Optofluidic waveguide bending by thermal diffusion for visible light control.

Optofluidics has inspired many promising optical devices. Among them, waveguide bending is an important element for guiding light. Here, we demonstrated the thermal-diffusion liquids, acting as a natural transformation optical material in an annular structure. Compared with conventional step-index waveguide bending, this thermal one enables real-time tunable visible light bends by extreme angles, with nearly no power loss and intensity distribution. This unique light bending is because gradient refractive-index profiles caused by thermal diffusion meet the requirements by transformation optics. The work demonstrates the thermal diffusion in liquids as a natural technology to realize optofluidic gradient-index designs and has potential for tunable optical systems.