  Energy of propagating electromagnetic waves can be fully absorbed in a thin
lossy layer, but only in a narrow frequency band, as follows from the causality
principle. On the other hand, it appears that there are no fundamental
limitations on broadband matching of thin absorbing layers. However, known thin
absorbers produce significant reflections outside of the resonant absorption
band. In this paper we explore possibilities to realize a thin absorbing layer
which produces no reflected waves in a very wide frequency range, while the
transmission coefficient has a narrow peak of full absorption. Here we show,
both theoretically and experimentally, that a wide-band-matched thin resonant
absorber, invisible in reflection, can be realized if one and the same resonant
mode of the absorbing array unit cells is utilized to create both electric and
magnetic responses. We test this concept using chiral particles in each unit
cells, arranged in a periodic planar racemic array, utilizing chirality
coupling in each unit cell but compensating the field coupling at the
macroscopic level. We prove that the concept and the proposed realization
approach also can be used to create non-reflecting layers for full control of
transmitted fields. Our results can have a broad range of potential
applications over the entire electromagnetic spectrum including, for example,
perfect ultra-compact wave filters and selective multi-frequency sensors.
