Optical computing made faster with silver nanometre long strips

Multiple colours, or frequencies of light, for photonic computing are expected to speed up the devices compared to the single channel, colour technology today.

Each light frequency is a channel of data and multiple colours will mean many channels and a capability for a broad bandwidth of information. With a microchip that could produce nanolaser light in many colours, a computer could be much faster. The use of light, or lasers to transfer and process data in a circuit is known as photonics. A Purdue University-led team has created a manufacturing process to produce multiple colours on a chip instead of a single colour.

Only using one colour of light at a time on an electronic chip limits sensor technologies. Different substances scatter light in different ways and only one frequency of laser light will limit what can be detected. This is true for using scattered colour when detecting viruses in biomedical samples or processing aerial photography of fields or forests.

"A laser typically is a monochromatic device, so it’s a challenge to make a laser tunable or polychromatic,” said Alexander Kildishev, associate professor of electrical and computer engineering at Purdue University. “Moreover, it’s a huge challenge to make an array of nanolasers produce several colours simultaneously on a chip.”

To have a microchip generate laser light, the chip has to have optical cavities. In a normal large-scale laser, the optical cavity is the space within which the photons generated by a source material, such as ruby, bounce between two mirrors, one of which is opaque. As the generated photons increase in number and fill the cavity, bouncing between each mirror, eventually a coherent stream of photons penetrates the opaque mirror to become a beam of light.

New ultrathin nanocavities with embedded silver strips have streamlined colour production, and therefore broadened possible bandwidth, for both today’s electronics and future photonics. Credit: Purdue University image/Alexander Kildishev

The Purdue researchers, along with their collaborators at Stanford University and the University of Maryland have created nanocavities on a chip. These have been created before and they can produce one frequency of laser light, which is determined by the cavity’s thickness. The Purdue nanocavities contain a silver strip which is referred to as a metasurface. A metasurface is thinner than a light wave. A light wave us measured by an angstrom, which is 0.1 nanometres. The metasurface allows the nanocavity to produce a range of light frequencies. "Instead of adjusting the optical cavity thickness for every single colour, we adjust the widths of metasurface elements," Kildishev said.

The metasurface also has another potential application, camera lenses. The Purdue researchers think that optical metasurfaces could replace traditional lenses in electronic devices such as smartphones. "What defines the thickness of any cell phone is actually a complex and rather thick stack of lenses," Kildishev said. "If we can just use a thin optical metasurface to focus light and produce images, then we wouldn't need these lenses, or we could use a thinner [smartphone] stack."

The work was supported by the Air Force Office of Scientific Research and the Defense Advanced Research Projects Agency's Defense Sciences Office Extreme Optics and Imaging programme.