Three researchers in Germany claim to have focused light down to the smallest spot size ever. Using a radially polarized beam from a helium-neon laser, the trio from the Univeristy of Erlangen-Nürnberg, produced a spot with an area of just 0.06 square microns. This is almost half the size of the previous record (R Dorn et al. 2003 Phys. Rev. Lett. 91 233901)
Many optical techniques, such as lithography, confocal microscopy and optical data storage, make use of sharply-focused light beams. As a tightly focused beam produces an intense electromagnetic field, this approach could also to probe or manipulate atoms.
The key to producing the record-breaking spot is the use of a radially polarized beam. To generate this, the researchers collimated a linearly-polarized, single-mode helium-neon beam and sent it through a pinhole followed by a polarization converter containing four half-wave plates. The resulting beam had a doughnut-shaped intensity pattern – a “hole” with zero intensity at the centre and the most intense light round the edges.
The team used an annular aperture to focus the beam. This caused the doughnut-hole to shrink and the majority of the electric field to cancel itself out, leaving an intense spot with an electric field pointing along the direction of the beam.
According to the authors, the minimum spot size for a radially polarized beam focused by an annular aperture is 0.16 square angstroms. This is considerably smaller than the theoretical spot size for a linearly polarized beam, which is 0.26 square angstroms and 0.22 square angstroms for circularly polarized light.