Abstract
Tradional antireflection coatings composed of dielectric layers usually require the thickness to be larger than quarter wavelength. Here, we demonstrate that materials with permittivity or permeability dominated by imaginary parts, i.e. lossy or gain media, can realize non-resonant antireflection coatings in deep sub-wavelength scale. Interestingly, while the reflected waves are eliminated as in traditional dielectric antireflection coatings, the transmitted waves can be enhanced or reduced, depending on whether gain or lossy media are applied, respectively. We provide a unified theory for the design of such ultrathin antireflection coatings, showing that under different polarizations and incident angles, different types of ultrathin coatings should be applied. Especially, under transverse magnetic polarization, the requirement shows a switch between gain and lossy media at Brewster angle. As a proof of principle, by using conductive films as a special type of lossy antireflection coatings, we experimentally demonstrate the suppression of Fabry-Pérot resonances in a broad frequency range for microwaves. This valuable functionality can be applied to remove undesired resonant effects, such as the frequency-dependent side lobes induced by resonances in dielectric coverings of antennas. Our work provides a guide for the design of ultrathin antireflection coatings as well as their applications in broadband reflectionless devices.
Highlights
Tradional antireflection coatings composed of dielectric layers usually require the thickness to be larger than quarter wavelength
Our theory predicts some other types of non-resonant ultrathin Antireflection coatings (ARCs), such as ARCs composed of strong gain media as the opponent of conductive films, and ARCs composed of zero-index media (ZIM) with tiny loss or gain
The additional layer is chosen as a quarter-wave dielectric layer with a relative permittivity of ε1 ε2 and a minimal thickness of d = λ0/[4(ε1ε2)1/4], as illustrated in Fig. 1(a). λ0 is the wavelength in free space
Summary
The ARC functionality only fails when the frequency increases to a regime where the approximation of constant conductivity is inaccurate To verify such an interesting feature, we plot the reflectance (defined as the ratio of reflected and incident energy flux) as the function of the incident angle and operating frequency in Fig. 6(a,b) for TE and TM polarizations, respectively. Thereby, we can stabilize the radiation patterns of antennas by applying the broadband and ultrathin conductive ARC
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