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 λ. 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 ~ 0.22λ 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.