Researchers dream of building crystals from the ground up to achieve tight control of their periodic structure. One approach to 3D patterning envisions steering atomic beams with a maze of laser light, but creating complicated light patterns isn’t easy. Now a team reports in the 25 February print issue of PRL that a so-called holographic crystal can efficiently generate more complex stencils for atoms. With a single incoming laser, the authors generated a three-beam interference pattern and etched a periodic design onto a gold surface. The method could theoretically accommodate 1000 beams and make intricate structures, such as photonic crystals–a new technology that may lead to “circuits of light.”
In atom lithography, a stream of atoms is focused onto a surface by laser light that runs parallel to the surface. The atoms can either scour channels into a carbon-based layer resting atop a gold film or pile up directly onto a material. To create patterns more complicated than simple arrays of lines or dots, atoms must pass through a gauntlet of many overlapping light waves. But generating and pointing all those light rays with conventional beam splitters and mirrors “just gets too complicated and too clumsy,” says Dieter Meschede of the University of Bonn in Germany.
Hoping to simplify things, Meschede and his colleagues shined their laser into a holographic crystal–a solid piece of doped with iron that can be “imprinted” with instructions on how to reconstruct a pattern of crossed lasers. As a first test, the team chose to create a simple interference pattern (the hologram) of three beams, one hitting the crystal and two reflected at slightly inclined angles to the first. They “recorded” these instructions by heating the crystal and focusing three intense rays upon it at the proper angles for three hours. Hitting this prepared crystal with a laser re-created the interference pattern, which looked something like a backgammon board. In the lithography experiment, the researchers sent a stream of cesium atoms through the interference pattern toward a gold surface covered with organic molecules. The atoms etched the same backgammon-like pattern onto the surface. “The beautiful thing about these holographic crystals is that you can use one laser beam to reconstruct many, Meschede says.
In principle, says team member Karsten Buse, now also at the University of Bonn, such crystals can store a pattern of up to 1000 overlapping beams. They could also hold separate holographic images for incoming light of different wavelengths or angles. To make a three-dimensional pattern, Meschede explains, the beam could contain several kinds of atoms, and the holographic template could be tuned to focus one component at a time, letting the others pass straight through. Alternating between stencils could yield highly periodic photonic crystals, which some researchers hope to make into all-optical circuits.
The holographic approach is a significant enhancement to a process that has remained mostly a curiosity since its inception, says Jabez McClelland of the National Institute of Standards and Technology in Gaithersburg, MD. Even so, “This method will never produce a pattern such as Lincoln’s face, or even a small dot in the middle of a blank field,” he says, because not all patterns are possible. Nevertheless, McClelland thinks that periodic or quasiperiodic 3D structures such as photonic crystals are exciting potential applications of the new technique.