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DOWNLOAD detailed description of the radial polarization converter

The Radial Polarization converter from Arcoptix is a worldwide unique device that converts a conventional linearly polarized beam into a beam that has a radial or azimuthal polarization distribution. As illustrated in the figure below the orientation of the polarization vector vary spatially but locally the polarization sate is considered as linear.

Principle of the Arcoptix polarization converter

Thanks to special alignment of the liquid crystal molecules, the polarization converter rotates locally the orientation of the linearly polarized beam. Depending of the orientation of the device in respect to the entrance polarization we may obtain either azimuthally or a radially polarization distribution as described in the figure above.

The polarization rotator can be delivered with adequate polarizers and housing (figure below). The specially designed housing permits to align our polarization converter precisely with the rest of your optical system. Also an additional electrical tunable phase shifter can be inserted in this housing.

Size aperture 10mm, housing 6 cm x 4 cm x1.5 cm Polarization converter between crossed polarizers.

For further information contact a ARCoptix engineer: info@arcoptix.com

Radial polarization converter Specs

wavelength range 400-1800 nm active area 10 mm diameter transmission better than 80% (in the VIS) retarder material Nematic Liquid-Crystal Substrates Glass non linear wavefront distortion lamda/2 temperature range 10°-35° Total size of the housing 6 cm x 4 cm x 1.5 cm

Principle

The radial polarization converter is a nematic liquid crystal cell composed of one uniform and one circularly rubbed alignment layer. The local alignment of the LC in the polarization converter is that of a twisted cell, with a twist angle given by the local alignment layers These twist angles are always smaller than pi/2. A thin disinclination line appears in the LC cell (line in the figure below) but is unnoticeable for most types of experiments. As shown in figure above, when linearly polarized light is shining through a polarization converter and the polarization direction is parallel or perpendicular to the uniform alignment layer, azimuthally or radially polarized light emerges on the other side. So by simple rotation of the entrance polarization the polaroptic polarization converter can switch from radial to azimuthal polarisation distribution. A more detailed description  can be found in “Stalder et.al., Optics Letters, volume 21, page 1948, published in 1996”.

Front view LC molecule twist inside the polarization converter

Applications

Dougnut focal point (or reduiced size focal spot)

For some applications such as confocal microscopy for example one is interested to produce a doughnut shaped focal point at the front focal plane of a high NA objective. Rigorous electromagnetic calculations shows that doughnut shaped focal points can be obtained by focusing beams having a radial polarization distribution. This may lead to interesting applications in the field of fluorescence microscopy.

Polarization axis finder (PAF)

When a polarization converter is used in combination with a polarizer, a device results that can be used as polarization axis finder (PAF). Watching the PAF a dark segment appears when the entrance polarization is linear. The orientation of the dark segment gives the direction of the polarization.

Inspection of birefringent materials

When placing a brefrigent material between two PAFS (two polarizers with two polarization converter), one can analyze the birefringent properties of the sample in one glimpse (characteristic interference colors and main axis). Neither the sample nor the polarizers have to be rotated.

Optical trapping

A Doghnut shaped focal point created by focusing a radial polarized beam may increase the traping force. Also it may enable trapping particles with lower refractive index than its surrounding fluid.

Laser cutting

The polarization direction of a laser beam when cutting materials is an important parameter. The cutting speed using p-polarized light is more then twice as fastcompared to using s-polarized light.Most cutting machines are therefore releasing circular polarized light which results in an average cutting speed and in cutting direction independnce. Radially polarized light may eventually increase cutting speed compared to circular polarized light...In principle the polarization converter can withstand high intensities (500W/cm^2).

Inspection of the polarization of the sky

The blue sky light due to scattering of sun light in theatmosphere is partially polarized and therfore be visualized with a PAF. Combined with a compass a sun dial could be built which indicates the local time.

References

We have alread sold our radial polarization converter all over the world.  Some groups have already obtained interesting results, here are some references of articles where the LC radial polarization converter has been used:

1) M. Stalder and M. Schadt, "Linearly polarized light with axial symmetry generated by liquid-crystal polarization
   converters," Opt. Lett. 21, p. 1948- (1996)

2) S.F Periera and al. “Frequency spectra and waveguiding of a family of daisy modes in vertical-cavity   
   surface-emitting lasers”, opt. comm. 179, p.485, (2000).

3) E. Descrovi and al., “Collection of transverse and longitudinal fields by means of apertureless nanoprobes with
   different metalic coating characteristics”, appl. phys. lett. 85 (22), p. 5340, (2004).

Articles may be obtained upon request.