School students the world over learn about the wave nature of light through Thomas Young’s famous double-slit experiment of 1800. In this physics classic, light is allowed to pass through two narrow slits, producing interference fringes on a screen placed behind the slits. Over the years Young’s experiment has been revisited many times, and in a contemporary form reveals the wave-particle duality of light as described by quantum mechanics. But whether the physics is classical or quantum, the laws and limitations of geometrical optics apply.
Often touted as the next big thing in optics, plasmonics describes the movement of surface plasmon polaritons (SPPs) – electromagnetic waves that propagate along and are bound to metal surfaces through their interaction with surface electrons. Plasmonics is seen by many as a bridge that will unite nanoelectronics with photonics, allowing the transmission of signals at light speed, but without the limitations imposed by the relatively large wavelength of light.
Researchers at Stanford University in California and Brown University in Rhode Island, US, have created an SPP analogue to Young’s double slit experiment, and with this shown that a simple analytical model can describe the wave nature of SPPs. The results, which may disappoint some, suggest that plasmonic devices cannot easily circumvent the limitations of conventional optics. Because they ride along the surface of wires and surfaces, the thinking has been that SPPs might get around the diffraction limit, which restricts the size of fibre-optics. The data, however, suggest otherwise.
In their experiment, Rashid Zia and Mark Brongersma generated an SPP and passed it across two narrow bridges of gold film on a glass slide. Exiting onto a broad sheet of gold film, the waves diffracted to create interference patterns analogous to those seen in Young’s original setup. The researchers then predicted with an optical model the expected diffraction patterns, and matched these closely with images taken from a scanning tunnelling microscope.
“This was a very exciting finding as it justifies the use of conventional photonic simulation tools to design and optimise a large class of plasmonic devices,” says Brongersma. “On the other hand, the unique optical properties of metals can also give rise to very different behaviour than is commonly observed in dielectric components.”
The experimental results, which are reported in the July issue of Nature Nanotechnology, demonstrate that SPPs propagating on patterned metal films have to obey the laws of diffraction. And, according to Brongersma, this concludes a lengthy debate over the very existence of a diffraction limit for SPPs.
Fascinating physics though it is, Zia and Brongersma’s work is of far more than academic interest, as the existence of a diffraction limit has important implications for photonic systems and how they interface with electronic devices. “Now that we have demonstrated that stripe waveguides have to obey the fundamental laws of diffraction, it is clear that they will not play an important role in nanoscale photonic devices,” says Brongersma. “Nanoscale photonic systems will have to rely on more strongly confining SPP waveguides that can support a deep sub-wavelength optical mode (beam diameter).”
Zia and Brongersma are now looking to create waveguide structures such as nanoscale V-grooves and slots in metal films, and see whether they can interface them with other nanoscale plasmonic components that can generate, switch, and detect light signals. If they succeed, many of these nanostructures could be fabricated using CMOS-compatible techniques, making them amenable to mass production.
Brongersma concludes: “You can couple stripes, you can make slits, you can make all sorts of other geometries that might work. But to see that potential through, you have to have a clear analytical theory and a way to test it.”
Further reading: Surface plasmon polariton analogue to Young’s double-slit experiment, Zia & Brongersma, Nature Nanotech. 2, 426 (2007).
Figure: Measured intensity of guided polariton waves on the left; numerical simulation on the right. The diffraction pattern is similar to that first seen in optical experiments carried out over 200 years ago by English physicist Thomas Young (image courtesy of Rashid Zia & Mark Brongersma).
Article first published in Nanomaterials News.