Pulsed laser damage resistance of nano-structured polarizers for 1064nm

Abstract

Non-diffracting surface relief grating structures combined with high refractive index films were designed as high efficiency, narrow-band, polarization selective high reflectors for the near infrared wavelength region. Such nanostructure polarizers (NSP) have the potential for increased laser damage resistance due to reduced absorption and the ability to create arbitrary refractive index layers with fewer defects and reduced electric field enhancement. Three NSP designs based on gratings in fused silica combined with tantala and magnesium fluoride films, were prototyped and characterized for efficiency, surface absorption and pulsed laser damage resistance at a wavelength of 1064nm. Most NSP prototypes exhibited <99.7% reflectivity for linearly polarized illumination over a several nm bandwidth with high transmission of the orthogonal polarization leading to extinction ratios greater than 300:1 for the best performers. NSP prototype performance was worse than predicted by the design models due to the imprecise replication of the fused silica grating surface in the film layers resulting from the deposition system configuration. Surface absorption measurements showed the expected low absorption in the 4 ppm range for film layers deposited on non-structured control substrates, but voids and growth defects revealed through scanning electron microscopy in the same films deposited over gratings, likely caused an observed 5 fold increase in NSP prototype surface absorption. Initial 1064nm wavelength, 6.2ns pulsed laser damage testing also showed a reduced damage resistance for NSP prototypes compared to the films deposited on non-structured control substrates. Follow-on work to eliminate the film defects for NSP designs is underway.