US scientists at the Georgia Institute of Technology have developed hydrogel-based nanoparticles whose optical properties can be precisely tuned by thermally adjusting their water content.

The particles can be custom-processed to form self-assembled photonic crystals. Potential applications include optical switching and optical limiting in telecommunications.

Lead researcher Andrew Lyon said: "The thermoresponsivity of the component particles creates a colour-tunable colloidal crystal. We have developed a polymer solution that can be processed in normal ways, such as spin-coating, which typically cannot be done with other types of colloidal photonic materials."

Lyon and colleagues have fabricated nearly 100 different types of hydrogel particles, which range from 50 nm to 1 micron in diameter. "We have shown that the transmission can be tuned from 400 nm to greater than 1 micron by changing the particle size or the particle swelling," Lyon told Optics.org.

Structures are produced using a two-step process. First, the nanoparticles are synthesized. After precipitation polymerization, a centrifuge separates the particles from the surrounding water.

The resulting film is then annealed, which reduces water content and also loss of photonic crystal order.After removing small amounts of water, the material is allowed to cool, reabsorb water and recrystallize.

This process is repeated up to 15 times to pack the hydrogel particles into an ordered three-dimensional hexagonal array. This produces the periodic structure needed to transmit specific wavelengths of light.

The particles are synthesized from poly-N-isopropylacrylamide (pNIPAm) lightly cross-linked with N,N-methylenebis(acrylamide)(BIS).

Although the team admits that real applications may be a long way off, they expect the structures to find uses in the telecommunications industry. Lyon says that the specifically-tuned photonic crystals could be used to extract information carried on optical fibres at specific wavelengths.