![]() The acoustic transmission of a single hole approaches a constant, 8/π 2, dislike the EM case, with decreasing the ratio r/λ. ![]() Consequently, a subwavelength hole has no cutoff for acoustic wave, but does for EM wave, which underlies the distinct transmissions of acoustic/EM waves through a hole in an ideally rigid/conducting screen. In nature, acoustic wave is a scalar longitudinal wave in inviscid fluids, while EM wave is a vector transverse wave. It is well known that acoustic and EM waves share a lot of wave phenomena, but they have something in difference. In sharp contrast, aperture-factor-related resonances have the wavelength determined mainly by the transversal/longitudinal dimensions of the aperture and are not sensitive to the incidence angle. Structural-factor-related resonances typically have the transmission wavelength comparable to the lattice constant and are dependent strongly on the incidence angle. Phenomenologically, various observed transmission resonances are associated with two geometrical factors: structural factor (SF) emerging globally from the lattice periodicity and aperture factor owned locally by the individual unit. Since then, the remarkable phenomenon has inspired a tremendous amount of attention and works on resonant transmissions of EM waves through various apertures on either metallic or dielectric structure. ![]() The holes, once organized or decorated, have the transmission cross section larger than themselves’ area, which is different radically from the theory by Bethe. However, in 1998, Ebbesen et al.has observed enhanced transmission of light through either a lattice of subwavelength holes or a single hole surrounded by surface periodical patterns on thin metallic films, where the optical transmission can be much larger than the area fraction of the holes at specific frequencies. For a hole of finite thickness, the transmittance is found to be reduced further, because no propagating mode exists inside the hole. ![]() ![]() This result shows that a small hole has extremely low transmission or negligible cross section for EM waves of very long wavelength. In 1944, Bethe found the transmittance of electromagnetic (EM) waves through a tiny hole in a perfectly conducting screen varies as being proportional to ( kr) 4, where k=2π/λ, λ is the wavelength and ris the hole radius. ![]()
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