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Millimeter-wave achromatic half-wave plate.
Shaul Hanany1, Johannes Hubmayr, Bradley R Johnson
1School of Physics and Astronomy, University of Minnesota, 116 Church Street, SE, Minneapolis, Minnesota 55455, USA. hanany@physics.umn.edu
Applied Optics
|August 4, 2005
Summary
We developed an achromatic half-wave plate (AHWP) for millimeter wavelengths. This sapphire plate stack achieves 96% modulation efficiency, significantly improving upon single plates for astrophysical polarimetry.
Area of Science:
- Optics and Photonics
- Astrophysical Instrumentation
- Materials Science
Background:
- Millimeter-wave astronomy requires precise polarization measurements.
- Existing wave plates often suffer from chromatic aberrations, limiting broadband applications.
- Sapphire is a suitable birefringent material for millimeter wavelengths.
Purpose of the Study:
- To design and construct an achromatic half-wave plate (AHWP) for the millimeter wavelength band.
- To evaluate the performance of the AHWP in terms of modulation efficiency.
- To assess the AHWP's suitability for astrophysical broadband polarimetry.
Main Methods:
- Constructed an AHWP using a stack of three a-cut sapphire plates.
- Rotated the optical axis of the middle plate by 50.5 degrees relative to the outer plates.
- Measured modulation efficiency at 110 GHz and analyzed its dependence on incidence angle.
Main Results:
- Achieved a measured modulation efficiency of 96 ± 1.5% at 110 GHz.
- Demonstrated significantly higher efficiency compared to a single sapphire plate (43 ± 4%).
- Observed constant modulation efficiency for incidence angles from 0 to 15 degrees.
Conclusions:
- The constructed sapphire AHWP effectively corrects for chromatic aberrations in the millimeter band.
- The high and stable modulation efficiency makes it ideal for broadband astrophysical polarimetry.
- The design principles discussed are crucial for future millimeter-wave polarimetric instrument development.