Silica’s compatibility broadens through self-assembly

Ashley N. Rice, ashley.rice@photonics.com

Silica, a rugged compound with unbeatable optical transparency over long distances, is about as easy to combine with other materials as oil is with water. But a novel manufacturing technique transforms this underused cousin of optical fiber into something that could resolve one of the largest issues remaining in optical communications – bridging the gap between electronic and photonic components.

An international team from the universities of Sydney and Paris-Sud are the first to expand silica’s compatibility through a microwire self-assembly process.

“Not only can we make uniform wires, but we can integrate anything into them,” John Canning, team member and a chemistry professor at the University of Sydney, told Photonics Spectra.

Figure (a) shows a roughly circular water droplet containing silica nanoparticles resting on an untreated glass surface. Figure (c) demonstrates how a UV laser can alter the surface texture, allowing the droplet to preferentially flow over the more wettable section, changing the droplet’s shape.

Silica microwires, if manufactured or self-assembled in place, have the potential to operate as optical interconnects. They also could achieve new functionality by adding various chemicals that can be introduced only by self-assembly. And, unlike optical fibers, silica wires have no cladding, which means greater confinement of light in a smaller structure better suited for interconnection. This ultimately minimizes losses and physical space.

To solve the silica incompatibility problem, the team devised evaporative self-assembly of silica nanoparticles at room temperature, Canning said.

“The setup is simple – we used a 193-nm-emitting ArF [argon fluoride] exciplex laser to process a glass slide in one area,” he said. “Onto this, we placed a drop of nanoparticles in solution, such that half the drop was on the processed area and the other half off. A microscope camera was used from the side to measure the contact angle on either side to confirm a difference, and from this, the shape was measured to be slightly elliptical and flattened at one end.

Self-assembled silica wires illuminated by HeNe laser light from one end.

“A second drop was placed on an unprocessed region and the contact angle measured and found to be uniform. The drops were allowed to evaporate and self-assembly monitored from the top using the microscope camera. The drop that was no longer uniform produced straight wires, whilst the uniform drop produced tapered waveguides as expected.”

The most challenging part, Canning said, was controlling the manufacturing process and forming straight wires.

“There are still key practical issues – when silica is this thin, it is sensitive to moisture in the environment and other factors which degrade properties such as mechanical strength over time,” Canning said, declining to discuss specifics.

The processing technology could allow complete control of nanoparticle self-assembly for various technologies, including microwire devices and sensors, photon sources and, possibly, silica-based integrated circuits, the researchers say. It also will enable the production of selective devices such as chemical and biological sensors, photovoltaic structures and novel switches in both optical fiber form and on waveguides – all of which could lead to technologies that seamlessly integrate microfluidic, electronic, quantum and photonic functionality.

“The first practical application will probably be in sensing, because the ability to put in any material and monitor it optically is straightforward,” he said. “We do have some other potential ideas, but we are not permitted to talk about them until they are protected.”

The results were published in Optical Materials Express.