Optica 2, 255–258 (2015)

Light splits into its component colours when it interacts with a surface composed of periodic features separated by a distance comparable to the wavelength of the incident irradiation. The phenomenon is commonly used in diffraction gratings and an external observer will see a different colour depending on their position relative to the grating. Connie Chang-Hasnain and colleagues at the University of California at Berkeley have now used this principle to make a thin and flexible display device that changes colour as the device is stretched.

The researchers etch silicon ridges of nanometre-thickness in a wafer and then transfer them to a polymeric substrate. The final surface contains ridges of a high-refractive index material (silicon) that are separated by about one wavelength and embedded in a low-index matrix (the polymer). Using this specific design, almost all of the power of the impinging light goes into a diffracted ray, rather than being simply reflected. The upshot of this special case is that under white light illumination and at fixed angle of observation, the colour perceived by an external observer can be modified by changing the separation distance between the ridges. In a striking demonstration (https://youtu.be/ppmx-PlsiT8), Chang-Hasnain and colleagues stretch their photonic material by less than 5% and change the perceived colour from green to red without an appreciable loss of power.