October 30th, 2007High Average Power Laser Program Workshop 1 Long lifetime optical coatings for 248 nm: development and testing Presented by: Tom Lehecka Penn State Electro-Optics Center Presented at: High Average Power Laser Program Workshop Naval Research Laboratory October 30-31, 2007

October 30th, 2007High Average Power Laser Program Workshop 2 Outline Problem Definition Optical specifications Optical design Test setup Results to date Near term goals

October 30th, 2007High Average Power Laser Program Workshop 3 Problem Definition High reflectivity (>99.5%) is required on optics after the final amplifier – direct impact on system efficiency High damage threshold (~10 J/cm 2 goal, ~1 J/cm 2 operation) optics required after final amplifier – direct impact on system cost and reliability Optical coatings and materials are known to “ darken” under continued UV exposure at 248 nm – leads to reduced reflectivity and perhaps lower damage threshold This problem is separate from final optic at target chamber Optics lifetime was considered a serious concern by the 2006 DOE review panel

October 30th, 2007High Average Power Laser Program Workshop 4 Optical Specifications 1.Reflectivity: Greater than 99.5% at 248 nm 2.Laser induced damage threshold: 10 J/cm 2 for 2 ns pulses 3.Operational fluence: 1 J/cm 2 for 2 ns pulses 4.Lifetime: Specified reflectivity and damage threshold maintained for one billion pulses at 1 J/cm 2

October 30th, 2007High Average Power Laser Program Workshop 5 Test facility Lens position, along with laser energy, determines fluence on optic DVR allows long term monitoring of surface condition – to date this has been sufficient for damage determination. Review of DVR allows accurate determination of onset of damage. Scattering detector can detect small changes in surface properties GAM Laser 200 cm f.l. lens Output to digital video recorder CCD Camera Photodiode Scattering monitor Diode laser (optional) Optic under test Power meter KrF – 248 nm ~100 mJ/pulse 18 ns pulse length 125 Hz max rep rate

October 30th, 2007High Average Power Laser Program Workshop 6 Laser Characterization Raw laser output provides a relatively flat top region for laser damage measurement Energy in the bucket measurement provided absolute fluence Power meter Adjustable aperture at equivalent test piece location Laser beam input CCD camera image of the beam 50 cm from focus Camera located at location of optic under test 3.4 X 1.9 mm beam dimension Provides 1.5 J/cm 2 with 100 mJ input Energy in the bucket measurement Varying aperture size determines fluence Lens

October 30th, 2007High Average Power Laser Program Workshop 7 Laser Fluence Fluence ranging from 10 J/cm2 can be readily accessed with spot size > 1 mm Normalized fluence ranges from to J/cm 2 /mJ Due to beam nonuniformities near focus, only the range from to will be used for testing

October 30th, 2007High Average Power Laser Program Workshop 8 Optical Anatomy Substrate Dielectric coating layers (not to scale) Nominal layer thickness = /4 Multi-layers of dielectric material with alternating indices of refraction are deposited on the substrate Fresnel reflections at each interface result in desired reflectivity Large variations between high and low index materials, or large numbers of layers required for high reflectivity Materials selection for deep UV are limited Laser light in Reflection out

October 30th, 2007High Average Power Laser Program Workshop 9 Optical Coating Issues 1.Materials selection for deep UV are limited a.Oxides – SiO 2, Al 2 O 3, Sc 2 O 3, HfO 2 b.Fluorides – MgF 2, AlF 3, CaF 2, Na 3 AlF 6 (cryolite), LaF 3, YF 3, GdF 3, NdF 3 2.Oxides, particularly SiO 2, are known from photolithography applications to darken after continued 248 nm exposure 3.Fluorides do not provide a large index variation resulting in many layers required for high reflectivity. a.Many layers often result in crazing due to thermal stresses b.Many layers can also increase scattering due to interaction with layers For HAPL development we have initially designed using fluorides. YF 3, LaF 3 and cryolite have been tried at Spectrum Thin Films. LaF 3, NdF 3, GdF 3 and cryolite resulted in high damage threshold coatings (~27 J/cm 2 ) for Japanese researchers in the 1990’s. Relatively low reflectivity ~97% with crazing observed for ~ 40 layers. (Izawa et. al, SPIE, 1990) COTS coatings are also being evaluated. These are typically oxide based.

October 30th, 2007High Average Power Laser Program Workshop 10 Results to Date (1) ManufacturerDesignDamage Threshold LifetimeNotes ThorlabsCOTS, unknown0.9 J/cm 2 NAHigh quality optical surface, no pinholes Alpine Research Optics COTS, details unknown, oxide based 1.5 J/cm 2 NAModerate quality, some pinholes. Vendor recommended by GAM – survive 1e9 shots at low fluence. Spectrum Thin Films Custom YF 3 /Na 3 AlF 6 (cryolite) 3.5 J/cm 2 NAPoor visible quality, many pinholes. 48 layers led to stress crazing Spectrum Thin Films Custom LaF 3 /Na 3 AlF 6 Not yet tested NA37 layers, lower reflectivity test. Some crazing and pinholes. Due to lower than desired damage threshold, no lifetime testing has been started. Desire minimum of 5 J/cm 2

October 30th, 2007High Average Power Laser Program Workshop 11 Results to Date (2) One inch optic with multiple damage sites Microscope image 1.5 J/cm 2 fluence damage site, single shot Microscope image 1.5 J/cm2 fluence damage site, 100 pulses Initial damage is small pits which quickly grows to large damage zone CCD camera easily detected the single shot damage Damage result from Alpine Research Optics mirror

October 30th, 2007High Average Power Laser Program Workshop 12 Near Term Goals 1.Try a more standard coating, likely Al 2 O 3 /AlF 3 to obtain high damage threshold from Spectrum Thin Films, then lifetime test 2.Continue to work with Alpine Research Optics to develop their capability (they are doing this work gratis, so far)

October 30th, 2007High Average Power Laser Program Workshop 13 Summary Lifetime of high fluence high reflectivity optics for 248 nm is likely to be a challenge for a laser fusion system We have developed a simple test facility to assess damage threshold and lifetime of sample optics Tests to date have not yielded optics near the desired 10 J/cm2 laser induced damage threshold Alternatives are endless, funding is not