Photonic jet with tunable focus based on water droplets freezing from the outside in

Water droplets are a highly abundant phase-change material to realize tunable optical lenses. We demonstrated for the first time that freezing mesoscale water droplets could be used as a tunable optical lens, such that freezing becomes an asset despite the low absolute values of the refractive indices of the shell and core materials and their optical contrast. It is shown that the dielectric shell of mesoscale water droplets in solid ice allows controlling both the maximum field intensity and the focus position of the formed photonic nanojet. The ice formation with air bubbles during the freezing of a water droplet is appropriate for a dynamic increase in the range of change of the focal position compared to solid ice. The proposed concept of a tunable spherical lens based on a freezing water drop can be used for microscopy and optical trapping in "green" mesotronics.