Fig. 1. Focusing performance of sGRIN. (a) RI profile of sGRIN; (b) map of electric near-field intensity profile at the focusing line in the frequency range of 4 to 20 GHz; (c) calculated electric near-field intensity distribution; (d) related FWHM; (e) focusing efficiency of proposed sGRIN.

Fig. 2. Principle of diffraction-limited far-field radiation of sGRIN. (a) sGRIN with RI distribution of 2.6/[1+(r/R)1.5]; (b) mean electric field amplitude as a function of distance from two-point sources (transverse electric polarization), EM waves experience strong interaction with gSIL and transport into the far field. Gradient periodic modulation in sGRIN is observed. (c) Far-field radiation pattern of two-point sources of 10 GHz; (d) map of normalized far-field intensity profile of two-point sources with distances of 0.56λ from 4 to 16 GHz; (e) related well depth; and (f) the distance between the two sources.

Fig. 3. Design and simulated results of sGRIN. (a) RI profile of sGRIN (dotted red line) and the discrete (solid blue line) one. The inset depicts three kinds of unit cells adopted based on the three regions in sGRIN. (b) Schematic of sGRIN; (c) map of simulated near-field intensity profile at the focusing line from 4 to 20 GHz; comparison between simulated (d) FWHM and (e) focusing efficiency of sGRIN with a discrete RI distribution (red solid line) and that of the original sGRIN with a continuous RI distribution (blue dashed line); (f) map of simulated normalized far-field intensity profile; (g) related well depth; distance between two feeding sources.

Fig. 4. Experimental results of subdiffraction focusing and diffraction-limited far-field radiation. (a) Photograph of the fabricated sample; (b) experimental map of near-field intensity profile at the focusing line from 7 to 13 GHz; (c) measured electric near-field intensity distribution and the corresponding FWHM of the combined lenses; comparison between the experimental (d) FWHM and (e) focusing efficiency of sGRIN (red solid line) with calculated results of sGRIN with a continuous RI distribution (dashed blue line); (f) schematic of the lens antenna; (g) map of measured far-field radiation intensity profiles fed by two sources with a subwavelength distance 0.56λ; (h) comparison between the measured well depths and the calculated results of sGRIN with a continuous RI distribution.

Fig. 5. Comparison of half of the light cone angle, operational bandwidth, and focusing efficiency among SIL, NRL, SOL, HPL, and our designed sGRIN.