PLANAR L-C BASED NEGATIVE-REFRACTIVE-INDEX METAMATERIALS

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

Recently there has been a proliferation of emerging new man-made materials with superior properties that cannot be found in nature. For this reason, these materials are referred to as "metamaterials" (the prefix "meta" means "beyond" in Greek). 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.




Materials with such exotic properties have the potential to radically change the world of wireless communications, radar and surveillance. Their unique characteristics will enable unprecedented levels of antenna miniaturization, antenna beam steering, and signal switching and routing. In addition, the technology may enable the creation of miniaturized RF lenses with a unique sub-wavelength resolution capability [2], as well as ultra-fast signalling between two points.

 
 

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 class of LHM metamaterials that take a step further than the original wire/SSR concept. These new LHM metamaterials consist of connected unit cells that do not excplicitly 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 offer large operating bandwidths, and are completely planar thus inherently supporting 2-D wave propagation making them well suited for RF/microwave device and circuit applications [5], [6].

 

We have verified through simulation that our structures exhibit negative refraction and focusing. Furthermore, a 30mm X 55mm LHM prototype has been implemented and tested around 2GHz. Recent experimental results demonstrated focusing of an incident cylindrical wave over an electrically small area, a phenomenon suggestive of near-field focusing.[5], [6].

 

Based on the same concept of devising left-handed metamaterials, a backward radiating planar antenna was implemented and tested at 15 GHz [7], [8]. This is perhaps the first experimental demonstration of backward wave radiation from a left-handed medium, an effect analogous to reversed Cherenkov radiation originally predicted by Veselago [1].

In the figures to follow, we provide a sample of the progress we have made so far.

Figure 2: Microwave circuit simulations showing a plane wave illuminating a RHM/LHM interface at an incident angle of 29°.

 

The refractive indices of the RHM and LHM are +1.2 and -2.4, respectively. Refraction is 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. The refractive indices of the RHM and LHM are +1.2 and -2.4, respectively. Focusing is observed in both phase and magnitude; the axes are labelled according to cell number.


Figure 4: Focusing device: LHM prototype interfaced with a parallel-plate waveguide (60mm X 95mm); the inset magnifies the surface of the LHM unit cell..

Figure 5: Correspondence of (a) full-wave field simulation results and (b) experimental results at 1.5GHz (normalized to the maximum respective focal amplitudes in each case).



Figure 6: Experimentally detected vertical E-field distribution 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, over which the index of refraction is determined to vary from -5.5 to -1.2, with a well-confined focal spot near 1.5GHz.


REFERENCES


 


[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 and G.V. Eleftheriades, "Negative refractive index metamaterials supporting 2-D waves."  IEEE International Microwave Symposium Digest, pp. 1067-1070, June 2-7, 2002, Seattle, WA

[6]     G.V. Eleftheriades, A.K. Iyer and P.C. Kremer, "Planar negative refractive index media using periodically L-C loaded transmission lines." Accepted in the IEEE Trans. on Microwave Theory and Techniques.

[7]     A. Grbic and G.V. Eleftheriades, "A backward-wave antenna based on negative refractive index L-C networks." Proc. of the IEEE Intl. Symposium on Antennas and Propagation, Vol. IV, pp. 340-343, June 16-21, 2002, San Antonio, TX

[8]     A. Grbic and G.V. Eleftheriades, "Experimental verificiation of backward-wave radiation from a negative refractive index metamaterial." Accepted in the Journal of Applied Physics.

[9]     G.V. Eleftheriades, "Planar Negative Refractive Index Metamaterials Based on Periodically L-C Loaded Transmission Lines."  Workshop of Quantum Optics, Kavli Inst. of Theoretical Physics, University of Santa Barbara, July 2002 (invited).

[10]    G.V. Eleftheriades, A.K Iyer, A. Grbic and O. Siddiqui, "Negative Refractive Index Metamaterials Based on L-C Loaded Transmission Lines."   Progress In Electromagnetic Research Symposium (PIERS), July 2002, Boston.

[11]    G.V. Eleftheriades, O. Siddiqui and A. Iyer, "Transmission line models for negative refractive index media and associated implementations without excess resonators."  Accepted in the IEEE Microwave and Wireless Components Letters.