Three-Dimensional Subwavelength Imaging by a Photonic-Crystal Flat Lens Using Negative Refraction at Microwave Frequencies

We experimentally demonstrate subwavelength resolution imaging at microwave frequencies by a three-dimensional (3D) photonic-crystal flat lens using full 3D negative refraction. The photonic crystal was fabricated in a layer-by-layer process. A subwavelength pinhole source and a dipole detector were employed for the measurement. By point-by-point scanning, we obtained the image of the pinhole source shown in both amplitude and phase, which demonstrated the imaging mechanism and subwavelength feature size in all three dimensions. An image of two pinhole sources with subwavelength spacing showed two resolved spots, which further verified subwavelength resolution.

Figure 1

(a) Conventional cubic unit cell of the bcc structure. (b) The modified unit cell after shifting by $(3/8)a$ along vertical direction. (c) Three-dimensional PhC fabricated layer by layer (20 layers in total).

Figure 2

(a) Band structure of the bcc lattice photonic crystal. (b) The constant-frequency surface of the crystal at $f=0.36(c/a)$ (or 17.0 GHz for our structure) in the first Brillouin zone.

Figure 3

Image of the point source achieved through the 3D PhC flat lens at image distance $d_i=11\mathrm{mm}$. The enclosed block shows the dimensions of the spot size measured by FWHM.

Figure 4

Measured intensity and phase distributions ($f=16.4\mathrm{GHz}$, $\lambda =18.3\mathrm{mm}$): (a) in the vertical plane, (b) in the horizontal plane.

Figure 5

(a) The image (intensity) of two pinholes 10 mm apart ($\lambda =18.3\mathrm{mm}$). (b) The intensity distribution along the blue line marked on (a). The image shows two resolvable spots with distance 10 mm.