Photonic Jet with Tunable Focus Based on Water Droplets Freezing from the Outside In

TL;DR

This paper introduces a novel tunable optical lens using freezing water droplets, demonstrating control over photonic nanojets and focus position through phase change, with potential applications in microscopy and optical trapping.

Contribution

First demonstration of a water droplet as a tunable optical lens utilizing freezing-induced phase change to control focus and field intensity.

Findings

Freezing water droplets can act as tunable lenses.

Ice formation affects the focus and intensity of photonic nanojets.

Air bubbles during freezing extend the focal range.

Abstract

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.