Feb 14, 2003
Spasers bring nanoscale optical fields
Lasers and masers are common acronyms in physics. Now, if two physicists are correct, these will be joined by a completely new acronym - the spaser, or surface plasmon amplification by stimulated emission of radiation. The idea, put forward by David Bergman at Tel Aviv University in Israel and Mark Stockman at Georgia State University in the US, could be an important step towards the development of a coherent source of surface plasmons for nanotechnology applications (D Bergman and M Stockman 2003 Phys. Rev. Lett. 90 027402).
The rapid growth in nanotechnology in recent years has sparked much interest in optical fields that are localized on the nanometre scale. Such fields have potential applications as nonlinear optical probes and in nano-photomodification. However, the scale of current optical devices such as lasers is limited by the wavelength of light.
One way round this problem is to exploit so-called surface-plasmon resonances. Optical fields are enhanced near nano-scale metal surfaces due to oscillating electromagnetic "hot-spots", which result from the collective motion of large numbers of electrons on the surface. Current surface-plasmon techniques mostly work by exciting these regions with an external optical field, but this only enables a small fraction of the excitation energy to be concentrated in the local field.
Bergman and Stockman have used techniques based on quantum field theory to quantize the surface-plasmon field equations, which results in an intense optical-frequency field that is localized on the nanometre scale. They then considered ways in which these fields could be emitted coherently.
A spaser is not a laser because it does not emit light, but there are many similarities. An active medium is formed by two-level emitters such as quantum dots, which can be excited by optical, electrical or chemical means. Once a population inversion is established, the excitation energy is transferred into the electric field of the surface plasmons, which are then amplified in a resonator. The key idea of Bergman and Stockman is that the resonator is built from a combination of dielectric and metal. This allows much shorter wavelengths to be generated due to "skin" effects in the metal surface, in which high-frequency currents are confined to a thin layer.
Their new "quantum-plasmonics" theory can also be used to study mechanical stresses in nano-scale systems, since it describes how forces are applied at the nano-scale by electric fields.
About the author
Matthew Chalmers is features editor of Physics World. This article first appeared in the February issue of Physics World.