Alumina, sapphire and silicon optical elements are used for the detection of cosmic microwave background radiation with its peak intensity between 0.5 and 10 mm. The reflectivity of such material is high but can be reduced with sub-wavelength structures acting as an anti-reflective coating. Required structure height of over 1 mm can be produced by laser machining. Using ultra-short laser pulses we machined > 1 mm high pyramidal structures on sapphire and alumina. The laser process has been optimized to achieve high area structuring speeds for 1 mm high structures, up to 425 mm2/h. The structured surfaces have been analysed with secondary-electron microscopy, optical confocal microscopy, and transmission spectroscopy in the 100 GHz range and provide an antireflection coating for optical elements in the millimeter wave-length.
Bibliographical noteFunding Information:
The authors acknowledge use of resources provided by the Minnesota Nanofabrication Center (http://nfc.umn.edu), the University of Minnesota Imaging Center (http://uic.umn.edu), and the Minnesota Supercomputing Institute (http://www.msi.umn.edu). The authors would also like to thank to Prof. M. Hasegawa for us to use the millimeter wave source. This work was partially supported by JSPS KAKENHI Grant Number 70625003, the Mitsubishi foundation (grant number 24 in JFY2015 in science and technology) and the ISAS strategic development fund from the steering committee for space science.
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- Cosmic-microwave background
- Ultra short pulsed laser processing