Specs and Prices

• Converts linear polarization to radial or azimuthal
• Can be used to create an optical vortex
• High damage threshold
• Nearly 100% efficiency in polarization conversion for dedicated wavelengths
• 50-90% transmission (AR coatings applicable)
• Large aperture possible (up to 10 mm or bigger; standard is 6 mm)
• No glued components – more resistant to heat
• No “ineffective center” problem
• No segment stitching

Benefits for laser micro-machining

• Helps achieving smaller spot size
• Ensures the same machining properties in all directions*
• Complex trajectories are made featuring the same track width**
• Ensures the same cutting speed in all directions
• Increases cutting speed
• Processed surface interacts with p waves which provides maximum absorption of radiation.

 *When processing materials with linearly polarized light, features are bigger in width, when machining is performed in the direction perpendicular to polarization of the beam and vice versa.

**This is useful for example in fabrication of microfluidics, whereas later chemical etching retains the same characteristics through all the channel.

Benefits for use in Optical Tweezers

• Increases trapping force
• Might trap particles with lower refractive index comparing to surroundings

Description

Prof. Peter G. Kazansky group from Optoelectronics Research Centre at Southampton University developed a direct laser writing technique for production of radial polarization converters also called as Z polarizer, Z-plate or q-plate.

Primary applications for such radial polarizers are in laser machining, microscopy and optical tweezers as well as Raman spectroscopy systems. Unique features of this converter are that it is made in the volume of monolytic fused silica (UVFS) window, therefore resistant against high power laser radiation; AR coatings might be applied; and a single element is used both for polarization conversion to radial/azimuthal and generation of optical vortices.

Fig.1 Radial or azimuth polarization beam intensity distribution.

Fig.2 Radial beam intensity distribution after polarizer rotated by 0 deg and 90 deg.

Method of Use

Cylindrically symmetric polarization (radial or azimuthal) generation

Following step-by-step procedure must be done in order to generate radial or azimuthal polarization beams.

Simplified approach

a)      Place the polarization converter directly into linearly polarized laser beam.

b)      Align the center of the converter with the optical axis of the incident laser beam.

c)      Check the alignment with linear polarizer placed after converter. The dumbbell shape must be symmetric for all polarizer angles.

d)     Polarization state of the output beam can be controlled by rotating the converter or the incident polarization (by rotating λ/2 waveplate placed before converter). If the dumbbell shape is aligned along linear polarizer transmission axis, the output polarization is radial. If the dumbbell shape is perpendicular to the polarizer transmission axis, the output polarization is azimuthal.

Universal approach

e)      Mount a λ/2 waveplate into a kinematic holder

f)       Place the polarization converter into the path of linearly polarized beam

g)      Align the center of the converter with the optical axis of the incident laser beam

h)      Check the alignment with linear polarizer placed after converter. The dumbbell shape must be symmetric for all polarizer angles

i)        Polarization state (radial/azimuthal) of the output beam can be controlled by rotating the converter or the incident polarization (by rotating λ/2 waveplate).

If the dumbbell shape is aligned along linear polarizer transmission axis, the output polarization is radial. If the dumbbell shape is perpendicular to the polarizer transmission axis, the output polarization is azimuthal.

Optical Vortex Generation Using Radial Polarization Converter

Radial polarization converter can also be used to generate optical vortex beam. Following step-by-step procedure must be done in order to generate optical vortex beam using radial converter:

1. Place the polarization converter into circularly polarized laser beam.
2. Align the center of the converter with the optical axis of the incident laser beam.

Note: The sign of the optical vortex charge „+“, „-“ is controlled by the handedness of the incident circular polarization.

Testimonials

Southampton University applied for patent application and appointed exclusivity in commercialising activities for Altechna R&D Ltd. Custom development of machining heads and optical assemblies incorporating the radial polarizer is possible on request.

References

[1] Rudolf Weber, Andreas Michalowski, Marwan Abdou-Ahmed, Volkher Onuseit, Volker Rominger, Martin Kraus, Thomas Graf, Effects of Radial and Tangential Polarization in Laser Material Processing, Physics Procedia, Volume 12, Part A, (2011), Pages 21-30.

[2] Cyril Hnatovsky, Vladlen Shvedov, Wieslaw Krolikowski, and Andrei Rode, Revealing Local Field Structure of Focused Ultrashort Pulses, Phys. Rev. Lett. 106, 123901 (2011).

[3] Yao Bao-Li, Yan Shao-Hui, Ye Tong and Zhao Wei, Optical Trapping of Double-Ring Radially Polarized Beam with Improved Axial Trapping Efficiency, Chinese Phys. Lett. 27 108701, (2010).

[4] Hong Kang, Baohua Jia, Jingliang Li, Dru Morrish, and Min Gu, Enhanced photothermal therapy assisted with gold nanorods using a radially polarized beam, Appl. Phys. Lett. 96, 063702 (2010).

[5] Gilad M. Lerman and Uriel Levy, Radial polarization interferometer, Opt. Express 17, 23234-23246 (2009) [6] Fake Lu, Wei Zheng, and Zhiwei Huang, Coherent anti-Stokes Raman scattering microscopy using tightly focused radially polarized light, Opt. Lett. 34, 1870-1872 (2009).

[7] Weibin Chen, Don C. Abeysinghe, Robert L. Nelson§ and Qiwen Zhan, Plasmonic Lens Made of Multiple Concentric Metallic Rings under Radially Polarized Illumination, Nano Lett., 2009, 9 (12), pp 4320–4325.

[8] Gilad M. Lerman and Uriel Levy, Effect of radial polarization and apodization on spot size under tight focusing conditions, Opt. Express 16, 4567-4581 (2008).

[9] D. W. Diehl, R. W. Schoonover, and T. D. Visser, The structure of focused, radially polarized fields, Opt. Express 14, 3030-3038 (2006).

[10] Tasso R. M. Sales, Smallest Focal Spot, Phys. Rev. Lett. 81, 3844–3847 (1998).

[11] A. V. Nesterov, V. G. Niz’ev and A. L. Sokolov , Transformation problem for radiation with radial polarization, Volume 90, Number 6 (2001).