Towards Very Compact Invisibility Cloaks

Jingjing Zhang (left) and Niels Asger Mortensen (right) of Technical University of Denmark

Authors: Jingjing Zhang¹, Shuang Zhang², and Niels Asger Mortensen¹

Affiliations:
¹DTU Fotonik - Dept of Photonics Engineering, Technical University of Denmark, Denmark
²School of Physics and Astronomy, University of Birmingham, UK

Much effort has been made to realize invisibility cloaks ever since the theoretical proposal based on transformation optics were first put forward in 2006 [1, 2]. There have been two main trends in the realization of invisibility cloaks: One is to extend the bandwidth of the cloak, and the other is to push the working frequencies to optical spectrum. These concerns have been addressed very recently where objects of millimeter size scale were successfully concealed over the whole visible range [3,4].

Shuang Zhang of University of Birmingham, UK

However, there is still one major problem that needs to be solved for the cloak design and realization, which is an important issue in practical applications. The size of the cloak device is usually much (one to two orders of magnitude) larger than that of the cloaked object, meaning that even hiding a tiny object requires a fairly large device. Our team, Jingjing Zhang, Liu Liu, and Niels Asger Mortensen from Technical University of Denmark, Yu Luo from Imperial College London, and Shuang Zhang from University of Birmingham makes the first attempt to address this concern and demonstrates at optical frequencies a cloak whose size is only four times that of the hidden object [5].

Past 2Physics article by Shuang Zhang:
February 13, 2011: "Macroscopic Invisibility Cloak made from Natural Birefringent Crystals"

Liu Liu of Technical University of Denmark (left) and Yu Luo from Imperial College London (right)

In the previous works on visible invisibility cloak, the ratio between the size of the object to be hidden and the size of cloak is limited by the small birefringence of the constituent calcite crystal (~10% difference between n_e and n_o), resulting in cloaks 20 times larger than the object. Here we construct an invisibility cloak from nano-structured homogeneous anisotropic composite materials, which are obtained by adopting semiconductor manufacturing techniques that involve patterning the top silicon layer of an SOI wafer with subwavelength gratings of appropriate filling factor (see Fig. 1). The effective media consisting of silicon gratings bypass the limitation of natural material at hand and give us extra freedom to design the devices as desired, especially those with miniaturized thickness, and the shape of the cloak can be custom designed by properly arranging the composing materials.

Fig. 1 Scanning electron microscopic image of a fabricated carpet cloak from (a) top view (b) oblique view. The inset shows the detail of the silicon gratings.


In the measurement, the light with TM polarization from a tunable laser is used to characterize the cloak. The cloak works by essentially disguising an object from light, making it appear like a flat ground plane. By precisely restoring the path of the reflecting wave from the surface, the cloak creates an illusion of a flat plane for a triangular bump on the surface and any objects underneath, hiding their presence over wavelengths ranging from 1480nm to 1580nm (see Fig. 2).

Fig. 2 The measured output image from a flat surface (left) and a cloaked protruded surface (right) at 1480nm (a), 1550nm (b), and 1580nm (c).

The cloak is made exclusively of dielectric materials, enabling the broadband and low-loss invisibility. In contrast to previous works based on nanostructures [6, 7], our cloak also shows advantages of easier design and fabrication. More importantly, the uniform grating profiles may be fabricated using large area interferometric lithography technique, and therefore, there is no size limitation on the invisibility cloak.

This design approach can be well extended to other frequency ranges. In microwave frequencies where we have access to dielectric materials with very high permittivity, the relative size of the cloak compared with the hidden object can be even more significantly minimized. With more precise fabrication, our scheme also holds promise for a true invisibility cloak that works in the visible parts of the spectrum and at a larger size, and has potential applications in integrated photonics and plasmonics.

References:
[1] J.B. Pendry, D. Schurig, D.R. Smith, “Controlling electromagnetic fields.” Science 312, 1780–1782 (2006). Abstract.
[2] U. Leonhardt, “Optical conformal mapping.” Science 312, 1777–1780 (2006). Abstract.
[3] Xianzhong Chen, Yu Luo, Jingjing Zhang, Kyle Jiang, John B. Pendry & Shuang Zhang, “Macroscopic invisibility cloaking of visible light.” Nature Communications, 2:176, (2011). Abstract.
[4] Baile Zhang, Yuan Luo, Xiaogang Liu, George Barbastathis,“Macroscopic invisibility cloak for visible light”, Phys. Rev. Lett. 106, 033901 (2011). Abstract.
[5] Jingjing Zhang, Liu Liu, Yu Luo, Shuang Zhang, and Niels Asger Mortensen, "Homogenous optical cloak constructed with uniform layered structures," Optics Express 19, 8625-8631, (2011). Abstract.
[6] J. Valentine, J. Li, T. Zentgraf, G. Bartal, X. Zhang, “An optical cloak made of dielectrics.” Nature Materials, 8, 568–571 (2009). Abstract.
[7] L.H. Gabrielli, J. Cardenas, C.B. Poitras, M. Lipson, “Silicon nanostructure cloak operating at optical frequencies.” Nature Photonics, 3, 461–463 (2009). Abstract.