Femtosecond laser pulses create 'black metal'

29 Nov 2006

Laser surface treatment promises more efficient detectors, as Chunlei Guo explains to optics.org.

Scientists in the US have found a way of dramatically increasing the light absorbing properties of metals. The team says that its femtosecond laser treatment could boost the efficiency of space telescopes and light meters among other applications.

'Black metal'

To create so-called 'black metal', the researchers use intense pulses of light from a Ti:sapphire laser. "The system consists of a femtosecond laser oscillator and a two-stage amplifier," Chunlei Guo of the University of Rochester told optics.org. "Pulses lasting about 60 femtoseconds are released from the amplifier at a repetition rate of 1 kHz."

Guo team

The bursts of light produce a mixture of micro- and nano-size structures on the metal surface. Black in appearance, the treated region can gather radiation more efficiently than the bare metal surface thanks to its vastly increased surface area.

Previously, researchers at Harvard University have been successful in using femtosecond pulses to create 'black silicon'. Guo believes that the potential benefits of treating metals could be even greater.

"Regular silicon already absorbs most of the visible light that falls on it, and so black silicon only offers about a 30 percent improvement," he said. "In contrast, metals absorb only a few percent of visible light before processing." Another difference between the treatments for silicon and metal is that Guo's process doesn't require the use of corrosive gas.

Pleased with the results so far, the next step for the Rochester team is to speed up the process. Currently, it takes around 30 minutes to treat a finger-sized strip of metal.

"The focused laser beam is small, so we have to scan the sample to treat a large area," Guo explained. "A multiple beam system would help, but our plan is to find a range of parameters that will allow us to reduce the focus of the beam and obtain a larger spot size."

Author

James Tyrrell is News Editor of Optics & Laser Europe magazine and a contributor to optics.org.