Radiation-induced negative optical nonlinearities in fused silica, sapphire, and borosilicate glass

Negative nonlinear absorption is reported in neutron- and gamma-irradiated high-OH content fused silica, low-OH content fused silica, sapphire, borosilicate glass, and radiation-resistant borosilicate glass irradiated to total neutron fluences of 3.4 × 10¹⁶ n ⋅ cm⁻² (42 Mrad γ), 1.7 × 10¹⁷ n ⋅ cm⁻² (211 Mrad γ), and 3.6 × 10¹⁷ n ⋅ cm⁻² (433 Mrad γ). Nonlinear absorption coefficients were measured via Z-scan and are on the order of −(10⁻¹⁴−10⁻¹⁰) m⋅W⁻¹. The Z-scan also reveals a negative nonlinear index of refraction in neutron- and gamma-irradiated radiation-resistant borosilicate glass on the order of −10⁻¹⁹ m⋅2W⁻¹. Thermal annealing at temperatures up to 800 ^∘C is shown to restore these nonlinearities to their positive unirradiated values in all four materials. All Z-scan measurements were performed at 532 nm with a nanosecond pulsed laser. The occurrence of negative nonlinearities may be attributed to the presence of metallic impurities and the resulting saturable absorption. A limited effect of photobleaching is observed in measurements and is quantified in multiple Z-scans.