Feb 23, 2007
Nanorod arrays polarize light
Periodic arrays of nanorods could be used as high-quality polarizers, according to new calculations by physicists in Sweden. The results, obtained by Aliaksandr Rahachou and Igor Zozoulenko of Linköping University, also show that one of the leading theories in optics cannot account for how light propagates in nanorod arrays.
People have marvelled at the optical properties of nanoparticles for centuries because of the colours in patterned, stained glass, which contains metallic nanoparticles. Over the past decade, scientists have become interested in how light propagates through nanoparticle chains and arrays, because of the unique electronic and optical properties of these materials. These properties could be exploited in areas such as sub-wavelength waveguides, novel optical chips, lasing and sensing. However, to realize such applications, a complete theory of how light behaves in nanoparticles is still needed.
The propagation of light through metallic nanoparticles (or their 2D equivalents, nanorods) embedded in a dielectric matrix can be described by so-called effective-medium theories – some of which were developed over 100 years ago. However, recent research has shown that these simple theories do not take into account some important properties of nanoparticles, like the size of the particles, their shapes, clustering and collective effects. Now, Rahachou and Zozoulenko have developed a new theoretical description of how light travels through periodic nanorod arrays that does account for some of these properties. The physicists have also shown that these parameters are of crucial importance for properly describing how light propagates through such structures.
Rahachou and Zozoulenko obtained their results by applying the recursive Green-function technique to an array of equally-spaced silver nanorods 10 nm wide, embedded in a gelatine matrix. This technique is a numerical tool developed by the researchers themselves, which allows them to study light propagation in complex photonic 2D systems.
The duo then used another theory, the Maxwell-Garnett theory, to estimate the effective dielectric function of the nanorods and calculate light transmission properties of the matrix. This theory is an effective-medium theory that provides information about the resulting dielectric function in a blend of two materials. Finally, the physicists compared these results with those obtained with the Green-function technique.
The researchers found that the Maxwell-Garnett theory can describe the propagation of light that is transverse electric (TE) polarized, but not that of light which is transverse magnetic (TM) polarized. In contrast, the Green-function technique appears to be valid for both. Moreover, the results show that geometrical factors, such as the size of the rods and their distribution in the array, can affect how light behaves in these materials. According to the Linköping University team, these results mean that nanorods could be used as high-quality polarizers because TE polarized light can freely propagate through the structures while the light with TM polarization gets fully reflected.
Rahachou and Zozoulenko are now developing an effective-medium theory that can describe nanorods of different shapes and sizes.
The researchers reported their work in J. Opt. A: Pure Appl. Opt..
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Belle Dumé is acting editor of nanotechweb.org