Fig. 3: Self-focusing thermal meta-emitter.

a The surface wave modulation design. S_c denotes the coupling surface region, which couples the output energy from the WG to the spoof SPPs. S_r represents the radiation surface region, where spoof SPPs are scattered at designed positions to form a focusing profile. b Eigenmode profile of the designed structure. The inset shows an enlarged view of the white dashed region, with the intensity rescaled to clearly display the WG mode pattern. Copper with conductivity of σ = 5.8 × 10⁷S/m is applied in the simulation. c Out-of-plane wavevector (k_y) dispersion of the meta-emitter. d Focal length of eigenmode as a function of k_y. The WG length is set to 2.9 mm in the simulation. e–g Ensemble-averaged intensity profiles in the xz, yz, and xy planes under random dipole excitation. h Schematic of the experimental setup. The sample coated with a blackbody layer on the back side is heated using a hot plate. The thermal radiation pattern is collected point by point using a millimeter-wave detector mounted on a 3D translation stage. A foil-wrapped board is employed to block background thermal noise from the hot plate. Upper inset: simulated and measured transmission spectrum of the sample under plane wave excitation at normal incidence. Bottom insets: photographs of the sample images on both sides. Bare and blackbody painted bottom surfaces are presented in the upper panel and bottom panel in the inset. WG: waveguide. The temperature is maintained at 120 °C during the measurement. i–k Measured intensity profile at xz, yz, and xy plane, respectively. Scalar bar: 3 mm. The detailed comparison of intensity profiles in the experiment and simulation can be found in Supplementary Note S9 and Supplementary Fig. 19. Fig. b, e, and i show focal intensity cross-sections in their insets, with FWHM marked on the profiles.