Are Photonic Nanojets Achromatic?

In this work we study the evolution of the photonic nanojet (PNJ) phenomenon as the ratio of the sphere's diameter over the illumination wavelength changes, for microspheres with diameters from 2 mu m to 100 mu m. We measure the focal point position by using 3D intensity scanning and identifying the maximum-intensity spot. We also measure the spot size at the focal plane using both intensity and phase data. Finally we use this information to compare the PNJ's behavior to that of a ball lens and evaluate the importance of chromatic effect in the PNJ phenomenon.