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Invisible display lights up metal

17 Feb 2005

Bang & Olufsen has used a femtosecond laser to create hidden displays in aluminium.

Danish audio and video systems maker Bang & Olufsen (B&O) has created a hidden display that works by shining light through a nanometer-thin layer of laser-machined aluminium. Invisible in the 'off' state, the display is activated by simply back-lighting the aluminium.

The Danish company is keen to move away from embedded information displays and instead work with an uninterrupted metallic surface. B&O engineers teamed up with researchers at the Technical University of Denmark to develop a machining process that would create the invisible metallic display.

The solution was to ablate sheet aluminium with a femtosecond laser to form micro-cavities in the backside of the workpiece. Because the holes stop just a few nanometers short of the metallic front surface, light is able to penetrate the thin aluminium layer.

Different symbols, representing actions such as 'play >' or 'record •', are written in the aluminium sheet by simply ordering the micro-cavities into the corresponding shape. Bang & Olufsen has made four demonstrator units that contain a battery, high brightness red and white LEDs, and four buttons to activate each of the test symbols. The display is visible only when backlit. A 15 micron thick, milky white oxide layer provides the mechanical strength to keep the symbol intact.

"Theoretically, it can be made in other metals too," Jan Prichystal from Bang & Olufsen told Optics.org. "We are focused on aluminium because that is the metal we use most." The team found that for the idea to work, the aluminium thickness would have to be reduced to below 12 nm. This gives a transmission of almost 6% when illuminated by a 550 nm light source. Because the transmission at 22 nm is only 1%, the pulsed machining process has to be controlled to within 10 nm.

To control the depth of ablation, researchers added a detection beam to the laser drilling set-up. By focussing a HeNe or diode laser onto the machining side of the aluminium sheet, engineers can use the transmission of light picked up by camera on the other side of the sample to monitor cavity depth.

Laser machining was conducted in two-steps at Laser Laboratorium Göttingen's facilities in Germany to verify the process. Rough machining is performed at full power until the laser/camera set-up detects the first transmission of light. This equates to an ablation of around 250 nm per pulse. The second step is executed at reduced power, ablating at just 10 nm per pulse to reach the desired transmission level.

Bang & Olufsen presented its work as part of the Lasers and Applications in Science and Engineering (LASE) session at Photonic West 2005 organized by SPIE.

James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.

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