Nanojet-induced modes in 1D chains of microspheres

We report on the light transport phenomena in linear chains composed of several tens of touching spherical microcavities. A new optical mode type, namely nanojet-induced modes (NIMs) is observed. These modes result from the optical coupling of microspheres acting as a series of micro-lenses, which periodically focus propagating wave into photonic nanojets. Theoretically, formation of periodic nanojets has been predicted in Z. Chen et al., Opt. Lett. 31, 389 (2006). The chains were produced by means of the self-assembly directed by micro-flows of water suspension of polystyrene microspheres. The mean size of spheres was varied in the 2-10 micron range. To couple light to NIMs we used built-in emission sources formed by several locally excited dye-doped microcavities from the same chain. Conversion of modes emitted by the light source into the NIMs results in losses of several dB per sphere in the vicinity (first few tens of spheres) of such sources. At longer distances we found an attenuation rate as small as 0.5 dB per sphere that reveals low intrinsic propagation loss for NIMs. The NIMs have potential applications for coupling and guiding of light in compact arrays of spherical cavities with extremely high quality (Q) whispering gallery modes.