Hydrogels have an extremely wide spectrum of applications, from soft contact lenses to selective drug delivery and from glues to moisture control elements. In this work the authors present yet another application of hydrogels in a novel type of photonic crystals whose response can be controlled faster than any previously reported similar structure. A photonic crystal is a periodic structure that prohibits light propagation in a range of frequencies, known as a bandgap. In experiments, this effect manifests itself in a high reflectivity of light within a bandgap, giving rise to the colorful appearance of photonic crystals. A team of researchers from the United States and from South Korea produced a hydrogel-based photonic crystal, whose bandgap can be varied across the whole visible spectrum by applying modest temperature change. They have produced a structure that does not melt as the temperature reaches 40 degrees Celcius, and demonstrates reversible tuning with the speed of the bandgap motion of 60 nm/s, which is several orders of magnitude higher than previously reported. The authors suggest that these photonic crystals have potential for numerous applications, including dynamically tunable organic lasers.
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