"This might open the way for real applications," says Max Rumler, a PhD student with supervisor Lothar Frey in Erlangen, Germany, and first author in the report of these results. His latest work filters light colours using the wavelength-dependent quantised collective responses of electrons in metal nanostructures—"plasmons".

Having free rein from Frey to find a PhD topic that would interest him, Rumler chose his subject after stumbling on Thomas Ebbeson’s report on extraordinary transmission in 1998, as well as a review of work applying the same principles in colour filters. "The effect is well known but lots of publications only deal with square millimetres or a centimetre at the most, so I thought I would try and increase the area for these," he adds.

Rumler and Frey work at the Fraunhofer Institute for Integrated Systems and Device Technology, the Graduate School in Advanced Optical Technologies, the Cluster of Excellence Engineering of Advanced Materials and the University of Erlangen-Nuremberg in Germany. Along with their colleagues they adopted a soft lithography approach to produce their arrays. Although soft lithography is not a new approach either, the technology to apply the approach reliably enough for colour filters has only recently become available.

The researchers optimized their soft lithography approach for producing aluminium colour filters with an active area of 145 cm2. They also tailored the thickness of the metal array and coating for improved performance, and identified the plasmon modes involved using frequency-domain numerical simulations.

Soft approach yields big results

The researchers produced a master stamp of an array of pits in silicon using laser interference lithography. The stamp can then be used to imprint the array into a polymer, such as the polydimethylsiloxane (PDMS) used by the researchers. Etching metal with this polymer array resist on top then transfers the original printed array structure onto the plasmonic metal.

The original master stamp can be used again and again for years making the reproduction of arrays simple and inexpensive. However lining up the resist by hand leads to distortions, and a way around these was only recently devised in work at Philips.

In addition structures produced by this kind of soft imprinting approach are prone to defects. While the defects would be problematic for some devices, Rumler suggests that photonics applications can be quite tolerant to defects.

Applications and outlook

The large plasmonic colour filters demonstrated by Rumler, Frey and colleagues may find use in devices as common as cameras. Since cameras only sense light and dark, they need filters to record colour. At present polymer chemical filters are used but there is a limit to how small and thin these can be made before their filtering function is compromised. The refractive index sensitivity of the plasmonic colour filters could also be applied to detect bacteria and other substances in biotech sensors.

The approach also shows potential for integration with sensing devices beyond the lab. "Aluminium is a standard CMOS material unlike gold or silver," adds Rumler. "So I don’t see any major problems with CMOS integration, but you would have to try it of course."

Thomas Ebbesen at the University of Strasbourg, who first reported extraordinary transmission in the late 1990s and was not involved in the current research, commented "This is an important breakthrough for the use of hole arrays as filters in a variety of applications."

Future work will aim to calculate the defect density and ways of controlling and reducing it. "Increased contact area usually means larger separation forces so I would suggest looking into this as the main source of defects, which then limits the lifetime of the mould," says Rumler.

Full details are reported in Nano Futures.