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The paradox 

of holes

Cut holes in gold film so fine as 

to be already semi-transparent, 

and you will have… a more 

opaque film! This is the conclusion 

of the team from the Physikalisches 

Institut of the Universität Stuttgart  

(DE). Surprising? Not totally, 

because we know that, being 

in the form of waves, light cannot 

pass through holes which are 

narrower than its wavelength. 

However, metals are at times 

an exception to the rule. Some 

10 years ago, researchers demon-

strated that in the presence of 

a particular arrangement of holes 

the incident light created waves, 

via the electrons of the metal, 

which spread over the entire 

surface. The interaction of these 

plasmons with the light caused it 

to be transmitted through the film.

The gold films used matched 

these transmission conditions 

– 20 nm thick and holes of 200 nm 

in diameter – but even so the 

percentage of transmitted light 

was lower than that of non-

perforated film. Scientists suspect 

the semi-transparent nature 

of the film which lets through 

40 % of the light. The remaining 

60 % would seem to be insufficient 

to allow the plasmons to play their 

role. Moreover, the transmission 

rate seems highly dependent 

on the wavelength as infrareds 

generate a very high absorption. 

The German study in this way adds 

a new relief to existing knowl-

edge. Mastery of these specific 

interactions could open the way 

to the development of integrated 

plasmonic chips for filtering 

certain wavelengths.