Direct Measurement of Total-Ionizing-Dose-Induced Phase Shifts in Commercially Available, Integrated Silicon-Photonic Waveguides
George N. Tzintzarov, Jeffrey W. Teng, Delgermaa Nergui, Brett L. Ringel, Stephen LaLumondiere, Daniele M. Monahan, Arielle Little, John D. Cressler
Abstract
In this work, integrated silicon-photonic waveguides are exposed to ionizing radiation. A microbeam X-ray source was utilized to deliver an isolated dose to a waveguide that behaved as one branch of an integrated Mach–Zehnder interferometer (MZI) structure, such that any radiation-induced phase shifts can be directly measured. It is found that the silicon waveguides used in the present study exhibit a nonmonotonic change in the effective index of refraction, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n_{\text {eff}}$ </tex-math></inline-formula> , for both <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$O$ </tex-math></inline-formula> - and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$C$ </tex-math></inline-formula> -band operation due to radiation exposure. <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n_{\text {eff}}$ </tex-math></inline-formula> first decreased with increasing dose, which is attributed to native SiO2 growth in the silicon core. At larger doses, this trend reverses and the observed net change in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n_{\text {eff}}$ </tex-math></inline-formula> eventually becomes positive. This increase in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n_{\text {eff}}$ </tex-math></inline-formula> can most likely be attributed to radiation-induced densification of SiO2. Overall, it was found that these commercially available silicon waveguides are sensitive to doses above <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\times 10^{{9}}$ </tex-math></inline-formula> rad(Si), and the nonmonotonic trends suggest that competing simultaneous mechanisms affect the waveguides under high dose levels.