Experimental Verification of Focusing in Compact, Planar Left-Handed Metamaterials

In the late 1960s, V.G. Veselago proposed that materials with simultaneously negative permittivity and permeability are physically permissible and possess a negative index of refraction [1]. Veselago termed these Left-Handed Media (LHM), because the vectors E, H, and k would form a left-handed triplet instead of a right-handed triplet, as is the case in conventional, Right-Handed Media (RHM). His conceptual exploration of this phenomenon revealed that, through negative refraction, planar slabs of such media would cause light or electromagnetic radiation to focus in on itself, as depicted in Figure 1.

Figure 1: Internal and external focusing using a LHM slab of thickness d.

Recently, novel electromagnetic metamaterials have successfully demonstrated negative refraction and suggest an approach whereby the permittivity and permeability functions are made to be simultaneously negative using an array of resonant cells consisting of thin wire strips and Split-Ring Resonators (SRRs), respectively [2]-[4].

Our research has yielded a new metamaterial architecture that consists of connected unit cells that do not rely on resonances to synthesize the required negative material parameters. The unit cells are equipped with (possibly tunable) lumped elements (inductors and capacitors), which permit them to be compact and therefore scalable from the MHz to the tens of GHz range. Moreover, these new metamaterials are planar and inherently support 2-D wave propagation, making them well suited for RF/microwave device and circuit applications [5].

We have also verified through simulation that our structures exhibit negative refraction and focusing, and a 30mm´55mm LHM prototype has been implemented and tested experimentally. Our results are to be presented at the IEEE MTT-S International Microwave Symposium in Seattle, Washington, on Wednesday June 5, 2002.

In the figures to follow, we provide a sample of the progress we have made in demonstrating negative refraction and, to the best of our knowledge, the first experimental verification of focusing phenomena using our planar metamaterials.

Figure 2: Microwave circuit simulations showing a plane wave illuminating a RHM/LHM at an incident angle of 29°. Refraction observed at -14°, in accordance with Snell’s Law. The axes are labeled according to cell number, and the right vertical scale designates radians.

Figure 3: Microwave circuit simulations showing a point source illuminating a RHM/LHM interface. Focusing is observed in both phase and magnitude; the axes are labeled according to cell number.

Figure 4: Experimentally obtained vertical E-field distributions over a 30mm´55mm LHM as the frequency is varied from 1 to 2GHz; focusing is apparent over a band extending from approximately 1.3 to 1.9GHz, with a well-confined focal spot near 1.5GHz.

The proposed media possess several desirable features including compactness, the ability to guide 2-D TM waves, practical operating bandwidths, scalability from RF to millimetre-wave frequencies, as well as the potential for tunability. The proposed concept has been verified with circuit and full-wave simulations, and a prototype focusing device has been implemented and tested experimentally. The experimental results demonstrated focusing of an incident cylindrical wave over an electrically short area, a phenomenon suggestive of near-field focusing. These results shall be presented at the IEEE MTT-S International Microwave Symposium in Seattle, Washington, on Wednesday June 5, 2002. We expect that new enabling RF/microwave devices can be implemented based on these proposed planar negative refractive index media for applications in wireless communications, surveillance, and radars.

[1] V. G. Veselago, “The electrodynamics of substances with simultaneously negative values of e and m,” Sov. Phys. Usp, vol. 10, no. 4, pp. 509-514, Jan.-Feb.1968.

[2] J. B. Pendry, A. J. Holden, D. J. Robins, W. J. Stewart, “Magnetism from conductors and enhanced nonlinear phenomena,” IEEE Trans. on Microwave Theory and Tech., vol. 47, no. 11, pp. 2075-2084, Nov. 1999.

[3] D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, S. Schultz, “Composite medium with simultaneously negative permeability and permittivity,” Phys. Rev. Lett., vol. 84, no. 18, pp. 4184-4187, May 2000.

[4] R. A. Shelby, D. R. Smith, S. Schultz, “Experimental verification of a negative index of refraction,” Science, vol. 292, 6 April 2001, pp. 77-79.

[5] A. K. Iyer, G. V. Eleftheriades, “Negative refractive index metamaterials supporting 2-D waves,” To be presented in the IEEE MTT-S International Microwave Symposium in Seattle, Washington, June 2–7, 2002.