Self-assembling silica microwires offer new platform for integrated optical devices

Silica microwires are the tiny and as-yet underutilized cousins of optical fibers. If they could be precisely manufactured, these slivers of silica could enable applications and technology not currently possible with the relatively larger optical fiber, says a team of researchers from Australia and France who recently reported their efforts to meet this goal.

By carefully controlling the shape of water droplets with an ultraviolet laser, the researchers have now found a way to coax silica nanoparticles to self-assemble into much more highly uniform silica wires. The international team describes their novel manufacturing technique and its potential applications in a paper published in the Optical Society’s (OSA) open-access journal Optics Materials Express.

This self-assembly technique could be important, according to the researchers, because it could, for the first time, enable silica to be combined with other materials to form integrated devices.

“We’re currently living in the ‘Glass Age,’ based upon silica, which enables the Internet,” said John Canning, team member and a professor in the school of chemistry at The University of Sydney in Australia. “Silica’s high thermal processing, ruggedness, and unbeatable optical transparency over long distances equate to unprecedented capacity to transmit data and information all over the world.”

Silica, however, is normally incompatible with most other materials, so giving it the capability to do more than just carry light has been a challenge. Further, bridging the gap between photonic components – such as optical switches, optical circuits, photon sources, and even sensors – requires some form of interconnect. But this transition is highly inefficient and interconnection losses remain one of the largest unresolved issues in optical communications.

Silica microwires, if they could be manufactured or self-assembled in place, have the potential to operate as tiny optical interconnects. Unlike optical fiber, silica microwires have no cladding, which means greater confinement of light in a smaller structure better suited for device interconnection, further minimizing losses and physical space. “So we were motivated to solve the great silica incompatibility problem,” explained Canning.

To this end, the researchers came up with the idea of using evaporative self-assembly of silica nanoparticles at room temperature. They recently reported this breakthrough in the journal Nature Communications, demonstrating single-photon-emitting nanodiamonds embedded in silica, which is a first step toward a practical photon source for future quantum computing.

The key to carrying this innovation further, as described in the new research, is perfecting the manufacturing process so that highly uniform wires can self-assemble from nanoparticles suspended in a solution. The challenge has been that, as naturally forming round droplets evaporate, they produce very uneven silica microwires. This is due to the microfluidic currents inside the droplet, which corral the nanoparticles into specific patterns aided and held together by intermolecular attractive forces. The nanoparticles then crystalize when the solvent (water) evaporates. Canning and his team realized that, by changing the shape of the droplet and elongating it ever so slightly, they could change the flow patterns inside the drop, controlling how the nanoparticles assemble.

The researchers did this by changing the “wettability” properties of the glass the drops were resting upon. The team used an ultraviolet laser to alter and pattern a glass made of borosilicate. This patterning made the surface more wettable in a very controlled way, allowing the droplet to assume a slightly more oblong shape. This subtle shape change was enough to alter the microscopic flows and eddies so as the water evaporated, the silica formed into straighter, more uniform microwires.

The researchers anticipate that their processing technology will allow complete control of nanoparticle self-assembly for various technologies, including microwire devices and sensors, photon sources, and possibly silica-based integrated circuits.

It also could 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 research has been published as “Laser tailoring surface interactions, contact angles, drop topologies, and the self-assembly of optical microwires,” J. Canning et al., Optical Materials Express, Vol. 3, Issue 2, pp. 284-294 (2013)

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