27 Nov 2003
Researchers use radially-polarized light to create what they say is the smallest spot size ever seen.
Three researchers in Germany claim to have focused light down to the smallest spot size ever seen. Using a radially polarized beam from a helium-neon (HeNe) laser, the trio produced a spot with an area of just 0.06 square microns, almost half the size of the previous record. (Physical Review Letters to appear)
This result could benefit a lot of applications. 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 recording breaking spot is the use of a radially polarized beam. To generate this, the researchers, from the Univeristy of Erlangen-Nürnberg, collimated a linearly-polarized, singlemode HeNe beam and sent it through a pinhole followed by a polarization converter containing four half-wave plates. The resulting beam had a donut-shaped intensity pattern – a “hole” with zero intensity at the center and the most intense light round the edges.
The team used an annular aperture to focus the beam. This caused the donut-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 0.16λ2. This is considerably smaller than the theoretical spot size for a linearly polarized beam is 0.26λ2 and 0.22λ2 for circularly polarized light.