Putting Optical Communications in the Black

Hank Hogan

The computers in an office someday might be networked with light, if recent research on black silicon pans out. A group of researchers from Harvard University in Cambridge, Mass., from Koç University in Istanbul, Turkey, and from SiOnyx Inc. in Beverly, Mass., have investigated the photoluminescence of black silicon and have found a broadband that is centered at 630 nm.

Photoluminescence, when combined with black silicon’s near-90 percent absorption of visible and near-infrared light, could make the material suitable for short-haul optical communications over a local area network.

Irradiating silicon with laser pulses in the presence of air creates black silicon, which strongly absorbs light. Shown here are a piece of black silicon (top) and a scanning electron microscope image of the light-absorbing spikes that form (bottom). Courtesy of Eric Mazur, Harvard University.

However, the investigators found that the photoluminescence intensity of black silicon decreases with increasing temperature, with quenching above 120 K. That implies certain operational requirements, noted Koç University physics professor Ali Serpengüzel.

“For now, it is necessary to cool the material,” he said. “However, it might be possible to optimize the material for room temperature operation.”

Eric Mazur’s group at Harvard fabricated the black silicon, and Serpengüzel’s group measured its photoluminescence.

SiOnyx is a startup for optoelectronic devices that will be based on black silicon, which is manufactured by shining a series of very short and intense laser pulses at a silicon surface in the presence of air or a variety of gases. The result is a spiked surface and near-total light absorption.

After fabricating the black silicon, the researchers attached it to a copper holder placed in a closed cryostat system. With this setup, they were able to maintain the sample temperature between 10 and 300 K. They used a second harmonic from an Nd:YAG laser from Spectra-Physics as a pump, firing the 532-nm beam with a pulse duration of 10 ns and a repetition rate of 10 Hz. They collected the resulting photoluminescence signal and measured it with a gallium arsenide photomultiplier tube from Hamamatsu sitting behind a monochromator from CVI Melles Griot. This gave them the photoluminescence spectra of black silicon in the 550- to 850-nm-wavelength region.

They found that the photoluminescence had a peak around 650 nm. It was most intense at 10 K and dropped off with increasing temperature. When the researchers boosted the pump laser intensity, the photoluminescence increased as well, although the relationship between the pump and the photoluminescence was not linear.

Serpengüzel said that future research would continue characterizing the material. After that will come material optimization and optoelectronic device fabrication.

Journal of Nanophotonics, Jan. 1, 2008, doi: 10.1117/12.798157.