Light, metal and molecules (II)

Periodic structures, such as arrays of cylindrical holes, in a metal film allow light to be coupled to surface plasmons at well-defined wavelengths (modes) for a given angle. The transmission spectrum of sub-wavelength arrays of holes in opaque films shows peaks corresponding to these plasmonic modes, i.e. at wavelengths where the electromagnetic field is exalted above the holes. The result is an extraordinary transmission that is 100 to 1,000 times higher than that expected from Hans Bethe's theory for sub-wavelength holes. In other words, the intense electromagnetic field of the surface plasmons compensates for the weak tunneling transmission of the holes. Since the discovery of the extraordinary transmission of sub-wavelength holes, various hole-based structures have been studied, ranging from single holes to holes surrounded by cylindrical periodic grooves (targets). Targets are particularly interesting because they can also focus transmitted light in a given direction defined by the dispersion curve. Hole-based structures have found a variety of applications, from ultra-fast photodetectors for optoelectronics to devices capable of trapping single proteins and observing their conformational changes.