Analytical, numerical, and experimental investigation of a Luneburg lens system for directional cloaking

Abstract

In this study, the design of a directional cloaking based on the Luneburg lens system is proposed and its operating principle is experimentally verified. The cloaking concept is analytically investigated via geometrical optics and numerically realized with the help of the finite-difference time-domain method. In order to benefit from its unique focusing and/or collimating characteristics of light, the Luneburg lens is used. We show that by the proper combination of Luneburg lenses in an array form, incident light bypasses the region between junctions of the lenses, i.e., the "dark zone." Hence, direct interaction of an object with propagating light is prevented if one places the object to be cloaked inside that dark zone. This effect is used for hiding an object which is made of a perfectly electric conductor material. In order to design an implementable cloaking device, the Luneburg lens is discretized into a photonic crystal structure having gradually varying air cylindrical holes in a dielectric material by using Maxwell Garnett effective medium approximations. Experimental verifications of the designed cloaking structure are performed at microwave frequencies of around 8 GHz. The proposed structure is fabricated by three-dimensional printing of dielectric polylactide material and a brass metallic alloy is utilized in place of the perfectly electric conductor material in microwave experiments. Good agreement between numerical and experimental results is found.

Multi-Directional Cloak Design by All-Dielectric Unit-Cell Optimized Structure

“…As it was noted, the designed structure has four-fold rotational symmetry, as seen in Figure 1 . Here the rotational and mirror symmetry concepts are realized to provide quasi-omnidirectional invisibility behavior for the structure [ 26 , 41 , 43 ]. In this regard, the proposed structure is optimized considering these symmetry constraints, and the corresponding results are given in Figure 3 c,d.…”

“…This approach has been further developed to design and analyze carpet cloaking methods [ 13 , 14 , 15 ]. Lately, different from the TO concept, new design strategies for optical cloaking are proposed such as metasurfaces [ 16 , 17 ], zero-refractive-index materials [ 18 ], plasmonics [ 19 , 20 , 21 ], thermally tunable semiconductors [ 22 ], transmission-lines made by parallel-plate waveguides [ 23 ], uniform dielectric claddings [ 24 ], woodpile photonic structures [ 25 ], graded index structures [ 26 ], and superluminal media [ 27 ] to operate in different wavelength regimes including microwave, terahertz, infrared and visible. Moreover, due to its perfect directional projection property [ 28 , 29 ] and transformation-invariant feature [ 30 , 31 ], optical cloaking can be realized by the so-called optic-null medium.…”

Optical cloaking of macroscopic objects by geometric-phase vortex processing

“…It should be noted that the preceding examples of electromagnetic concealment of macroscopic objects that can be described and understood from ray-tracing analysis are not unique to the optical domain. See, for example [22,23], which report on graded index lensing concealment strategies in the microwave and radio wave domains, respectively, however, with objects whose size is comparable to the wavelength.…”

Modified structural Luneburg lens for broadband focusing and collimation

“…Similar to its counterpart of optical and acoustic Luneburg lenses, SLL has a gradient refractive index profile that decreases radially from the lens centre to its outer surface. Optical Luneburg lens [39][40][41] has been well studied and its focusing and collimation capabilities have been applied to many applications, such as communications and nuclear scattering [42][43][44]. In recent years, acoustic Luneburg lens has been developed for omni-directional acoustic wave focusing and collimation by using phononic crystals and acoustic metamaterials [45][46][47][48][49][50].…”