  Water droplets are a perspective highly abundant phase-change material to
realize tunable optical lenses. We demonstrated for the first time that
freezing mesoscale water droplet could be use as tunable optical lens, such
that freezing becomes an asset despite both the low absolute values of the
refractive indices of the shell and core materials and their optical contrast.
It was shown that the dielectric shell of mesoscale water droplet in the form
of solid ice allows controlling both the maximum field intensity and the focus
position of the formed photonic nanojet. The formation of ice 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 tunnelable spherical lens based on a freezing water drop can be
used for microscopy, optical traping in "green" mesotronics.
