Scientists in Europe have demonstrated thin, flexible, paper-like mirrors whose polarization and peak wavelength of reflection can be tuned (Opt. Express 21, 20821–20830; 2013). As these narrow-band (bandwidth ∼100 nm) reflectors are easy and cheap to fabricate, they are potentially well suited for use in low-cost applications such as e-readers, smart credit cards, labels and dosimeters. They offer an attractive alternative to silver-coated reflectors. Gia Petriashvili and co-workers from the Georgian Technical University (Georgia), University of Calabria (Italy) and Hewlett Packard Labs (UK) made the reflectors by mixing a cholesteric liquid crystal with a reactive monomer (RM 257), an optically active dopant (ZLI-811) and an ultraviolet photoinitiator (Irgacure 2100). A thin layer of the mixture is applied to a paper substrate, and it is covered with a protective polymer film. The entire assembly is then irradiated with ultraviolet light to polymerize it and thereby make it mechanically robust and flexible.

Once polymerized, further irradiation with ultraviolet light can be used to change the pitch of the photosensitive cholesteric mixture, and thus tune the spectral response of the mirror's reflection across the visible range — longer durations of ultraviolet exposure give shifts to longer wavelengths. Furthermore, by using a mask during ultraviolet exposure, an array of miniature reflectors with different spectral properties can be realized; for example, a pattern of red, green and blue mirrored pixels can be realized on the one substrate.

Credit: © OSA 2013

The polarizing properties of the reflectors can be controlled by varying the thickness of the polymer cover film, which also acts a phase retarder. In this manner, the reflection polarization can be changed (for example, between linear and circular polarization).

As the reflectors are flexible and tunable, they offer considerable more opportunities for applications that traditionally used silver-coated reflectors. For example, their properties can be tailored to suit any kind of backlight or reflective display design.