Thin low-loss dielectric coatings for free-space cloaking

Abstract

We report stereolithographic polymer-based fabrication and experimental operation of a microwave X-band cloaking device. The device is a relatively thin (about one wavelength thick) shell of an air-dielectric composite, in which the dielectric component has negligible loss and dispersion. In a finite band (9.7-10.1 GHz), the shell eliminates the shadow and strongly suppresses scattering from a conducting cylinder of six-wavelength diameter for TE-polarized free-space plane waves. The device does not require an immersion liquid or conducting ground planes for its operation. The dielectric constant of the polymer is low enough (ϵ 2.45) The possibility to make large and nontransparent objects electromagnetically undetectable has been the subject of intense research following the introduction of the transformation optics (TO) concept [1,2] and its microwave implementation [3]. In theoretical TO scenarios, an inhomogeneous shell around an object eliminates backward reflection and forward shadow, thus by virtue of the optical theorem also suppressing the scattering in all other directions. Currently, however, no technique is available for fabrication of invisibility devices capable of hiding objects with cross sections exceeding 5 free-space wavelengths (λ) [4]. Inherent in TO-based proposals are exotic material properties, such as large or near-zero magnetic permeability and/or dielectric permittivity, both with negligible imaginary parts. To date, virtually all TO medium proposals involved metallic components behaving as conducting [3][4][5] or plasmonic [6,7] media. Propagation loss is a major limitation for cloaks already at microwave frequencies [3,4], and it becomes overwhelming at visible wavelengths.The attenuation issue prompted a search for invisibility scenarios that could be based entirely on low-loss dielectric materials with a dielectric constant ϵ < 1. Naturally birefringent dielectric crystals such as calcite [8,9] were used in visible-wavelength cloaking experiments. Having refractive indexes n o;e > 1, such media are only capable of cloaking if submerged in a high-index immersion liquid whose refractive index n im satisfies 1 < n o∕e < n im . Immersion helps avoid the most fundamental limitation of TO-based cloaking-the need for superluminal phase velocities [10,11]-while rendering these cloaks useless for free-space applications such as sensing countermeasures.In this Letter, we present a general design and fabrication methodology that yields 2D microwave cloaks composed of only one, virtually lossless, dispersionless, dielectric medium with ϵ 2.45. The cloaks are multidirectional, i.e., invisible with respect to several propagation directions, are capable of hiding objects many wavelengths in diameter, and are thinner relative to the object size than in any previous experimental demonstrations [3][4][5]. Here, we apply the technique to TE-polarization waves, however, it also yields cloaks with similar performance metrics for TM polarization. Our choice of TE polarization is dictated by the testin...

CLOAKING AND INVISIBILITY: A REVIEW (Invited Review)

“…These different kinds of carpet cloaks can be invisible either unidirectionally or in multiple directions [121][122][123], and implemented exactly with natural bi-refringent dielectric crystals [121] or by other means at microwaves [122,123]. The compromise here is to sacrifice all-angle invisibility to gain some simplicity in the design.…”

Rapid Prototyping across the Spectrum: RF to Optical 3D Electromagnetic Structures

“…Several reports have documented the use of additive manufacturing to fabricate 3D EM devices, including gradient index lenses [25,26] at both microwave [27,28] and optical frequencies [26], and radio frequency lenses that attain resolution beyond the diffraction limit [29,30]. These methods may be utilized to construct 3D EM designs [31,32] that incorporate non-planar geometries and material inhomogeneity [33]. The entire process of designing, fabricating and characterizing these devices and structures is discussed in the next sections along with recent examples of applications of such media in practical systems.…”

“…Truly bulk 3D media have been also demonstrated with such stacking techniques although they are restricted to certain propagation directions and/or polarization [73]. Most of the currently demonstrated designs are very thin materials which in the wavelength of operation act like a monolayer of atoms and suffer from high losses which make them unviable options in practical devices [33]. The biggest challenge lies in implementing such structures as large-scale materials that are thick, or 3D with low losses.…”

“…In the designed band, the shell eliminated the shadow and significantly reduced scattering from a large cylinder for incident plane waves in free space without the use of an immersion liquid or conducting ground plane [33]. It should be noted that fabrication of passive structures such as those used in FSS, filters, beam steering arrays, cloaks, etc.…”

An elasto-mechanical unfeelability cloak made of pentamode metamaterials

“…The theory underlying cloaking has either been based on analytical coordinate transformations [3][4][5][6] or on numerical topology optimization 13,[23][24][25] . The analytical theory for mere invisibility and analogues thereof based on simple spherical or cylindrical core-shell geometries has been worked out a long time ago [26][27][28] .…”