Fig. 1. (a) Plot of the FOM versus the effective confinement area enables a comparison of the nanostructure, optical lens focusing, plasmonic lenses (hollow square [4], solid square [5], hollow circle [6], solid circle [7], hollow triangle [8], solid triangle [8]) and nanofocusing (hollow pentagon [10], solid pentagon [11], hollow diamond [12], solid diamond [13]). The gray area marks the diffraction-limited. The curve of the FOM for the nanostructure is calculated when d=250  nm and s varies in the range from 1 to 100 nm. (b) Schematic of the nanostructure. Three low-index nanoslits are embedded in the high-index ChG block. nc is the refractive index of the ChG block, and ns is the refractive index of the nanoslits. The refractive index of the background is nb. L, s, and d are the length, width, and depth of the nanoslits, respectively. w is the width of the nanoridges. Inset: 2D cross section of the nanostructure.

Fig. 2. Dependence of (a) effective confinement area Aeff, (b) energy confinement efficiency η, and (c) FOM of the mode on depth d and width s of the nanoslits. The white-dashed line marks the critical transformation length for mode coupling. (d)–(i) x–y and x–z cross sections of the energy density distribution of the nanostructures (d), (e) [s,d]=[100,50]  nm, (f), (g) [s,d]=[100,250]  nm, and (h), (i) [s,d]=[1,250]  nm.

Fig. 3. Normalized energy density (a) along the x direction at y=0, z=zf for the nanoslit with d=250  nm, (b) along the y direction at x=0, z=zf for the same d, and (c) along the z direction at x=0, y=0, for the nanoslits with a fixed s=1  nm. The coordinates are marked in the figures. (d) Effective confinement volume Veff and the FWHM in the z direction as a function of s when d=250  nm.

Fig. 4. (a) Spectral response of the nanostructure ([s,d]=[1,250]  nm) with different nc. Two enhancement peaks are shown in the curve. (b) Dependence of the enhancement peak wavelength (solid line) and Aeff (dashed line) on nc for the two peaks. (c) Dependence of η (solid line) and the FOM (dashed line) on nc for the two peaks.

Fig. 5. Normalized energy density at the point of (x=0, y=0, z=zf) versus the wavelength for different refractive indices of the nanoslits ns ([s,d]=[1,250]  nm). The black-dashed lines mark the positions for two peaks for different values of ns while nc=2.5, nb=1.