Tunable transformation optical waveguide bends in liquid

Optical waveguide bends are indispensable to integrated optical systems, and many methods to mitigate bend loss have thus been proposed. Transformation optics (TO) causes light to travel around a bend as if it was propagating in a straight waveguide, eliminating the bend loss. Many reported TO waveguide bends have utilized solid materials, but there are fundamental difficulties for real applications because of their complex fabrication, lack of reconfiguration, and the so-called effective medium condition. Here, we develop a method to overcome these problems using the convection–diffusion of liquids. It enables real-time tunable transformation optical waveguide bends using natural liquid diffusion while still exhibiting the major merits of quasi-conformal mapping. We have experimentally demonstrated bending in visible light by 90 and 180° while preserving the intensity profile at a reasonably high level of fidelity. This work bridges fluid dynamics and optics and has the potential for application in on-chip biological, chemical, and biomedical measurements, as well as detectors and tunable optical systems.