Investigation of neutral atom and ion emission during laser conditioning of multilayer HfO2-SiO2 coatings
Abstract
It has been recently shown that the optical damage thresholds of multilayer coatings can be increased by up to a factor of two by conditioning the coating at laser fluences below the optical damage threshold. The laser conditioning mechanism of HfO2-SiO2 coatings was studied by observing the laser induced emission of neutral constituents of the coatings during laser conditioning. They observe the ejection of Hf, Si and oxygen (O or O2, or both) neutrals starting at fluences of about 10 J/sq cm, considerably below our measured unconditioned damaged threshold of about 16 to 18 J/sq cm. A threshold pulse fluence was observed for laser conditioning, which is also about 10 J/sq cm. This identical threshold for neutral emission and laser conditioning suggests that the neutral emission is caused by the laser conditioning process. The emitted neutrals have extremely high kinetic energies, on the order of several tens of eV. On the basis of this information, they propose that the laser conditioning process is caused by microscopic damage that involves cracking of the coating layers, resulting in the ejection of the high energy particles via a fractoemission process. This microscopic damage apparently raises the damage threshold by either reducing the absorption of coating defects or by relieving the thermal stresses that cause cracking, particle emission, and possible plasma formation by laser ionization of the neutrals.
- Publication:
-
Presented at the 22nd Boulder Damage Symposium
- Pub Date:
- 1990
- Bibcode:
- 1990boda.symp...25S
- Keywords:
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- Coatings;
- Fluence;
- Hafnium Oxides;
- Ion Emission;
- Laser Damage;
- Neutral Atoms;
- Radiation Effects;
- Silicon Oxides;
- Atomic Structure;
- Cracks;
- Damage;
- Defects;
- Electric Fields;
- Ionization;
- Irradiation;
- Kinetic Energy;
- Thermal Stresses;
- Atomic and Molecular Physics