Quasi-guiding Modes in Microfibers on a High Refractive Index Substrate

Light confinement and amplification in micro- and nanofibers have been intensively studied, and a number of applications have been developed. However, typical micro- and nanofibers are usually free-standing or positioned on a substrate with lower refractive index to ensure the light confinement and guiding mode. Here we numerically and experimentally demonstrate the possibility of confining light within a microfiber on a high refractive index substrate. In contrast to the strong leakage to the substrate, we found that the radiation loss was dependent on the radius of the microfiber and the refractive index contrast. Consequently, quasi-guiding modes could be formed and the light could propagate and be amplified in such systems. By fabricating a tapered silica fiber and dye-doped polymer fiber and placing them on sapphire substrates, the light propagation, amplification, and consequent laser behaviors have been experimentally studied to verify the quasi-guiding modes in a microfiber on a higher index substrate. We believe that our research will be essential for the applications of microfibers and on-chip photonic circuits.