Photonic crystals are an integral part of optoelectronics due to their ability to guide and confine light. The periodically varying refractive index structures that form photonic crystals can be constructed in a variety of ways, but one of the most appealing is a low-cost, bottom-up approach involving the self-assembly of colloidal crystals from suspension. Seiichi Furumi and colleagues1 from the National Institute for Materials Science and Yokohama National University in Japan have now prepared a stable colloidal crystal gel that produces mechanically tunable laser light.

Furumi and his colleagues had the idea that photons generated inside a colloidal crystal could be resonantly enhanced under certain conditions to form a single, narrow emission peak similar to that produced by a laser. To test this hypothesis, the researchers created a colloidal crystal gel film consisting of ordered arrays of polystyrene microparticles embedded in a gel stabilized by an ionic liquid. They then introduced into the matrix a fluorescent organic dye with a suitable emission spectrum. Optical excitation of the dye molecules gave rise to a single, narrow laser emission peak, confirming their idea.

On applying pressure to the colloidal crystal, however, they made another surprising discovery. “We serendipitously found a new potential use for colloidal crystal films in laser applications,” says Furumi. “The single laser emission peak can be continuously and reversibly tuned over a wide wavelength range by applying mechanical stress to the colloidal crystal film, which changes the photonic bandgap.”

Fig. 1: An optically excited colloidal crystal gel film produces narrow laser emission peaks with a tunable wavelength determined by an applied mechanical stress, which changes the periodicity of the ordered microparticle arrays © 2011 Wiley-VCH

The key to producing the lasing action in the colloidal crystal film was the creation of a suitable photonic bandgap, which is determined by the physical structure of the microparticle arrays. Changing the structure by compressing the film alters the bandgap and shifts the lasing wavelength, giving rise to mechanically tunable lasing functionality (see image).

Polymer hydrogels similar to the colloidal crystal gel prepared by Furumi and his colleagues are notoriously susceptible to drying out and otherwise degrading under ambient conditions. However, thanks to the ionic liquid used to maintain the gelling state, this new colloidal crystal gel is permanently stable, maintaining its useful optical properties even under unregulated, ambient humidity and temperature conditions.