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

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

August 1, 2008

Air Force Research Laboratory

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.

Figure 1. Near-Field Light would be generated from incident far-field light. The light would be blue-detuned to exert a dipole force on atoms in a beam to focus the beam to a spot having a width of the order of several nanometers.

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.

Experimental lenses based on this principle were fabricated on silicon-on-insulator surfaces, using standard techniques of photolithography and etching to form the holes, and vacuum evaporation to apply reflective coats of gold. At the time of reporting the information for this article, the lenses had not yet been tested in operation, but computational simulations of operation under representative conditions had been performed. In these simulations, the spatial distribution of 780-nm-wavelength near-field illumination produced by a lens having a 500-nm-diameter circular cross section exit hole was computed by use of commercial software that solves the applicable equations of electromagnetism by finite-difference time-domain analysis techniques. Then the spatial distribution of the near-field light was used in solving the Schrödinger equation for a Gaussian wave packet of a cold beam of rubidium atoms incident on the hole at an axial velocity of 1 m/s. From the numerical results of the simulation, the full width at half-maximum of the zeroth diffraction pattern of the deBroglie waves of the beam, which width is considered to approximate the width of the focal spot, was estimated to be 2 nm — comparable to the deBroglie wavelength.

Figure 2. A Prism and a Slit would be used to generate highly concentrated two-color illumination for two-step photoionization of atoms of interest: Near-field two-color illumination would be obtained through interaction between the slit and the total-internal-reflection evanescent fields of the two color beams.

In a proposed related development, slits having widths of tens of nanometers would be used to generate near-field illumination in a scheme for detecting small numbers of atoms with high sensitivity and high spatial resolution. In this scheme, two-color illumination needed for two-step photoionization of atoms of a species of interest would be made to undergo total internal reflection in a prism, and the interaction of the evanescent field with the slit would concentrate a portion of the two-color illumination into the near field in the vicinity of the slit (see Figure 2). Some atoms of the species of interest entering this vicinity would become ionized and would be counted by use of a channel electron multiplier or other suitable detector.

This work was done by Haruhiko Ito of Tokyo Institute of Technology for the Air Force Research Laboratory.

AFRL-0081