Photonic crystals are periodic dielectric structures that can be used to prohibit, confine or control light propagation in a particular wavelength band (known as the photonic bandgap). Consequently, they are an important building block for all-optical integrated circuits. The ability to tune their photonic bandgap dynamically across a wide wavelength range is highly desirable, but it has proved difficult to achieve to date. Now, Tsung-Hsien Lin and co-workers from Taiwan and the USA have demonstrated optical tuning of the bandgap of a liquid-crystal blue-phase (BP) photonic crystal across the entire visible spectrum (Adv. Mater. http://dx.doi.org/10.1002/adma.201300798; 2013).

Credit: © 2013 WILEY

BP photonic crystals are easy to fabricate — a three-dimensional periodic cubic lattice with dimensions of several hundred nanometres can be created through self-organization in a liquid crystal. The researchers found that the photonic bandgap of a compound consisting of chiral azobenzene (1.7%), commercially available nematic liquid crystal E48 (54.3%), chiral dopants S811 (29%) and R811 (15%) could be tuned over a wide wavelength range by irradiating the compound with blue light.

The compound exists in two phases, BP I and BP II, whose photonic bandgaps lie in different regions of the visible spectrum. A reversible transition between these two phases can be driven by either temperature changes or exposure to blue light (wavelength, 408 nm). The compound initially exhibited Bragg reflection at a wavelength of approximately 470 nm. During irradiation with blue light (intensity, 13 mW cm−2), the reflection band of BP II was observed to shift continuously to longer wavelengths (from 470 nm to 520 nm), until a phase transformation to BP I occurred. On further exposure to blue light, the reflection band of BP I continuously shifted to longer wavelengths (up to 630 nm). After 15 s of irradiation, the reflection band of BP I no longer shifted; rather, it remained constant at 630 nm. Although natural thermal relaxation from BP I to BP II took a few hours, it could be accelerated by irradiation with 532 nm light (intensity, 24 mW cm−2). Increasing the concentration of the light-driven chiral switch in the mixture from 1.7% to 3.5% broadened the optical tuning range of the photonic bandgap from 470–630 nm to 420–710 nm.