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Recently, tunable photonic nanojets (PNJs) utilizing a mask have been investigated to produce narrow PNJs and curved PNJs (photonic hooks, PHs). Herein, a dielectric nano-layer is employed to tune the PNJs from a micro-cylinder. Symmetric and asymmetric illumination caused by the nano-layer are studied separately with finite element method. The results show that, for the symmetric illumination, narrow PNJs and twin PNJs (t-PNJs) can be generated through controlling the width of the layer. For the asymmetric illumination, it has the ability to produce PHs and asymmetric t-PNJs by changing the thickness of the nano-layer. Based on these results, an optical switch for light-route selection depending on the incident light wavelength is presented. This work may have great potential in the fields of optomechanical manipulation, super-resolution imaging and optical nanolithography. [ABSTRACT FROM AUTHOR]