Advantageous properties of metamaterials enable a quite general broad range of applications in numerous components and systems.

From the beginning of XXI century, interest in metamaterials has grown explosively. The potential take-up of these structures in applications like communication and sensing systems is primarily due to the control of the amplitudes, frequencies and wavenumbers of propagating and non-propagating electromagnetic modes enabled by metamaterials to an extent that was not previously possible. The control of electromagnetic modes in the application fields (indoor and outdoor communication systems, communicating mobile objects, transport systems (high-speed trains), space/earth communication and microelectronics; at µ-wave and sum-mm wave frequencies) are relevant to the Metamorphose goals.

Significant progress in understanding the physics of metamaterials is achieved in the past few years. Along with this understanding, siginifcant research efforts on metamaterials have started all over the world. We briefly review the recent developments.

In the DNG and MNG metamaterials research area, a great effort is devoted to push the metamaterial properties from the microwaves to the IR and optical regime. In 2006 a number of papers on experimental demonstration of IR metamaterials were published: negative magnetic permeability in the far-infrared frequency regime of 6 THz [1], double negative (SRR+wire) metamaterial operating at 100 GHz [2], single-slit SRRs with magnetic resonance at a wavelength of 900 nm [3]. Related theoretical work demonstrated the ability of negative values of µ [4] in single-ring multi-cut SRR designs up to optical frequencies of 500 THz. In this last paper it is concluded that the highest resonance frequency at which µ < 0 increases with the number of cuts in the SRR. The idea of multi-cut SRRs is discussed in [5]. It is observed that increasing the split width, gap distance and metal width causes magnetic resonance frequency to shift to higher frequencies. A review covering many of the developments over the whole spectral range has been recently been published [6].

An alternative to the SRR design for the achievement of negative permeability (and, under certain conditions, negative refractive index), is a pair of short metallic wires or plates, separated by a dielectric layer. This design, which was originally proposed by Sarychev et al. [7] for optical metamaterials, has been recently exploited in the area of microwave metamaterias [8]. The design introduces a new topology for the one-dimensional metamaterials, in which the proapagation direction of the electromagnetic wave and the layer stacking direction of the metamaterial coincides, and thus allowing the fabrication of multilayer metamaterials at submicron scale with existing fabrication techniques. In 2006, negative refraction in the few GHz regime was demonstrated for pairs of H-shaped wires [9], as well as for combinations of pairs of short and continuous wires [10], which conform to this topology. Symmetrized alternatives to the wires-pair design were also proposed, like a system of parallel plates [11] and a cross-like structure [12]. Moreover, combination of plate pairs connected with continuous wires (fishnet structure) have been studied [13], and have been demonstrated to show negative refraction with low losses and peculiar propagation characteristics in the 1.5 µm regime [14].

[1] Gundogdu TF, Tsiapa I, Kostopoulos A, Konstantinidis G, Katsarakis N, Penciu RS, Kafesaki M, Economou EN, Koschny T, Soukoulis CM, "Experimental demonstration of negative magnetic permeability in the far-infrared frequency regime." Applied Physics Letters 89, 084103 (2006).
[2] Gokkavas M, Guven K, Bulu I, Aydin K, Penciu RS, Kafesaki M, Soukoulis CM, Ozbay E , "Experimental demonstration of a left-handed metamaterial operating at 100 GHz." Physical Review B 73, 193103 (2006).
[3] Klein MW, Enkrich C, Wegener M, Soukoulis CM, Linden S , "Single-slit split-ring resonators at optical frequencies: limits of size scaling," Optics Letters 31, 1259 (2006).
[4] Zhou J, Koschny T, Kafesaki M, Economou EN, Pendry JB, Soukoulis CM , "Saturation of the magnetic response of split-ring resonators at optical frequencies," Phys. Rev. Lett. 95, 223902 (2005).
[5] Aydin K, Bulu I, Guven K, Kafesaki M, Soukoulis CM, Ozbay E , "Investigation of magnetic resonances for different split-ring resonator parameters and designs," New Journal of Physics 7, 168 (2005).

[6] Johnson NP, Khokhar AZ, Chong HM, De La Rue RM, Antosiewicz TJ and McMeekin S, "A review of size and geometrical factors influencing resonant frequencies in metamaterials" Opto-Electronics Review 14(3), 187-191, 2006
[7] K. Sarychev, R.C. McPhedran and V.M. Shalaev, "Electrodynamics of metal-dielectric composites and electromagnetic crystals," Phys. Rev. B 62, 8531-8539 (2000); V.A. Podolskiy, A.K. Sarychev, and V.M. Shalaev, "Plasmon modes and negative refraction in metal nanowire composites," Opt. Express 11, 735-745 (2003).
[8] J. Zhou, E.N. Economou, Th. Koschny, and C.M. Soukoulis, "Unifying approach to left-handed material design", Opt. Lett. 31, 3620-3622 (2006).
[9] Zhou JF, Koschny T, Zhang L, Tuttle G, Soukoulis CM, "Experimental demonstration of negative index of refraction," Appl. Phys. Lett. 88: 221103 (2006).
[10] K. Guven, M.D. Caliskan, and E. Ozbay, "Experimental Observation of left-handed transmission in a bilayer metamaterial under normal-to-plane propagation," Opt. Express 14, 8685 (2006).
[11] G. Dolling, C. Enkrich, M. Wegener, J.F. Zhou, C.M. Soukoulis, and S. Linden, "Cut-wire pairs and plate pairs as magnetic atoms for optical metamaterials," Opt. Lett. 30, 3198 (2005).
[12] C. Imhof and R. Zengerle, "Pairs of metallic crosses as a left-handed metamaterial with improved polarization properties," Opt. Express 14, 8257 (2006).
[13] S. Zhang, W. Fan, K.J. Malloy, S.R. Brueck, N.C. Panoiu, and R.M. Osgood, "Near-infrared double negative metamaterials," Opt. Express 13, 4922 (2005).
[14] G. Dolling, C. Enkrich, M. Wegener, C.M. Soukoulis and S. Linden, "Observation of simultaneous negative phase and group velocity of light," Science 312, 892 (2006).

