Tunable liquid optics: electrowetting-controlled liquid mirrors based on self-assembled Janus tiles

In optofluidics, an area that has experienced explosive growth over the last decade, dynamical control of light propagation is achieved by manipulating the curvature of the interface between two immiscible liquids. Among the first demonstrated optofluidic components were tunable liquid microlenses, which exhibit high reliability, low cost, low energy consumption, and large dynamic range. The functionality of optofluidic devices could be greatly enhanced if their tunable surfaces were decorated with micromirrors, microgratings, or optoelectronic microchips. As a first step in this direction, we show here a tunable liquid mirror based on self-assembly of anisotropically- functionalized, micron-sized, hexagonal micromirrors ("Janus tiles") at the liquid-liquid interface to create a high-reflectance surface. The use of reflection instead of refraction overcomes the limited available refractive-index contrast between pairs of density-matched liquids, allowing stronger focusing for the same geometry. Our liquid mirror is mechanically robust and retains its integrity even at high levels of vibrational excitation of the interface. Electrowetting was employed to dynamically vary the focal length of the mirror. Janus tiles with complex optical properties can be manufactured using our approach, thus potentially enabling a wide range of novel optical elements.