Colour-converting crystal goes 3D

Manipulating the frequency of light, and thus changing its colour, is crucial for improving displays and optical telecommunications. Optical physicists often use specially engineered one- and two-dimensional structures, called ‘nonlinear photonic crystals’, to convert the frequency of laser light passing through them. Now, an international team of physicists has created the first three-dimensional nonlinear photonic crystal, opening the door to more efficient optical devices.

The problem with 1D and 2D nonlinear photonic crystals is that they can only change the colour of the input laser beam for a limited range of incoming angles. “The full potential of frequency conversion can only be achieved by going to 3D,” says lead researcher Wieslaw Krolikowski, an optical physicist at Texas A&M University at Qatar, in Doha. “It would allow an entirely novel class of devices,” he explains, including more stable memory storage devices for computers and more efficient devices for on-chip data processing.

Krolikowski and colleagues used an ultrashort laser beam to slightly shift the position of specific ions deep inside a barium calcium titanate crystal. This enabled them to fashion a 3D material from the crystal that changes the frequency of input light as required. 

“It was crucial that we did not destroy the crystal’s structure,” says Krolikowski. “This took us a few years.” Their 3D nonlinear photonic crystal is only around 100 microns across, and the team’s next challenge is to engineer a bigger, millimetre-sized crystal, using a cheaper material, to make it more appealing for commercial applications.

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