2, Physics, Korea University, Seoul, , Korea (the Republic of)
All-optical operation holds promise as the future of computing technology, and key components will include miniaturized waveguides (WGs) and optical switches that control narrow bandwidths for logic operations and wavelength multiplexing. Nanowires (NWs) offer an ideal platform for nanoscale WGs, but their utility has been limited by the lack of a comprehensive coupling scheme with band selectivity. Controlled in-coupling of light to NW WGs thus still is problematic and has relied on scattering at NW end facets or end-on parallel coupling to the NWs. Mie resonances of NWs have been considered as a potential route to enable controlled coupling into guided modes, but the interplay of the two orthogonal modes remains yet unclear. Here, we introduce a NW geometric superlattice (GSL) that allows controlled, narrow-band guiding in Si NWs through direct coupling of a Mie resonance with a bound guided state (BGS) under normal incidence illumination with transverse-magnetic (TM) polarization. Periodic diameter modulation in a GSL creates a Mie-BGS coupled-excitation that manifests as a scattering dark state with a pronounced scattering dip in the Mie resonance envelope. We analyze scattering characteristics of NW GSLs using Temporal Coupled-Mode Theory (TCMT) and compare with numerical finite-element modeling. The coupling strength between the Mie and BGS modes, which is zero in the absence of the GSL geometry, becomes non-zero with periodic modulation and increases with an increasing modulation depth. Experimental extinction spectra of individual NW GSLs are measured by a home-built laser microscope and presented with numerically simulated spectra. The frequency of the coupled mode, tunable from the visible to near-infrared, is determined by the pitch of the GSL and exhibits a Fourier-transform limited bandwidth. Using a combined GSL-WG system, we demonstrate spectrally-selective guiding at telecommunication wavelengths with bandwidths of ~50 nm, which aligns with the spectral positions of the dips observed in extinction spectra. We also present optical switching characteristics of a GSL with an index change of the surrounding medium, highlighting the potential to use NW GSLs for the design of on-chip optical components.