ScienceWise - Sep/Oct 2008

Clearly Infrared

How the high infrared transparency of chalcogenide glass promises technological innovation

ANU researchers are setting new world records in processing a range of glass materials used to manipulate infrared light. Their results set the scene for a revolution in infrared technology that promises increased internet speed and much more besides.

Infrared light is that region of the electromagnetic spectrum that extends from the red end of visible light out to what we feel as radiant heat. It's a range of light wavelengths at the centre of many amazing applications that include remotely detecting explosives, chemicals and biological agents; dramatically speeding up internet communications; and even helping us detect earth-like planets in distant solar systems.

But working with infrared light has always been a challenge because, unlike other wavelengths in the visible spectrum, it doesn't transmit well through standard glass. However, there is a range of materials known as chalcogenide (pronounced chal - koj - enide) glasses that are excellent performers when it comes to the transmission of infrared light. If we could build tunable infrared sources and optical chips out of chalcogenide glass it would open up a new world of infrared usage. The reason it hasn't happened yet is because chalcogenide glass is devilishly difficult to work with.

"Chalcogenide glasses are basically alloys that contain one of the chalcogon elements on the periodic table," says Dr Steve Madden from the Laser Physics Centre. "These include sulphur, selenium or tellurium, and they're typically alloyed with elements like germanium, arsenic, gallium, antimony and silicon. These glasses are already used in thermal infrared night vision systems but up till now it's been a major challenge to take them the next step and use them in thin films as part of optical chips and waveguides.

"The problems are two fold - producing a film of chalcogenide material with minimal defects and of the desired composition, and then processing it without introducing imperfections that will destroy its ability to transmit and manipulate infrared light."

And researchers at the Laser Physics Centre have made significant advances in both areas. First, in order to create superior quality chalcogenide films they employ an ultrafast pulsed laser to ablate chalcogenide glass targets. Atoms dislodged from the target are then deposited as a thin film.

"The material being deposited is quite literally ripped apart at the atomic level by a laser pulse about a millionth of a millionth of a second short with a peak power around a million Watts squeezed into a tiny fraction of a square millimetre," explains Dr Andrei Rode, the scientist who developed the unique ultrafast pulsed laser deposition system along with co-inventors Barry Luther-Davies and Eugene Gamaly at the Laser Physics Centre.

"Laser is the only clean tool where all the energy is harnessed in depositing the material you want without any contamination from the surrounding environment," says Dr Rode. "Our system is ideal for the growth of high-quality films of the desired composition without defects, which usually spoil the quality of light transmission along the films."
The second area of challenge is then taking these films and processing them into waveguides in ways that don't damage the glass. The team has crafted new techniques for chalcogenides which enable high resolution patterning and etching of the material without damaging it or introducing loss to the final device.

"Chalcogenide glasses are very sensitive to the chemicals traditionally used to define patterns in photoresist onto chips," Dr Madden, leader of the Planar Integration team at the Laser Physics Centre. "We've developed special etch recipes that enable us to get very smooth etch surfaces, far better than anyone else has ever demonstrated, and we've tailored the photolithography process to prevent the developer from attacking the chalcogenide. So, by combining those two steps we've been able to do very high quality handling and etching of the chalcogenide. And our methods use standard industry equipment so our techniques can be scaled up for industry.

"The advances we've made in producing higher quality films and more effective processing have produced results four to ten times better than anyone else worldwide."

The work is being undertaken within the ARC funded Centre for Ultrahigh Bandwidth Devices for Optical Systems (CUDOS). Their results are a huge leap towards making commercial infrared optical chips. The ANU group has successfully made optical wires up to 22 cm long that are essentially loss free.

"The work we've been doing with CUDOS has been to trying to make long waveguides measured in the tens of centimetres," says Dr Madden. "Up until now the longest chalcogenide waveguides have been 6 cm long. We've now made them up to 22 cm and we're aiming for 50 cm. With wavelengths this long it's possible for non-linear processes to take place with the power of light coming out of an optic fibre. This makes these things perfect for optical chips."
In conjunction with other colleagues within the CUDOS Centre, the researchers are investigating the potential of the devices for ‘All Optical' processing of ultra-high speed data traffic for telecommunications networks. As there are no electronics involved in this type of processing, it is expected that such devices may be able to process data at rates hundred times faster than electronic systems. In collaboration with the Danish Technical University all optical multiplexing at 640 Gb/s has already been demonstrated using an ANU device, illustrating the promise of this revolutionary technology.



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