Nanocrystalline silicon optical microcavities

Abstract

We investigated optical microcavities using silicon-rich silicon oxide (SRSO), one of the interesting materials for Si photonics, consisting of nanocrystal silicon (nc-Si) embedded in SiO$_{2}$ matrix. The microcavity consisting of two Si/SiO$_{2}$ DBRs and the central SRSO active layer was realized, and the effects of this DBR-based microcavity on nc-Si luminescence are investigated. Photoluminescence at room temperature reveals an enhancement of the luminescence intensity emitted along the optical axis of the cavity and a spectral narrowing, leading to cavity Q factor of 52. Furthermore, we measured the refractive index of SRSO active layer based on the calculation of microcavity reflectance. However, since the Q factor of this structure is too low to obtain a very accurate measurement for the optical properties such as losses, we investigate the microsphere cavity with higher Q factor. To investigate the effect of nc-Si layer on the cavity Q of silica microspheres, 140$\\pm$10 nm thick SRSO films with excess Si content ranging from 5 to 14 at. $\\%$ were deposited on silica microspheres formed by CO$_{2}$ laser melting of an optical fiber, and subsequently Q of the spheres with the SRSO layer was measured at 1.56 $\\mu$m using a tunable external cavity coupled laser diode and tapered fiber coupling. We find that the presence of SRSO layer reduces the cavity Q factor from $\\ge$ 2$\\times$10$^{7}$ to 2-5$\\times$10$^{5}$. However, the observed Q factors are nearly independent of nc-Si size, density, or even the presence of nc-Si. We investigate three possible origins of the Q factor lowering by SRSO layer. First, we estimated the effect of varying the nc-Si size distribution and increasing the average nc-Si radius on the nc-Si scattering loss. However, the calculated Q factors remains still $>$ 10$^{6}$ even with these effects, indicating that nc-Si scattering may not be responsible for the lowering of Q factor. Second, we investigated the possibility of abso...