Window materials used in the current generation of high-energy laser (HEL) systems cannot be
used for next-generation systems since these aim to operate at or near the megawatt level under
CW operation. Related intensities will lead to beam distortion and possibly damage of current
optical materials. These materials include polycrystalline spinel and single-crystal sapphire – both
have undesirable optical absorption and scattering near the 1 μm wavelength that limit their
performance in future HEL applications. However, phenomena associated with optical absorption
and scattering have not been comprehensively studied for the crystalline materials of interest and,
as a result, the physical mechanisms underlying both optical losses in these materials are not well
understood.
The overall goal for this research is to obtain a comprehensive understanding of low-level
absorption and scattering in sapphire and spinel at and near the 1 μm wavelength region using the
commercially available samples. In this work, ultraviolet-visible spectroscopy and photothermal
common-path interferometry are used to measure bulk and surface absorption losses in different
samples in the UV-visible-near-infrared wavelength regions. The absorption coefficient values
obtained are in the range of 10-5 to 100 cm-1 and increase as the wavelength decreases. In addition,
scattering measurements on samples with different surface polishing conditions are made at 405,
532, 633, 1064, and 1550 nm using an instrument developed to assess the bidirectional scatterance
probability distribution function. The total integrated scatterance is in the range of 10-5 to 10-1 and
increases for all samples as the wavelength decreases. Different absorption and scattering models
are applied to interpret the measured data. Materials characterization techniques including positron
annihilation lifetime spectroscopy, laser ablation inductively coupled plasma mass spectroscopy,
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secondary ion mass spectroscopy, Raman spectroscopy, atomic force microscopy, and optical and
electron microscopies are utilized to characterize bulk and surface defects in different samples.
Overall, the measurement results indicate that both weak absorption and scattering losses are
strongly related to defect structures such as lattice disorder and impurities that were introduced
during crystal growth or post-growth processing. Understanding these defects and their
contributions to optical loss can lead to improved manufacturing and processing methods