Sub-wavelength focusing of cylindrical vector beams has attracted much attention because of its special properties. On this basis, an all-dielectric concave grating lens with negative refractive index is proposed in this paper. Through discussing the influence of equivalent negative refractive index (neff) and preset focal length on the focusing properties, the parameters of the structure is optimized. The results show that when neff is -1, the focal size is the smallest. A smaller preset focal length results in a smaller the transverse focal size. Furthermore, the effect of higher order diffracted light is also studied. It is found that, sub-focusing can be suppressed effectively by removing the structural part that supports the higher order diffraction. Finally, not only the focusing property is improved, but also the structure is simplified. This work provides a flexible and feasible method for the design of negative-index lens, and offers a reference for manipulating the focus of CVBs. Therefore, it provides a scheme for focusing artificial microstructure.
Hybrid plasmonic waveguides (HPWs) have attracted wide attention in recent years, because it makes a better compromise between the low loss of dielectric waveguide and the constraint capability of surface plasmonic waveguide. In this work, a hollow HPW with slat metal layer is analyzed to further reduce the loss and maintain constraint capability, then Bragg grating is designed and studied. By changing the waveguide width to further analyze the mode. The results show that normalized mode area is around 0.01, and propagation length (Lp) is up to 3500 μm, for TM polarized mode at operating wavelength of 1550 nm. For TE mode, Lp keeps millimeter level. Based on hollow HPW, Bragg grating is constructed by alternating the waveguides with different widths. Since the effective index of waveguide mode is quite sensitive to the change of the width and the trends of TM and TE modes are different from each other, Bragg gratings with different filtering characteristics and polarization properties can be designed by choosing combinations with different width. Simulations prove the validity of the design. HPW and Bragg structure proposed in this work would provide a reference for designing related photonic devices and have the potential applied value in the field of optical communication and integrated optics.
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