Getting optical signals to bend around sharp corners has remained an obstacle to developing all-optical integrated circuits and better opto-electronic devices. But now researchers have created a new process for making complex miniature waveguides that can steer optical signals in three dimensions through solid materials.
Paul Braun, a materials scientist at the University of Illinois at Urbana-Champaign, and his colleagues have demonstrated a technique that uses focused laser light to carve out intricate waveguides within photonic crystals–materials that can be used to manipulate photons in much the way that semiconductors direct electrons. Recently, there have been advances in using conventional lithography to create two-dimensional optical waveguides, says Braun. “But what’s very hard to do is take light and manipulate it in 3-D.”
The work will spur the development of a range of optical devices, says Steven Johnson, an applied mathematician at MIT who has carried out research on the use of photonic crystals as waveguides. A 3-D waveguide carved into photonic crystals, he says, “can be used to trap and control light, and has potential applications in everything from more-efficient lasers to optical signal processing for telecommunications or other applications,” he says.
Photonic crystals can be made by packing together beads of silica. When they’re packed together in a precise three-dimensional arrangement, it is possible to create what is known as a complete photonic bandgap material. This material, says Braun, will act as a perfect reflector for a particular narrow band of light–dictated by the size of the beads. “It’s a perfect reflector for all angles of incidence.”
If channels can be created within the material, any light entering the material via these channels will not be able to escape, except through the channels. So once in the material, it becomes possible to manipulate the light in unusual ways, such as by trapping it or bending it around very sharp corners without fear of it escaping.
A number of research groups have been working on using the materials to create 3-D optical waveguides, says Johnson. But one of the problems has been the low refractive properties of the polymer materials used, which makes them unsuitable for completely trapping light, he says.
Braun’s group has gotten around this by using the polymer as a template for creating a complete photonic bandgap material out of silicon, which has a higher refractive index.
The group starts off with a stack of precisely arranged silica beads. The stack is then immersed in a light-sensitive monomer, which solidifies into a polymer when hit by pulses of focused laser light.
By carefully directing pulses of laser light, it is therefore possible to create continuous paths out of the polymer material. Then, after the researchers rinse out the remaining monomer, they fill the voids between the beads and the polymer material with a silicon-based material using a process called chemical vapor deposition. The entire structure is then bathed in hydrofluoric acid to dissolve all but the silicon.
What’s left is a solid structure of silicon with a network of waveguides within it, says Braun.
In the current issue of Nature Photonics, the group reports its findings and shows that by starting off with beads that are 725 nanometers in diameter, it is possible to create waveguides for a narrow band of wavelengths in the near-infrared range. This is potentially extremely useful, since this is the range that is currently employed for most optical communications, says Braun.
The work is more of an evolution than a revolution, says Johnson. And he notes that while Braun’s structures are not yet useful for making working devices, they are an important first step toward creating more complex and functional optical devices.
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