The Radial Polarization converter from Polaroptic (or Arcoptix) is a worldwide unique device that converts a conventional linearly polarized beam into a beam that has a smooth and continuous radial or azimuthal polarization distribution. As illustrated in the figure on the right the orientation of the polarization vector vary spatially but locally the polarization sate is considered as linear.
Thanks to special twisted alignment of the liquid crystal molecules inside the system (see principle below), the polarization converter rotates locally the orientation of the incoming linearly polarized beam. Depending of the settings of the device we may obtain either azimuthally or a radially polarization distribution as described in the figure.
Depending of the chosen configuration the system also includes other liquid crystal cells like a polarization rotator (TN cell) or also called TN cell and a phase compensator. These both cell need to be actively driven.
Radial Polarization converter specifications:
| wavelength range | 350-1700 nm |
| active area | 10 mm diameter |
| transmission | better than 75% (in the VIS) |
| retarder material | Nematic Liquid-Crystal |
| Substrates material | Glass bk7 |
| Local extinction ratio (input Intensity/ouput intensity) when placed between crossed polarizers | ~100 @ 633nm |
| Output intensity homogenity | < 1/100 RMS variation |
| temperature range | 15°-35° |
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Save operating limit |
500 W/cm2 CW |
| Total size of the housing | 6 cm x 4 cm x 2.0 cm (without alignment screws) |
Principle
The theta cell, which is the heart of the radial polarization converter, is a kind of twisted nematic liquid crystal cell composed of one uniform and one circularly rubbed alignment layer. The local alignment of the LC is variable and consequently the twist angle of the LC molecules is also locally variable (see figure). The linear polarization entering the cell will follow adiabatically the the twsit of the LC molecules and comes out with a radial or azimuthal distribution. Notice that due to the two possible twist directions, a thin disinclination (singularity) line appears in the LC cell (line in the figure) which is unnoticeable when using white light but has its importance when using coherent light (creates a diffraction pattern). The iundesired effect of this discinlination line can be compensated in the system with the phase compensator (optional with the system).
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 theta-cell
Driver (optional)
The Polarization converter can be driven with the Arcoptix LC (Liquid Crystal) driver which is a USB computer controlled electrical power supply optimized for driving the polarization converter. The Phase compensator (phase step compensation) and the TN cell (switch between azimuthal and radial polarization) inside the polarization converter can be activelly driven with the two outputs of the LC Driver.
Also the user can computer control the labview or any other custom program the driver.
The LC driver has two independent outputs (Lemo connectors). They are controlled via a simple windows compatible software. The output has a variable square amplitude with polarity inversion and a frequency of 1.6 KHz. This guarantees a homogenous variation of the LC layer inside the cell. An external trigger input can be provided on demand.
Downloads:
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DOWNLOAD Detailed description of the polarization converter |
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DOWNLOAD User Manual of the polarization converter |
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DOWNLOAD Application notes the polarization converter |
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DOWNLOAD Drawing of the polarization converter |
Applications
Dougnut focal point (or reduced 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 Doughnut 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 fast compared 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 independence. 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 the atmosphere is partially polarized and therefore be visualized with a PAF. Combined with a compass a sun dial could be built which indicates the local time.
References:
We have already sold our radial polarization converter to many research groups around the world. Here are some references of articles where the LC radial polarization converter has been used:
1) 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).
2) M. Stalder and M. Schadt, "Linearly polarized light with axial symmetry generated by liquid-crystal polarization converters," Opt. Lett. 21, p. 1948- (1996)
3) J. S. Ahn, H. W. Kihm, J. E. Kihm, D. S. Kim, and K. G. Lee, "3-dimensional local field polarization vector mapping of a focused radially polarized beam using gold nanoparticle functionalized tips", Optics Express, Vol. 17, 14, P.2280 (2009)
4) 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).
5) D. Ivanov, V. Shcheslavskiy, I. Märki, M. Leutenegger and T. Lasser, "High volume confinement in two-photon total-internal-reflection fluorescence correlation spectroscopy", appl. phys. lett. 94, 083902-1 (2009).
6) R. Martinez-Herrero, P.M. Mejias *, G. Piquero, V. Ramirez-Sanchez, "Global parameters for characterizing the radial and azimuthal polarization content of totally polarized beams", Optics Communications 281, p. 1976–1980, (2008).
7) H. Tomizawa, H. Hanaki, T. Ishikawa,"NON-DESTRUCTIVE SINGLE-SHOT 3-D ELECTRON BUNCH MONITOR WITH FEMTOSECOND-TIMING ALL-OPTICAL SYSTEM FOR PUMP & PROBE EXPERIMENTS", Proceedings of FEL 2007, Novosibirsk, Russia.
8) F. Snik, T. Karalidi, and C. U. Keller "Spectral modulation for full linear polarimetry", Applied Optics, Vol. 48, Issue 7, pp. 1337-1346 (2009).
9) V. Ramirez-Sanchez, G Piquero and M Santarsiero "Generation and characterization of spirally polarized fields", J. Opt. A: Pure Appl. Opt. 11 (2009) 085708 (6pp).
10) Hong Kang, Baohua Jia, Jingliang Li, Dru Morrish, and Min Gu " Enhanced photothermal therapy assisted with gold nanorods using a radially polarized beam", APPLIED PHYSICS LETTERS 96, 063702 (2010).
11) Eyal Shafran, Benjamin D. Mangum, and Jordan M. Gerton, "Energy Transfer from an Individual Quantum Dot to a Carbon Nanotube", Nano Letters 2010 10 (10), 4049-405
12) I. Rajapaksa, K. Uenal, and H. Kumar Wickramasinghe " Image force microscopy of molecular resonance: A microscope principle", APPLIED PHYSICS LETTERS 97, 073121 (2010).
13) Zachary D. Schultz, Stephan J. Stranick and Ira W. Levin, "Advantages and Artifacts of Higher Order Modes in Nanoparticle-Enhanced Backscattering Raman Imaging", Anal. Chem., 2009, 81 (23), pp 9657–9663
14) Gilad M. Lerman, Liron Stern, and Uriel Levy "Generation and tight focusing of hybridly polarized vector beams", OPTICS EXPRESS Vol. 18, No. 26 (2010).
15) Ignacio Moreno, Jeffrey A. Davis, Isaac Ruiz, and Don M. Cottrell " Decomposition of radially and azimuthally polarized beams using a circular-polarization and vortex-sensing diffraction grating",Optics Express, Vol. 18, Issue 7, pp. 7173-7183 (2010).
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17) , , , Vol: 35, Issue: 17, PP: 2873-2875
20) Han Lin, Baohua Jia, and Min Gu, "Generation of an axially super-resolved quasi-spherical focal spot using an amplitude-modulated radially polarized beam", Optics Letters, Vol. 36, Issue 13, pp. 2471-2473 (2011).
and many more...