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Femtosecond laser creates subsurface structures

11 Feb 2008

Laser irradiation followed by selective etching can fabricate microchannels and self-assembled nanostructures in sapphire, and could lead to microreactive devices for chemical and biological measurements.

German researchers have used an ultrashort pulsed laser to create subsurface nanostructures in a sapphire crystal. The team believes that the techniques could be used to fabricate microfluidic devices as well as 3D photonic structures. (Optics Express 16 1517.)

Rather than using the laser for direct machining of the sapphire surface, the team exploited the fact that irradiation with ultrashort laser pulses generates highly nonlinear light-matter interaction beneath the surface of the material. Subsequent etching with hydrofluoric acid produces self-assembled nanoplanes within the material, perpendicular to the beam's polarization.

"The irradiation induces some physical effects in the material which lead to the formation of these structures, but there is no beam-shaping or special processing technique involved," Dirk Wortmann of RWTH Aachen University explained to optics.org. "Hence we describe the structures as being self-assembled."

There are several theories for what lies behind the self-assembly effect in sapphire, ranging from an interference effect between laser-induced plasmon waves, to a "density wave" in the partially molten material that becomes rapidly frozen in place. "If you ask ten people working on the topic, you will get at least eight different answers," said Wortmann.

Another result of the laser pulse is to drastically accelerate the speed of the etching, but here the cause is more clearly understood. After using a 1045 nm source with a pulse duration of 400 fs and a repetition rate between 100 kHz and 5 MHz to irradiate sapphire, Wortmann found that the irradiated areas were etched up to 10,000 times faster than the unaltered areas.

"Regions irradiated by the laser become more amorphous, and the etch rate for amorphous Al203 is much better than for crystalline sapphire. There are no nanostructures in unirradiated regions, since the unmodified material will not etch."

SEM images after etching clearly showed nanostructures formed to a depth of 500 µm in the sapphire without interruption, and feature lengths of up to 1 mm were achieved by using a numerical aperture of 0.8.

"For each pulse, non-local effects distort the waveform and lead to an elongation of the focal volume in the propagation direction," noted Wortmann. "But in the focal volume there are many nanostructures: the calculated spot size is about 1 µm, and within this region there are 3-5 nanochannels."

Wortmann now plans to do more research into the initial formation of nanostructures, including time-resolved measurements of the irradiated material's atomic and electronic properties. Detailed study of the effects of the laser's pulse duration, repetition rate and wavelength should also help to control the structures' periodicity and increase the reproducibility.

"The biggest potential is in manufacturing microreactive devices for chemical and biological measurements, producing channels for different liquids and a meeting point where they can react," said Wortmann. "Beyond that, micropumps or even turbines are possible, but for such complicated devices a much more detailed understanding of the process is necessary. We're working on this, and I am sure that we are not the only ones."

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