Until now, researchers believed that surface plasmons polaritons (SPPs) – surface excitations that involve billions of electrons – were responsible for helping to transmit light through tiny openings in metal films. However, the explanation has never been completely convincing because such extraordinary light transmission (EOT) was also seen at longer wavelengths, where the involvement of SPPs is difficult to conceive. While classic electromagnetic theory, based on "modal expansion", also provided a global description of EOT, it did not describe exactly how surface waves behave between the holes when light is transmitted.
Now, Lalanne and colleagues have calculated that quasi-cylindrical waves must also play an important role in EOT. They have shown that, at optical frequencies, cylindrical waves, as well as SPPs, contribute to the EOT – both at short and long wavelengths.
The physicists obtained their results by abandoning the classic modal expansion approach that has been used since the 1960s for analyzing light diffraction by gratings. Instead, they considered SPP scattering events that occur in the 2D hole array at the level of every individual ID chain of holes.
"We combined 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," Lalanne told nanotechweb.org. "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."
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," explained Lalanne.
The researchers now hope to experimentally validate their theory in experiments by studying SPP scattering by 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.