Quantifying Proton-Induced Optical Absorption in Silicon Using Ultrahigh-Q Nanocavities for Space Silicon Photonics

To enable the use of silicon photonic devices in space, it is important to quantify radiation-induced increases in optical absorption. Here, we directly quantify the optical absorption induced in silicon by proton irradiation by analyzing the degradation of the quality factor (Q) of ultrahigh-Q silicon photonic crystal nanocavities. Irradiation with 20 keV protons at a fluence of 7.5 × 10^{12} cm^{-2} reduced the Q-factor of a nanocavity from 2.5 × 10^{6} to 8.1 × 10^{5}, corresponding to an induced absorption coefficient of approximately 1.6 × 10^{-1} cm^{-1}. The use of nanocavities with Q exceeding two million enables quantitative evaluation of such extremely small absorption. Because the cavities contain no intentional p–n doping, the measured increase reflects the intrinsic response of bulk silicon. These results provide guidelines for designing silicon photonic components for spacecraft, and the method offers a sensitive means to quantify absorption increases caused by different types of radiation.