Side-lobes-controlled photonic nanojet with a horizontal graded-index microcylinder

A photonic nanojet generated by the transparent dielectric microparticle (microsphere or microcylinder) is a sub-wavelength focused beam. The properties of the photonic nanojet have been modified by changing the refractive index and structure of the micropaticles. In this Letter, a super-narrow photonic nanojet with a full width at half-maximum waist of approximately 116.6 nm (λ/4.3, with 500 nm excitation wavelength) is obtained by a horizontal graded-index microcylinder, which is divided by multilayers parallel to the direction of light propagation. The method for side lobes controlling by the waves' superposition from the modified graded refractive index and that of the center layer is proposed and discussed. Also, a structure with the composition of different height plates approaching the desired circular section is suggested for decreasing the difficulties in fabrication, and generates a similar photonic nanojet.