Researchers at the Georgia Institute of Technology have developed a family of hydrogel-based nanoparticles that can be used to form photonic crystals, the optical properties of which can be precisely tuned by thermally adjusting the particles’ water content.
The soft conformable spherical particles could be the basis for a ‘photonic fluid’ that would be custom-processed to form self assembled periodic structures able to transmit specific wavelengths of light. Potential applications include optical switching and optical limiting in telecommunications and new types of medical diagnostics.
‘We have a very simple processing method for taking one type of particle and creating a whole host of optical materials from it, as opposed to having to synthesise a new particle for each optical material you would like,’ says Andrew Lyon, Assistant Professor of chemistry and biochemistry at Georgia Tech. ‘We have a polymer solution that can be processed in normal ways that typically cannot be done with other types of colloidal photonic materials.’
The nanoparticles are synthesised from poly-N-isopropy-lacrylamide lightly cross-linked with N,N-methylenebis (acrylamide). After precipitation polymerisation in aqueous media, the particles are separated from the surrounding water by centrifuging. The resulting gelatinous material has a faint blue, green or red hue.
To give it desirable optical properties, the material is annealed by heating it above the volume phase transition temperature of the component hydrogel particles, at which point the photonic crystal loses its order and the nanoparticles begin to give up water. After removing small amounts of water, the material is allowed to cool, re-absorb water and recrystallise. This thermal cycling process serves to pack the soft hydrogel particles into an ordered 3-D hexagonal array, which produces the periodic dielectric structure needed for optical properties. This is repeated up to 15 times until the resulting crystalline structure has the desired optical properties.
By closely controlling the hydration of the particles, the researchers can tune the colours by one nanometer steps over a wavelength range of more than 200 nanometres. When heated above the transition temperature, the material becomes a liquid and can be worked using standard polymer processing techniques.