LASER DAMAGE - A BALANCED VIEW Difficult to take the long considered and balanced view when your work stops dead with that flash of light or “ping” Rona Belford

We would like to thank Sean Kudesia for his continued support and useful insights in our LIDT research work and to SELEX ES for its support Thanks also to Scottish Enterprise for funding our most recent research project

To cause damage we need to break bonds DEFINITION OF LASER DAMAGE Any permanent laser-radiation-induced change in characteristics of either the Substrate or the Coating To cause damage we need to break bonds Permanent Change means alteration in the morphology has occurred and that means a massive change at the molecular level which has its origin in the breaking of bonds What bonds? and how much energy will it take?

Band Diagram for a Crystalline Semiconductor/ Insulator AO MO MO* Increasing Electron Energy Increasing Electron Energy Conduction Band Edge Valence Band Edge N atoms

SiO2 Si 8.9 eV 1.1 eV Increasing Electron Energy Insulator Semiconductor Conduction Band Edge 8.9 eV Increasing Electron Energy 1.1 eV Valence Band Edge

Si

We are dealing with Amorphous materials (Glasses) are formed by cooling from the liquid phase melt or by sputtering. The thermal history determines the material’s structure Structure determines electronic states; thermal history “freezes in” these states

Typical Distribution of Electronic States in an Amorphous SC/insulator Localized States Tail States Defect States Typical Distribution of Electronic States in a Crystalline SC/insulator Crystalline SiO2 Typical Distribution of Electronic States in an Amorphous SC/insulator Amorphous SiO2 Structure determines electronic states Annealing differences Optical glasses have much reduced defect states

Origin of these Additional Energy States The Structure (Amorphous =lack of Periodicity) Dictates the Electronic Structure “Freezing-in” non-equilibrium features such as bond angle or bond length give states of higher energy bonding electrons and lower energy anti-bonding states

Typical Distribution of Electronic States in an Amorphous SC/insulator Localized States Tail States Defect States Typical Distribution of Electronic States in a Crystalline SC/insulator Crystalline SiO2 Typical Distribution of Electronic States in an Amorphous SC/insulator Amorphous SiO2 Structure determines electronic states Annealing differences Optical glasses have much reduced defect states

Free H2O (highly Hydrogen bonded) Modes of Absorbing IR Through Vibration Symmetric Stretch Asymmetric Stretch Deformation (bend)

≡ Si- O H + H O-Si ≡ Elevated Temp ≡ Si-O-Si ≡ +H20

BAND STRUCTURE FOR LITHIUM NIOBATE

http://phys.strath.ac.uk/12-370/sld006.htm Allister Ferguson

http://phys.strath.ac.uk/12-370/sld006.htm Allister Ferguson

http://phys.strath.ac.uk/12-370/sld006.htm Allister Ferguson

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