Optical amplifiers are one of the central devices in optical information science and technology – especially in the field of optical communications in the infrared, explains co-team leader Xiangfeng Duan of the University of California at Los Angeles. Most optical devices working in this important band of the electromagnetic spectrum are larger than a micron in size, which means they cannot be integrated into circuits. Although researchers have succeeded in making smaller devices, these either leaked too much light or did not amplify light strongly enough for practical applications.

The team overcame these problems by designing a novel amplifier based on a central nanowire core of silicon measuring just hundreds of nanometres in size surrounded by a thinner shell of erbium silicate material. When they shine laser light onto the structure, the high refractive index of the silicon core (3.5 at a wavelength of 1550 nm) tightly confines this light within a submicron region to minimise the amount of light lost as it propagates through the core waveguide. The single crystalline erbium-silicate shell, for its part, acts as a highly efficient gain medium and amplifies the light that leaks out into the shell.

Flexible design

In their best device (that is, one with a 600-nm core and 300-nm shell), the researchers measured a gain of up to 31 dB mm−1. This value is more than 20 times larger than that reported for optical amplifiers measuring microns across.

“The key advance in our work lies in the fact that our design is flexible,” says Duan. “It allows us to tune the size of the core and shell independently to simultaneously minimise the optical losses in the device while maximising the gain,” he tells nanotechweb.org.

The researchers say that they will now be looking at fabricating their amplifier using well-established guided-growth, directed-assembly and standard lithography techniques for making integrated circuits. “If successful, we might be able to integrate these structures into practical devices quite soon,” adds co-team leader Anlian Pan of Hunan University Changsha in China. “Such devices would have faster processing speeds and require lower operating powers than current technologies.”

The work is detailed in Physical Review Letters http://dx.doi.org/10.1103/PhysRevLett.115.027403.

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