Optical beams with radial or azimuthal polarization are of interest for applications such as particle acceleration and imaging beyond the diffraction limit, but generating such beams is not a trivial matter. Martynas Beresna and co-workers from Southampton University and the Lithuanian firm Altechna have now fabricated a monolithic glass polarization converter that promises to greatly simplify this task (Appl. Phys. Lett. in the press; 2011). The team's optical vortex converter is capable of transforming the polarization of an input beam from circular to either radial or azimuthal, depending on whether the input beam is left- or right-handed.

Credit: University of Southampton

Previous studies have demonstrated that illuminating a silica glass substrate with femtosecond pulses can generate self-assembling nanostructures that exhibit birefringence. The researchers calculated that a silica substrate covered with a spiral pattern of such nanostructures could be used to convert circularly polarized light into either radially or azimuthally polarized light.

They tested their prediction by focusing a train of intense pulses from an amplified femtosecond Yb:KGW laser system (pulse length of 270 fs, wavelength of 1,030 nm and optimal repetition rate of 200 kHz) onto a 2-mm-thick fused silica sample mounted on a computer-controlled translation stage. They moved the sample in 1 µm steps during exposure to produce the required spiral pattern. The resulting design of nanostructures provided a retardance as large as 260 nm, which is sufficient for performing the required polarization conversion at visible and near-infrared wavelengths. The researchers say it takes around 1.5 hours to produce a 1.2-mm-diameter converter. They characterized the converters using a quantitative birefringence measurement system; the images above show the spiral pattern of the glass nanostructures (left) and the resulting spatial variation in retardance (right).