Gallium is a liquid metal with fascinating, yet underexplored, properties that are appealing for optics. As a metal, gallium can be used for a wide variety of applications such as reflective surfaces, antennas, and photonic structures. As a liquid, it can be manipulated at room temperature into useful shapes in ways that simply are not possible with solids, such as injection into cavities and 3D printing. It is also easy to break up into small particles, which have optical properties that depend on their size and shape. Liquids are also soft and deformable. Consequently, the shape of liquid gallium can be dynamically manipulated in a variety of ways to control its optical properties. Furthermore, other metals can be mixed with gallium to tune optical and physical properties, such as melting point. Such tuning and manipulation capabilities open the path to conveniently setting the spectral region of applications from ultraviolet to radio frequencies, as well as to developing dynamically tunable devices operating at room temperature and below. Importantly, gallium has low toxicity and effectively zero vapor pressure, making it safe to handle. The review by Datta et al. in Optical Materials Express is a great summary of the fascinating properties and applications of this liquid metal.

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