A Groovy Optical Switch — with Plasmons
Subwavelength-scale device may enhance Raman spectroscopy.
A team from the Grenoble-based Laboratoire d’Études des Propriétés Electroniques des Solides, part of France’s Centre National de la Recherche Scientifique, has devised a groovy subwavelength optical switch. The method uses two rectangular cuts in a metallic surface — grooves that are both much narrower and spaced much more closely together than the wavelength of the light that the device controls.
The researchers, led by Tomas López-Rìos, noted that the construct could be used to concentrate incoming light within one groove or the other, which would be useful in optical switching applications, and could be used for enhanced spectroscopy of molecules in very small cavities.
Key to the technique is the interaction between a beam of light and surface plasmons, highly localized surface energy excitations that are induced when light of a particular wavelength strikes a conducting surface, such as that of a metal, at the proper angle. Plasmons are thus a near-field phenomenon that can be created from a distance. Although the mechanism is not completely understood, plasmons are known to play a part in surface-enhanced Raman scattering, by amplifying the typically weak Raman signal millions of times. It is this effect that the researchers propose exploiting.
In their theoretical investigation, the scientists calculated the effect of two parallel grooves cut into a gold surface upon light at 4 to 20 μm. In particular, they considered rectangular channels 1.5 μm deep and 200 nm wide, with a separation between them of 500 nm.
The two grooves, they noted, acted like resonating cavities but with some unusual properties because of their closeness and metallic nature. They found that, because of the interaction between the incoming beam and surface plasmons, light squeezed into the cavities in one of two resonant states. As a result, the intensity of the light concentrated within the grooves was amplified several thousand times over that of the incoming beam.
Next, the researchers considered what would happen if they introduced a second beam of the same wavelength as the first but arriving at a different angle and phase. The second beam created new states within the system and allowed the investigators to force the light into one cavity or another, so that one groove contained the entire electromagnetic field, while the other was dark. They had, therefore, created a switch.
The ability to operate a subwavelength optical switch could be important in surface-enhanced Raman spectroscopy research as well as in other applications. Although the current study concerns infrared radiation, the scientists said that the technique could be extended to shorter wavelengths — provided that the dimensions of the grooves are scaled properly to the wavelength.
Physical Review Letters, July 2006, 036405.
- Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
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