Charged particle radiation induced changes to optical properties of acousto-optic materials

We report on the measurement of the transmittance and reflectance of unpolarized light (425-700 nm) in three birefringent, acousto-optic materials, including quartz, lithium niobate, and tellurium dioxide, after exposure to varying fluences of proton radiation ($ {10^{14}} {-} {10^{18}}\;{\rm protons}/{{\rm cm}^2} $10¹⁴-10¹⁸protons/cm²) delivered by a 10 keV hydrogen ion beamline. We observe a general monotonic decrease in transmittance with increasing fluence for all three materials, but with varying rates of change and critical points of change. Reflectance measurements also exhibit a general monotonic trend with fluence, but increases in quartz are observed versus decreases in both lithium niobate and tellurium dioxide. These observations are used to assess the suitability of the materials for acousto-optic applications in the space environment where charged particles from the solar wind are dominant and pose a threat to device operation. Our measurements agree with previously reported work concluding that tellurium dioxide is suitable for space applications at low fluences (below $ {{10}^{16}}\;{\rm ions}/{{\rm cm}^2} $10¹⁶ions/cm²), but our findings also raise previously unreported concerns for higher accumulated fluences observed for longer mission lifetimes of greater than five to 10 years in space in an unshielded configuration.