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Self-assembled photonic crystal confines light in 3D

20 Dec 2007

A photonic crystal that can confine and guide light in all three dimensions offers new possibilities for novel laser designs.

US scientists have created a three dimensional pathway for light inside a 100 nm x 100 nm photonic crystal made using self-assembly techniques. Once inside the crystal, light can be directed through tiny waveguide-like structures to travel along a straight line, follow a curve and even bend around very narrow corners – without sustaining large losses (Nature Photonics doi:10.1038/nphoton.2007.252).

"Our goal was to confine light within a photonic crystal in all three dimensions," said Paul Braun of the University of Illinois at Urbana-Champaign, who led the research group. "There are lots of schemes for the planar control of a light beam on a substrate, and we wanted to go further."

Photonic bandgap materials make for good laser components, because they can have internal defects that allow specific frequencies of light to pass through. In this sense, the defects act as optical cavities that can repeatedly bounce off incoming photons until they acquire the energy needed for lasing.

However, one of the biggest challenges is to control the fabrication of defects in the photonic crystal. Braun's team chose to create their 3D photonic structure using self-assembly techniques – which are cheaper and less time consuming than lithography-based methods – but needed to develop a method that could create an orderly set of defects anywhere in the crystal structure.

The first step was to pack silica spheres in a tight lattice structure to make an artificial opal structure. This was achieved using a vertical deposition process, in which the spheres are assembled onto a substrate by vaporizing the liquid out of a colloidal suspension. A coating of a resin-like material was then applied across the substrate to provide the monomer raw material for subsequent polymerization.

"To get the accuracy and precision needed for this design, we used a process called two-photon polymerization (TPP)," Braun said. In TPP, two photons of identical frequency and energy must strike the substrate at exactly the same time to polymerize the liquid resin and turn it into a transparent solid.

If this same process is applied across the entire crystal, the polymerization can be replicated in three dimensions. The key, however, is to use a laser-focusing system with a resolution of a few hundred nanometers. "By using self-assembly to build most of the structure, our process is much more efficient," explained Braun. "We only need to define the waveguide through TPP, not the entire structure."

Once the desired polymeric structure is obtained, chemical vapor deposition is used to create a thin layer of amorphous silicon in the spaces between the spheres. This is followed by etching processes that remove the template and unused monomer remnants.

According to Peng Jiang, a nanophotonics scientist at the University of Florida, this work represents a significant advance in controlling the fabrication of defects in self-assembled photonic crystals. "By combining colloidal self-assembly with simple two-photon polymerization, it opens the path for precise control of the location, shape, dimension, and alignment of embedded defects in self-assembled inverted opals," he said.

Braun is confident that these 3D photonic crystals offer a number of new possibilities for novel laser designs. "When you are using a system like this, the laser can be built in any direction," he said. "There is no limitation for it to be in a direction parallel or perpendicular to the surface of the substrate. In addition, the lasing threshold can be much lower."

However, translating this prototype to a commercial device will require rigorous minimizing of the optical losses associated with the crystal, and a standardized process to generate identical defects across substrates.

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