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New model illuminates extraordinary optical transmission

15 Apr 2008

Nano-optical devices could benefit from research into the sub-wavelength behaviour of light.

Researchers in France have developed an analytical model that sheds new light on extraordinary optical transmission (EOT), a surprising phenomenon in which an array of subwavelength holes in a metallic film transmits significantly more light than expected. Finding an explanation for this mysterious effect is not just important for understanding the fundamental physics at play, but also for designing nano-optical devices that could be used in high-impact applications such as lithography and sensing.

The EOT phenomenon has intrigued researchers since it was first observed over a decade ago, not least because the holes in the array are so small that they should only support evanescent light waves. However, an accurate description of the electromagnetics behind EOT has so far proved elusive.

Some scientists have focussed on the role of electron density surface waves in the metal film that are known as surface plasmon polariton (SPP) modes. But these SPP modes, which are characterized by collective oscillations of free electrons in the metal, cannot account for all the features observed in EOT.

As a result, scientists have long debated the importance of SPP waves for determining the overall response from an array of nanoholes in a metallic film. Some researchers believe that interference-based mechanisms dominate, while others hold that SPP modes play a more significant role - and the existing experimental evidence supports both schools of thought.

Current theories also fall short of explaining the phenomenon. Models based on electromagnetic mode expansions cannot account for the surface waves in EOT systems, while full-vector calculations that solve Maxwell's equations give no useful insight into why light goes through the subwavelength holes - even though they are able to reproduce the EOT spectral features.

In contrast, the model proposed by Haitao Liu of Nankai University in Tianjin, China, and Philippe Lalanne at the Institut d'Optique in Palaiseau, France, makes it possible for the first time to determine the relative contributions from SPPs and other electromagnetic waves to the overall EOT process. "We think that it is the first time that a model is capable of describing the SPPs that are excited in between the holes and of discriminating their role with respect to other waves in a quantitative manner," Lalanne told optics.org.

Liu and Lalanne's approach is to consider the multiple SPP scattering events that occur within a 2D hole array. They achieve this by first obtaining an elementary description of individual 1D chains of holes, and then combining these elementary events in an SPP coupled-mode model to obtain a closed-form expression for the transmission of light through a 2D hole array.

"By comparing the SPP model predictions with computational results for the transmission, we inferred what contribution was due to the SPP in the EOT and what was due to other waves," said Lalanne.

Liu and Lalanne found that multiple-scattering SPP modes tend to dominate the response at higher frequencies, such as the visible range. But at lower frequencies, including the near-IR band, other electromagnetic modes such as quasi-cylindrical waves play a more important role.

The results will allow scientists to better understand EOT, in particular the resonance behaviour of light on metallic surfaces at subwavelength distances. "In general, it stresses the importance of waves other than SPPs in many optical phenomena," said Lalanne.

The researchers now hope to experimentally validate their theory in by studying SPP scattering from 1D hole chains. They would also like to use the result to design metallo-dielectric metamaterials for new nano-optics devices.

The work was reported in Nature.

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