Opt. Lett. 36, 4311–4313 (2011)

Credit: © 2011 OSA

Although the use of transformation optics and metamaterials to create optical analogues of black holes has already been suggested, experimental realization of the required material structures remains a significant hurdle. Khachatur Nerkararyan and co-workers from Yerevan State University in Armenia and the University of Southern Denmark have now suggested that spherical metal surfaces brought into contact may provide an alternative approach for investigating optical black holes. The researchers showed that plasmons with an appropriate trajectory in the gap between two metal spheres (or between a sphere and a planar surface) can be concentrated into a 'black hole' around the point of structural contact. The team used an effective-index approximation to simplify the problem, thereby reducing the three-dimensional problem down to two dimensions. In one example, they considered a 100-μm-radius gold sphere in contact with a planar surface and 800-nm-wavelength light. The effect relies on the fact that reducing the gap width increases the effective permittivity experienced by the gap plasmons, thanks to an increasing Coulomb attraction across the gap. This increased effective permittivity draws the light towards the point of contact, which therefore acts as an optical black hole. The researchers hope that this configuration can be realized in practice to allow the table-top exploration of phenomena such as Hawking radiation.