Polarizer Converts Arbitrary Beam to Radial or Azimuthal Polarization

Breck Hitz

Radially or azimuthally polarized light can be useful in high-resolution photolithography, in coupling into hollow-core fibers and in other applications. Scientists in several laboratories have designed intracavity devices to generate radially or azimuthally polarized monochromatic laser beams. Recently, scientists at Technion — Israel Institute of Technology demonstrated an extracavity polarizer that can convert 50 percent of the incident light at any wavelength to either radial or azimuthal polarization.

Figure 1. A cylindrical sheet polarizer, wrapped around the convex reflector, polarized the emerging beam either radially or azimuthally. Images reprinted with permission of Optics Letters.

The simple device comprises a pair of conical mirrors, one convex and one concave, and a rolled-up sheet polarizer forming a cylinder between them (Figure 1). Incoming light reflects off the convex mirror and passes through the cylindrical polarizer. If the preferred polarization of the sheet is along the device’s optic axis, the transmitted light will be radially polarized after reflecting off the convex conical mirror. On the other hand, azimuthal polarization will result if the sheet’s preferred polarization is perpendicular to the device axis. The in-between case — when the sheet’s axis is aligned arbitrarily — will result in spiral polarization.

Figure 2. The annular, radially polarized beam emerging from the polarizer had uniform intensity (a). But when viewed through a vertical polarizer, its radial polarization was evident (b).

To roll the plastic polarizing sheet into a cylindrical tube, the scientists heated it to 100 °C and carefully formed it around the convex mirror, which they purchased. Because they were unable to find a vendor for the outer, concave mirror, they fabricated it themselves by diamond turning it from a block of aluminum and hand-polishing it. The end result “could no doubt be improved by professional production,” they note. Nonetheless, the polarizer performed nicely, producing an annular, radially polarized beam (Figure 2).

Optics Letters, Dec. 1, 2006, pp. 3405-3407.