Optical coatings for lightweight, highpower mirrors

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Optical coatings for lightweight, high-power
mirrors
Joseph Talghader Department of Electrical and
Computer Engineering University of Minnesota
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Project Scope

• Development of Optical Coating Materials
• Negative thermal expansion thin films
• Mechanical properties of oxides
• Modeling of High-Power Coatings
• Thermal and strain sensing in optical membranes
• Optomechanical and thermal modeling
• High-Power Adaptive Mirrors
• Study and characterization of heat transfer
• Development of improved structures and actuators

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Negative Thermal Expansion (NTE)

• Few materials show NTE over wide temperature
  range including room temperature. We are the
  first to deposit NTE thin films by design.
• ZrW2O8 is well known because of large NTE
  (approx. -10 ppm/K) over wide temperature range
  (0.3K to 1050K)
• Transverse motion of O in M-O-M bond
• Strong M-O bond
• Increased temperature causes metal-metal distance
  to decrease
• Requires high oxygen mobility in lattice

Room Temperature
Elevated Temperature
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Thermal Expansion Results

• Negative expansion for evaporated films
• Sputtered film showed positive expansion

X-Ray Diffraction

• Films deficient in Zr, and all films amorphous
• Evaporated films have a lower, near-crystalline
  density

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NTE Vibrational Spectrum
Raman Scattering Analysis

• Dramatic differences between thin film and bulk
• Current work is to identify vibrational
  differences between NTE and positive films
• Using Synchrotron X-ray analysis to characterize
  local film structure

PTE
NTE
transverse vibration?
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Thermal and Strain Maps
Contact
Pads
Optical Coating
Membrane
Strain Gauge
Temperature
Sensor

• Able to extract stresses in
• optical coatings in real time
• during HEL excitation

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Major Accomplishments I

• Negative thermal expansion (NTE) thin films
  based on ZrxWyOz
• Large NTE -10 x 10-6/oC
• Thermal expansion stable (after annealing) to
  140oC
• Film density appears to dictate behavior
• Raman spectrum shows vibrational differences
  between thin film and bulk
• Thermally invariant deposition of NTE films on
  4 U of A
• mirror substrates
• Measurements of coefficients of thermal
  expansion,
• elastic modulus, hysteresis, and hardness on
  optical thin films
• Fabrication of membranes for optical coating
  models
• Integrated strain and temperature sensors on
  coating membrane
• Successfully measured stresses in coating during
  HEL excitation

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Major Accomplishments II

• Numerical, predictive mechanical model of
  multilayer coatings
• on thin membranes
• Initial investigations of heat transfer in
  micromachined
• adaptive mirrors
• Design and initial processing of MEMS adaptive
  mirrors with
• enhanced heat transfer and very long throw

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Recent Air Force Interactions

• Developing heat transfer testing of adaptive
  mirrors with Mark Gruneisen (AFRL)
• Testing optomechanical coating models on membrane
  mirrors from Dan Marker (AFRL)
• Informational visit by Charles Lee (AFOSR), Mark
  Gruneisen (AFRL), and Dan Marker (AFRL)
• Informational visit by Maj. Mike Dolezal (JTO)
• Invited by David Reicher (S. Systems - AFRL
  contractor) to speak at ICMCTF