Tunable multimode and narrowband in a photonic quasicrystal waveguide.

In this work, we propose a photonic quasicrystal waveguide, which contains a hollow core surrounded by coaxial dielectric quasiperiodic multilayer. Due to the self-similarity in the cladding structure, multiple omnidirectional photonic band gaps (PBGs) exist in the waveguide. The light waves with the frequencies within the omnidirectional PBGs are totally reflected, thereafter, the transport of multimodes is achieved in the quasiperiodic waveguide. Further, it is shown that the centre frequency and the width of the omnidirectional PBG can be tuned by the refractive indexes or the generations of the quasiperiodic sequence in the cladding multilayer. As a consequence, both the quality factor and the confinement performance of the waveguide can be significantly enhanced by decreasing the width of the omnidirectional PBGs. The investigations make it possible to design miniaturized multifunctional optical devices, such as on-chip narrowband waveguide-based filters and laser resonators.

Influence of the interlayer on coupling of surface plasmons in a sandwiched structure with periodic array of nanoapertures.

In this work, we investigate the optical properties of a multilayer structure, where a SiO2 film is sandwiched by silver films with periodic array of sub-wavelength apertures. Due to the coupling of surface plasmons (SPs) between different layers, electric and magnetic resonances have been observed. By varying the thickness of the interlayer SiO2, we can modify relative phase of the SPs resonance and control the shifts of transmission peaks. Experimentally the multilayers are fabricated by magnetron sputtering and the array of apertures is milled by focused-ion-beam facility. The measured optical transmission spectra reasonably agree with our numerical calculation, which bases on three-dimensional finite-difference time-domain method. To understand the shifts of the peaks, we present a phenomenological explanation, considering the transmission peaks as energy levels, and the coupling of localized surface plasmons as perturbation. These results may have potential applications in designing plasmonic devices and tuning electromagnetic wave in nanophotonics.