Optical losses in pure crystalline silicon in the IR band measured using WGM microresonators
The need for semiconductor technology for crystalline silicon of the highest purity and homogeneity has provided samples exhibiting low optical absorption in the infrared range. Such silicon has become the basis for photonic elements in the telecommunication band, including high-Q microresonators, which are particularly important. However, at longer wavelengths, the loss mechanisms have not yet been sufficiently studied. At the same time, this range is extremely important, especially for biological and medical applications and for fundamental research. We used optical microresonators with whispering gallery modes made from various types of silicon crystals as a tool to study the loss mechanisms. The study involved the pump wavelengths 1.5, 2.6, 6.1, and 8.6 $\mu$m and the maximum measured Q-factors were $1.5\cdot10^9$, $5\cdot10^8$, $1.6\cdot10^7$, and $5\cdot10^4$, respectively. We showed that the conductivity type does not noticeably influence the optical losses, while resistivity and the growing method are defining factors. Our study confirms the utility of WGM microresonators as loss measurement tools and provides significant potential for the development of silicon microresonator-based photonics in the mid-IR band.