The research findings are published in Nanomaterials (Q1; IF:5,719).
Freezing water droplet is a promising natural object studied by mesotronics (mesoscale photonics). In particular, the droplets can serve as a novel ground for many optical devices due to the fact that the solid and liquid states of water droplets make it possible to create and make use of various optical effects.
Researchers from Tomsk Polytechnic University have designed an optical switch based on a curved light beam of a specific shape, also known as the photonic hook effect. For this purpose, they used a cooled water droplet, placed on a cold superhydrophobic (water-repelling) surface to retain its spherical shape to the maximum. The freezing drop can be used as an optical switch due to its temperature sensitivity and relatively fast switching dynamics.
In the freezing process, ice spreads upward along the droplet, thus changing the state of water. Since the optical properties of ice and water are different, when we shine a laser on the one side of a droplet, it triggers the formation of a photonic hook on the other side. The level of ice in a droplet is constantly changing during the freezing process, which makes the photonic hook bend in different ways. The key property of the photonic hook that makes the optical switch function is the dependence of its beam curvature on the position of the water-ice interface during the droplet freezing process. If we place two detectors in the radiation locus, then first the signal from the hook will reach the first detector, and then, in the process of freezing, the second detector. In other words, the light will give a signal to one or the other detector. This is how the optical switch works,
An optical switch based on the photonic hook effect is a forward-scattering device. That means that the position and shape of the field localization is sensitive to the spatial position and the distance between the droplet and the detectors. Thus, the photonic hooks in this kind of a switch function as switching channels, making it possible to create a two-channel switch based on a freezing water droplet. Whilst microscopic water droplets of about 5-10 microns in size are abundant in fog and clouds.
In addition, the device is temperature sensitive. The unique property of the photonic hook to change its curvature depending on the temperature makes the switch applicable for miniaturized "on-a-chip" devices, in green optoelectronics and mesotronics. In this case, no additional micromechanical devices, electrical signal or multi-wavelength light control are required.