Photonic crystals deposited on-chip

17 Jun 2002

A self-assembly technique for making practical photonic crystals on a chip is reported in today's issue of Nature.

Using a technique known as self-assembly, researchers at Princeton University and NEC Research Institute, US, have made photonic crystals with opal-like structures on a silicon wafer (Nature 414 289).

These crystals operate at the crucial 1.3 and 1.55 µm wavelengths and may allow cost-effective manufacture of the next generation of photonic devices for telecommunications and computing.

"While we know many practical applications for photonic crystal, the real challenge has been to fabricate them in a form that is practical for use in photonic devices," said David Norris, leader of the NEC research team.

The self-assembly technique has been reported previously by a Spanish collaboration (Nature 405 437) where three-dimensional periodic structures were created using a colloidal solution. However, this research yielded irregular, polycrystalline photonic crystals that were difficult to incorporate into a photonic device, as they were not mounted on a chip. Structural defects present also destroyed the photonic bandgap.

Norris and his colleagues also used a colloidal solution. The researchers deposited silica spheres of diameter approximately 1 µm directly onto a silicon substrate in a face-centered cubic structure by applying a small temperature gradient to the colloidal solution during deposition.

They then used commercial deposition apparatus to fill in the gaps around the spheres with silicon. The original spheres, that formed an opal template, are then removed leaving a porous material called an inverted opal. The resultant planar, single-crystalline silicon photonic crystals have defect densities low enough that the bandgap survives.

The researchers also doped the original opal lattice with different diameter spheres. This changed the repeating pattern in the lattice and a different diameter cavity was produced.

By changing the size of the spheres and controlling their position in the lattice, the cavity can be potentially tuned to trap specific wavelengths. The researchers believe this is the first step in the development of silicon microlasers.

Richard De La Rue from the University of Glasgow believes that "this is excellent work which has promise for more good things to come".

"What would be really exciting would be to make similar inverted structures out of III-V semiconductor compounds that are intrinsically good for light emission, which silicon is not. Work in this area is being carried out, but there is still some way to go," said De La Rue.