The energy is coupled from the slit to the subwavelength corrugated structure on the output surface of the metal to propagate spoof SPPs and radiate a narrow broadside beam with high directivity. The antenna is fed at center of the structure through a slit, connecting to a feeding waveguide, as illustrated in Fig. Here we apply the concept of surface impedance to the subwavelength corrugated metal structure, which originally supports the spoof SPPs and modulate the surface impedance using the cosine function and triangle-wave function, respectively, so that the modulated structures could radiate propagating waves as a periodic leaky-wave antenna. Conversely, the periodic structures satisfying the radiation condition could also be used to convert a propagating wave to surface wave 33 due to the reversibility of light. As spoof SPPs on the 1D structure is also a kind of transverse magnetic (TM) surface wave 11, 13, one could treat the subwavelength corrugations on metal as a kind of metasurface, the surface impedance of metasurface has been widely discussed and utilized to generate leaky waves, especially the sinusoidally modulated metasurfaces 27, 28, 29, 30, 31, 32. However, the radiations of leaky waves by these subwavelength corrugations have not been discussed.
In recent years, subwavelength corrugations or grooves on metals supporting spoof SPPs with a period of approximately 1/6 wavelengths have been analyzed for slow-wave phenomena 21, 22, 23, 24, 25, 26. However, in the microwave region the period of the corrugated metal structures is about one wavelength to support leaky waves and the angle of radiation beam was determined by the empirical formula 12, 16, 19 or complicated mode expansion method 13.
Similarly, electromagnetic beams shaped by the corrugated metal structure on the output surface 12 have been observed, followed by the analysis of leaky-wave 13 and the design of bull’s eye antennas in the microwave frequency 14, 15, 16, 17, 18, 19, as has been done in the optical frequency 2, 20. The one-dimensional (1D) corrugated metal structure has also shown to propagate spoof SPPs 11. In the microwave region, a two-dimensional (2D) metal with drilling holes was proposed and investigated to support spoof SPPs 10. The field on the exit surface has also been analyzed using the leaky surface-plasmon theory 5, 6, 7, 8 and transforamtion optics 9.
After the phenomenon of extraordinary transmissions through subwavelength holes was investigated by surface plasmon polaritons (SPPs) between metal and air in the optical region 1, 2, it was found that the corrugated metal structure on the output surface could shape the transmitted light emerging from the aperture 2, 3, 4.
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