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Nanostructures absorb light from all directions

More efficient light absorbing materials on the microscale are needed for a variety of technology applications, such as optical interconnects, light-emitting sources and shields for micro-optical devices. In particular, materials that absorb light from all directions are required, which are not easy to make. Now,[size=1][color=red][b] researchers in Spain have shown that they can achieve total omnidirectional light absorption in nanostructured metal surfaces that contain localized optical excitations called plasmons.[/b][/color][/size] What's more, the effect can be seen over the full range of incident angles and can be tuned throughout the visible and near-infrared regions. Such surfaces could also be crucial for making solar cells.Y%Z ~0A"Z
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[url=http://nanotechweb.org/cws/article/tech/34494/1/080609][img]http://images.iop.org/objects/nano/news/thumb/7/6/9/080609.jpg[/img][/url]S!v%w` s ?#j
Metals make excellent mirrors on the macroscale but are poor reflectors of light when structured at subwavelength scales. One example of this is "black silver". This behaviour is best explained by light absorption by "plasmon excitations". 2q9T_.HA G%y
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Since the 1970s, researchers have shown that surface excitations at optical, infrared and microwave frequencies can produce full light absorption using linear gratings. Problem is, absorption in gratings relies on delocalized surface excitations that are very sensitive to the angle of incidence of light falling on them. This limiting factor has prevented applications in photovoltaic cells and microscale lighting where broad light collection angles, and thus omnidirectional light absorption, are ideally required. VCt@r|R?3Y|dy

"S!Pb ?a?{K [b]All-angle photovoltaics[/b]
$zs^v?Xwj Javier Garcia de Abajo of the Instituto de Optica in Madrid and colleagues in France and the UK, have now shown that omnidirectional light absorption is possible on any opaque surface when localized plasmon resonances are specifically engineered on the surface. The researchers identified gold nanoporous samples that exhibited this effect. b}5Rmb#E-j+p0~P?
"The result opens up a new avenue for all-angle photovoltaics," de Abajo told [i]nanotechweb.org[/i]. "The ability to absorb light from all directions is very important for photovoltaic cells, provided that the samples are covered with semiconductor materials capable of gathering all of the absorbed light to produce electron-hole pairs (excitons)." fuq+BoC5gJ
The researchers say that they still need to overcome a major problem – that the reported effect only occurs for specific light wavelengths. Broad-band applications will require absorption over a range of light wavelengths. $AS~H2o[[
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[b]Kirchoff's law[/b]
qB8A6ubQxd%V$j "Another interesting application – which we are currently considering – is the emission of light of specific colours triggered by our surfaces," added de Abajo. "This emission could either be thermal or like that in light-emitting diodes. The latter would require pn junctions for electron-hole recombination in the vicinity of the gratings in our structures (see figure)." Such surfaces would provide the optimum solution for omnidirectional black-body emission, thanks to Kirchoff's law – the light emission rate from an object being proportional to its light absorption rate. m:\ ?{ob
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The work was reported in [i]Nature Photonics[/i].
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vT-{D9D Source： [i]nanotechweb.org[/i]

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