Near-Field Light Lenses for Nano-Focusing of Beams of Atoms

Air Force Research Laboratory

Friday, August 01 2008

Lenses like these could contribute to development of nanodevices in general.

Near-field light lenses for focusing beams of light and beams of atoms to spots having a width of no more than several nanometers are undergoing development. There are numerous potential applications for such lenses:

•A capability to focus beams of light to nanometer-sized spots is essential for development of proposed nanoscale optical devices (e.g., optical switches and logic gates) based on near-field optical interactions.
•A capability to focus beams of atoms to nanometer-sized spots could contribute to the development of nanophotonic devices, including quantum dots, which must be sized and positioned with precision.

The principle of operation of a near-field lens of the present type is best described by reference to Figure 1. The lens is a microscale structure that includes a hole that is approximately a truncated pyramid and has a reflective surface everywhere except at the edge of the hole at the narrow end of the pyramid. Optionally, the hole can be made to taper from a square to a circular cross section toward the narrow end. The edge of the hole at the exit end has a radius of curvature <100 nm. Far-field laser light is delivered to the interior space between the reflective surfaces via optical fibers. The size and shape of the lens are such that a significant portion of the incident far-field illumination is converted into near-field illumination near the edge of the hole. The wavelength of the illumination (780 nm in the case of rubidium atoms) is chosen to be slightly lower than the wavelength of a resonance of the atoms — a condition called “blue detuning.” In the presence of blue detuning, the near-field illumination exerts a repulsive dipole force on atoms that enter the near-field region, causing the atoms to become concentrated into the focal spot.