Abstract
As a fundamental phenomenon in electromagnetics and optics, material absorption has been extensively investigated for centuries. However, omnidirectional, reflectionless absorption in inhomogeneous media has yet to be observed. Previous research on transformation optics indicated that such absorption cannot easily be implemented without involving gain media. A recent theory on wave propagation, however, implies the feasibility to implement such absorption requiring no gain, provided that the permittivity profile of this medium can satisfy the spatial Kramers–Kronig relations. In this work, we implement such a profile over a broad frequency band based on a novel idea of space–frequency Lorentz dispersion. A wideband, omnidirectionally reflectionless absorption is then experimentally observed in the gigahertz range, and is in good agreement with theoretical analysis and full-wave simulations. The proposed method based on the space–frequency dispersion implies the practicability to construct gain-free omnidirectionally non-reflecting absorbers.
Highlights
As a fundamental phenomenon in electromagnetics and optics, material absorption has been extensively investigated for centuries
Based on the space–frequency Lorentzian dispersion proposed in this work, we experimentally implement a permittivity profile that obeys the spatial K–K relations over a broad band by constructing a passive gradient mesoscopic structure
We further demonstrate a wideband, omnidirectionally non-reflecting absorption that is in good agreement with theoretical analysis and full-wave simulation
Summary
As a fundamental phenomenon in electromagnetics and optics, material absorption has been extensively investigated for centuries. Non-reflecting absorptive media designed by the parity-time symmetric property have the same requirement of gain regions[10,11,12] Different from these methods, a recent research has suggested that a class of isotropic, inhomogeneous one-dimensional (1D) susceptibility profile (permittivity ε(x) or permeability μ(x) along the x direction) can be used to design a non-reflecting absorptive material that requires no gain[13]. The design of such profiles initiates a new research direction for non-reflecting wave absorption[14,15,16,17,18]. Based on the space–frequency Lorentzian dispersion proposed in this work, we experimentally implement a permittivity profile that obeys the spatial K–K relations over a broad band by constructing a passive gradient mesoscopic structure. Based on the concept of dispersion engineering[19, 20], our approach is expected to realize gain-free omnidirectionally reflectionless absorbers
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