RadiationResistant Transmissive Optics

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Radiation-Resistant Transmissive Optics

• S.J. Zinkle
• HAPL IFE Program Workshop
• San Diego, April 4-5, 2002

NRL IFE 2/2001
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Methodology for selecting candidate
radiation-resistant transmissive optics

• Initial list of 100 optical materials was
  screened to select materials with high
  transparency between 200 and 500 nm
• Numerous optical materials rejected due to too
  low of band gap energy (e.g., carbides and most
  nitrides)
• Requirement of Eggt4 to 6 eV (UV cutoff ?lt200-300
  nm) eliminates many promising candidates,
  including SiC, ZnO, TiO2, LiNbO3 and SrO (DPSSL
  and KRF) and MgO, ZrO2, Y2O3 and zircon (for
  KrF)
• Radiation effects literature reviewed for
  remaining candidates to select most promising
  candidates

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Original List of Candidate Optical Materials
(transparent at 200-500 nm)
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Candidate Radiation-resistant Optical Materials
(no radiation-induced absorption peaks near 248
or 351 nm)
Alkali halides (NaBr, KCl, etc.) are less
promising due sensitivity to radiolysis
(displacement damage from ionizing radiation)
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Anti-reflective protective coatings

• Transparent anti-reflective coatings can be used
  to protect the surface of IFE mirrors.
• Mechanical damage to the anti-reflective coating
  from debris would not effect the reflective
  properties of the underlying mirror surface.
• Roughening of the anti-reflective coating is not
  necessarily detrimental to its operation.
• Radiation induced change to absorption in the
  coating would still be an issue, but the coating
  would be much thinner than a transmissive optic.

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Subwavelength Mirrors

• Subwavelength mirrors use periodic features of
  order l/3 to l/2 to form a surface waveguide
  which reflects light in a narrow waveband with
  very high reflectivity (as high as 99.9).
• Higher reflectivity allows the use of smaller
  mirrors.
• Current research is for near-IR wavelengths.
  Near-UV wavelengths would simply require smaller
  feature size.
• Anti-reflectivity coatings can be used to protect
  the mirror surface.
• This technology is only in the development stage.