Light splitting function of branched chains of transparent microspheres

To observe optical properties of propagation light through branched coupled-resonator optical waveguides (CROWs), we fabricated ordered chains of transparent microspheres that have 30°- and 60°-branched structures on a lithographically patterned substrate using a self-assembly process. The spectra of propagation light within the chains were directly measured by guide-collection-mode near-field scanning optical microscopy (NSOM) techniques. Moreover, we perform a finite-difference time domain (FDTD) simulation to explain the spectra. The spectrum of light propagating to the 60°-branch shows some sharp peaks, which seem to be associated with whispering gallery modes (WGMs). On the other hand, the spectrum of light propagating to the 30°-branch shows rather broad peaks. The FDTD simulation suggests that the WGM component propagates to both of the 30°- and 60°-branches, and also suggests that almost all of the nanojet-induced mode (NIM) component propagates to the 30°-branch and little propagates to the 60°-branch. Therefore, the most plausible cause of the difference in the NSOM spectra is that the difference in propagation efficiency of the NIM component between the 30°- and 60°-branches. These results suggest that the microspheres' branching chain itself has a splitting function, which is a kind of wavelength-selective filters.