microcylinders to elongate the PNJs. We show that a properly designed five-layer microcylinder structure can generate an ultra-long PNJ with a beam length ~ 107.5 times the illumination wavelength λ0. We then optimize five-layer microcylinders to narrow the waist of PNJs. We show that a PNJ with a full-width at half maximum waist of ~ 0.22λ0 can be obtained outside the surface of the optimized microcylinder. In addition, curved PNJs with subwavelength waist are also obtained. We finally optimize the five-layer structures for refractive index sensing based on the beam length of PNJs. The estimated minimum detectable refractive index variation when using this sensing method is ultra-small. Our results could potentially contribute to the development of a new generation of devices for optical nanoscopy and biophotonics, and greatly promote the practical applications of PNJs.