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UV laser builds structures atom by atom

17 Jul 2008

A 326 nm laser system is the latest tool for researchers trying to create precise 3D nanostructures via atomic nanofabrication.

A laser system emitting at 326 nm could be ideal for manipulating Group three atoms such as indium (In), say researchers from the University of Bonn in Germany. The source looks particularly promising for atomic nanofabrication (ANF), an application involving the precise handling and direct deposition of atoms using laser light (Optics Express 16 10803).

"We are experimenting with a laser-cooled In atomic beam," researcher Jae-Ihn Kim from Bonn's Institute of Applied Physics told optics.org. "The goal of our research is to generate a fully 3D structured (InAl)As crystal with periodically modulated In concentration. It is conceivable to generate fully 3D nanostructures with ANF."

While the potential of ANF is clear, it relies on the availability of lasers emitting at the short ultraviolet wavelengths that match the transitions found in group three atoms. Indium, for example, has a transition at 325.6 nm. This is where Kim and his colleague Dieter Meschede come in with their fibre-based source that frequency triples 977 nm light to 326 nm.

Kim and Meschede's system starts with an external cavity diode laser emitting at 977 nm. This emission is then split and fed into two independent fibre amplifiers, each based on a Yb-doped double-clad fibre.

"Yb-doped fibre sources are not usually operated at 977 nm," commented Kim. "This was the greatest technical challenge. Two independent fibre amplifiers are used to escape photodarkening effects. Also, because of the large absorption peak of Yb ions at 977 nm, a special fibre with a small inner cladding was required to reduce the threshold pump power and suppress unwanted emission at 1040 nm."

On exiting the fibre amplifiers, one beam is passed into an external cavity where it is frequency doubled to 488 nm. The 977 nm light from the second fibre amplifier and the 488 nm beam are then coupled into a doubly resonant cavity in order to produce 326 nm via sum frequency generation.

According to Kim, the 12 mW of useful output power at 326 nm is continuous-wave, tunable over 18 GHz and TEM00 in nature. With further improvements to the system, Kim believes that the output power will approach 100 mW.

With a practical 326 nm source in place, Kim has several goals. "In the short-term, an indium atomic beam will be laser collimated to enhance the beam flux and reduce the transverse velocity of indium atoms in the beam, which is the requirement to generate narrow structures," he said. "In the long-term, a source of single indium atoms may be constructed to generate structures more accurately on an atom by atom basis."

Kim and Meschede also plan to extend the ANF technique to Group five elements, with a view to creating novel composite materials.

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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