Nanoribbon Waveguides Suited for Photonic Circuits

Daniel S. Burgess

A team of researchers at the University of California, Berkeley, and at Lawrence Berkeley National Laboratory has demonstrated that crystalline oxide nanoribbon waveguides can be used with other nanowire optical components to create rudimentary systems that suggest the feasibility of nanowire-based photonic integrated circuitry. Photonic integrated circuits promise applications as active and passive components in telecommunications, optical computing and a wide variety of other sectors.

The researchers focused on SnO₂ and ZnO nanoribbons in the work, but Peidong Yang of the university's department of chemistry said that there are many other suitable material systems. The highly flexible and robust single-crystal structures are roughly rectangular in cross section -- hence their name -- with sides varying from 5 × 15 nm to 2 × 1 µm and with lengths of up to 4000 µm.

Researchers have assembled flexible nanoribbon waveguides such as this one and nanowire light sources and detectors to demonstrate the feasibility of creating nanowire-based photonic integrated circuits. Courtesy of Peidong Yang.

Nanoribbons with sides on the order of 100 to 400 nm are suitable for waveguiding visible and ultraviolet radiation, although the researchers found that their synthesis process predominantly yielded ribbons for blue and green wavelengths. Measured losses at 450 to 550 nm ranged from 1 to 8 dB/mm, depending on the area of a given ribbon in cross section and the presence of scattering centers.

To demonstrate the potential application of the waveguides in photonic circuitry, the researchers examined approaches to coupling the ribbons and linked them to nanowires serving as input and output components. They found that staggering the ribbons for several microns with an air gap between them led to efficient evanescent tunneling and outperformed butt-coupling. Using a near-field scanning optical microscope, they confirmed that light could be injected into the ribbons from an optically pumped ZnO nanowire source and detected by a similar wire.

However, Yang said that integration remains a serious hurdle. The scientists assembled the systems serially, manipulating the nanoscale components into their desired position one by one. An efficient, parallel fabrication process would be necessary to enable the use of nanoscale photonic elements in commercial systems.