Fig. 1. Profile of plane-concave lens and schematic of focusing process. (a) Cross-section of plane-concave lens in r-z plane; (b) schematic of focusing process

Fig. 2. Intensity distributions of focal field with predetermined f=8 μm. (a) Radial polarization incidence; (b) azimuthal polarization incidence

Fig. 3. Intensity distributions of focal field with predetermined f=8 μm and different refractive indices of n=2, 2.20, and 2.36, respectively. (a) Intensity distribution of field along axial direction; (b) intensity distribution of field along lateral direction (radial direction)

Fig. 4. Intensity distributions of focal field with predetermined f=8 μm and different equivalent negative refractive indices of n_eff=-0.8, -0.9, -1.0, -1.1, and -1.2 respectively. (a) Intensity distribution of field along axial direction; (b) intensity distribution of field along lateral (radial) direction

Fig. 5. Intensity distributions of focal field with predetermined f=6, 7, 8, and 9 μm, as n=2.36 and n_eff=-1.0. (a) Intensity distribution of field along axial direction; (b) intensity distribution of field along lateral (radial) direction

Fig. 6. Analyses of focal field with f=6 μm and f=9 μm. (a)(b) Contour plots of |E|²; (c)(d) contour plots of |E_z|²; (e)(f) distributions of |E|², |E_z|², and |E_r|² along lateral (radial) direction