Selection of Materials for UV Optics

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Selection of Materials for UV Optics

• James Johnson
• OPTI 521
• December 8, 2008

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Overview

• Introduction
• Considerations for UV Optics
• Criteria for Selecting a Material
• Material Properties

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UV Optics

• UV Range 10-300nm
• Applications including lithography, space
  science, scanners, medical

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Why They are Different

• Most Materials Strongly Absorb below 300nm
• Atomic Transitions in this range are common
• Ideal Materials do Have a Resonance Deep into the
  UV

Ee
h?
Eg
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Problems with UV Optics

• Material Absorption Follows Beer's Law
• II0e-aL
• Damage from high intensities
• Solarization of Material after Long Exposure

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Scattering

• Larger Scattering needs smoother surface
• Rayleigh ?-4
• ?/10 Surface Error not easily achieved
• Must be polished

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Criteria

• What to consider
• Cut-Off Wavelength
• Cost
• Transmission at Wavelength
• Durability

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UV Transmissive Mediums

• Fused Silica
• Fluorides
• Calcium Fluoride
• Magnesium Fluoride
• Sapphire

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Fused Silica (SiO2)?

• Most common UV material
• Relatively Inexpensive
• Most grades will not work below 190nm
• Strong Sources will cause Compaction

From Ultraviolet Radiaion(Koller)?
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Calcium Fluoride(CaF2)?

• Best Alternative when a Farther Wavelength is
  Needed
• Expensive
• Grown Crystal
• Difficult to Polish
• Hygroscopic

From Melles-Griot
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Magnesium Fluoride(MgF2)?

• Similar Range to CaF2
• Less Transmissive
• Doesn't Absorb Water
• Makes good AR-Coating

From EKSMA
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Sapphire (Al2O3)?

• Extremely Hard Making for Good Windows
• Stable
• Large bandwidth
• Chemical Resistant
• Costly
• Extremely Difficult to Remove Material

From Melles-Griot
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Summary

• It comes down to where you need it and what you
  can spend
• Stick with Fused Silica if it works
• Fluorides offer the widest range, but can cost
  significantly more
• Sapphire offers dimensional stability and
  material strength

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Questions?
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Overage - Reflectors

• Most materials that reflect in the visible work
  in the UV
• Exception is Silver
• Metals like Aluminum, SiC are excellent choices