We are all familiar with the tiny lasers that live inside our DVD drives and pointers, but these devices are huge compared to a recently developed form of optical amplifier. While the size of lasers is generally diffraction limited, such that the confining cavity can only be as small as half the wavelength of the light, plasmon lasers can be much smaller by making use of surface plasmon polaritons. These devices exist on the nanoscale, and thus operate an order of magnitude below the diffraction limit.
Just as phonons are quasiparticles composed of the collective vibrations of atoms, plasmons consist of plasma oscillations; in this case the free electrons within a metal. Plasmons can be excited by electromagnetic waves, and when the coupling is strong, the resulting mix between the photon and plasmon can be regarded as another quasiparticle, named a polariton.
Unfortunately, a lot of the radiation is lost by escaping through the cavity and from being absorbed by the metal. The only way to achieve a gain high enough for lasing is to cool the device down to around 10 K. Such a limitation means that it is unfeasible to integrate these lasers into useful electronic devices.
However a team from the University of California, Berkeley appear to have cracked this weakness, as they have recently demonstrated the operation of a plasmon laser at room temperature [Ma et al., Nat Mater (2010) doi:10.1038/nmat2919]. The device consists of a 45 nm thick CdS square, followed by a 5 nm MgF2 layer, on a silver surface. The MgF2 gap acts as the cavity, and because it is so thin, the oscillation modes of the CdS square can strongly hybridize with the surface plasmon polaritons at the metal interface. The new laser makes uses of total internal reflection, as Prof. Zhang explained, “the plasmons are mostly reflected and confined into a small cavity - this reduces the laser energy losses and makes the lasing easier”.
Due to the symmetry of the square device, several higher order harmonics were also supported by the cavity. By introducing a curved surface to break the symmetry, it was possible to allow only a single mode to propagate.
Zhang is confident that such a laser could eventually be applied to “single molecular spectroscopy, ultra-high density of data storage, and nano-lithography”. However the authors hope that these devices will also provide an avenue for pure research, as “the intense fields that are generated and sustained in the gap region make such lasers highly useful for investigating light-matter interactions” [Ma et al.].
Stewart Bland