Planar 2-D periodic rectangular lattices of reactive L-C circuits have been considered as the wave channeling structure [1-3]. The generic conditions of wave propagation on such lattice networks have been established. The only anisotropic lattice with the constituent reactances of dissimilar types (capacitive or inductive) can support propagating waves. Such lattices guide the waves from a point source only along certain directions confined to the narrow sectors, which are referred to as channels. The parallel and series L-C circuits acting as the lattice reactances provide opportunities for altering handedness of the channelled waves without physical interchange of the L and C positions in the lattice. Therefore such lattice arrangements appeared to be particularly attractive for implementation of tunable medium where the parameters of constituent reactances can be externally controlled by the bias applied to the inclusions containing ferroelectric, liquid crystal or magnetic materials.

The mechanism of wave channelling by 2-D anisotropic lattice of lumped reactances suggests that the channel direction, impedance and phase can be controlled by the tunable elements embedded in the lattice reactances. The performance of finite lattices with tunable inclusions, comprising 12×12 unit cells have been modelled in ADS circuit simulator using hierarchal networks [4]. The analysed unit cell configurations (Fig. 1) included double series (SSM), double parallel (PPM) and mixed parallel-series (PSM) L-C circuits and the reference basic L-C unit cell. The choice of the unit cell configurations was dictated by a trade-off between the circuit simplicity and tuning capability in both arms of the unit cell. The channel direction and handedness of the channelled wave can be tuned in the designated frequency band by varying only capacitance of the L-C circuits. The tunable lattices can be applied to beam steering or phase compensation.

Another approach is an array of wire dipoles with tunable L-C loads, which can be designed as a periodic array of thin wire dipoles loaded by parallel L-C circuits. These arrangements are of practical importance for the realisation of metasurfaces with tunable parameters. The generic framework for the rigorous self-consistent analysis of metasurfaces composed of double periodic arrays of wires loaded with lumped linear and nonlinear inclusions has been developed. In particularly, it was shown that a purely inductive load reduced the array resonance frequency while the purely capacitive load has an opposite effect. Switching between L and C loads in the parallel L-C circuit arrangement allows the array to operate as the dual band shutter. When both L and C are engaged in the loading L-C circuit, resonances of the loaded array are offset from the unloaded wire array resonance where the loaded array becomes transparent. Thus it has been demonstrated that transparency of such metasurface can be readily controlled by triggering the elements of the parallel L-C circuit loading periodic array of thin wire dipoles.

Many microwave devices are based on tunable metasurfaces (high-impedance surfaces) [5]:
- electrically controllable conformal ("horizontal") antennas (printed dipole and patch antennas) as tunable antenna substrates improving the antenna matching and the efficiency (compared to the conventional metal-backed dielectric substrate) [6];
- leaky-wave antennas (scanning and pattern control in both E and H planes) [7,8];
- antennas for efficient radiation beam steering [7, 9-11];
All-angle and both polarization absorbers based on metasurfaces, where the absorption frequency can be efficiently controlled are of high interest [6,7,12]. As the tunable components, p-i-n-diodes, semiconductor / ferroelectric varactors, and MEMS are used. Tunable metasurface controlled by MEMS designed for mm-wave range was investigated in [13, 14].

1. E. Brennan, A. Gardiner, A. G. Schuchinsky, V. F. Fusco. "Channelled Waves in Anisotropic Mesh Metamaterials", Proceedings of the IET Microwaves Antennas and Propagation, vol.154, no.1, Feb.2007.
2. E. Brennan, A. G. Schuchinsky, V. F. Fusco. "Waves in 2-D Anisotropic L-C Lattice Metamaterials: Phenomenology and Properties" Microwave and Optical Technology Letters, vol.48, no.12, Dec.2006, pp.2538-2542.
3. J. K. H. Wong, K. G. Balmain, G. V. Elegtheriades "Fields in Planar Anisotropic Transmission-Line Metamaterials," IEEE Trans. Antennas Propagation, vol.54, no.10, pp.2742-2749, Oct.2006
4. A.Schuchinsky, E. Brennan, V. Fusco, "Wave Channelling in 2-D Anisotropic Lattice Metamaterials with Tunable L-C Parameters", accepted for presentation on EuMC37, 2007, Munich
5. O. Luukkonen, C. Simovski, S. Tretyakov, "Microwave devices based on tuneable metasurfaces," accepted for presentation on EuMC37, 2007, Munich
6. N. Engheta and R. Ziolkowski, Eds, Metamaterials: Physics and Engineering Explorations, Wiley, NY, 2006.
7. C. Caloz and T. Itoh, Electromagnetic metamaterials: transmission line theory and microwave applications. Wiley: NY, 2006.
8. D. Sievenpiper, J. Schaffner, J. J. Lee, and S. Livingston, "A Steerable Leaky-Wave Antenna Using A Tunable Impedance Ground Plane," Antennas and Wireless Propagation Letters, vol.1, no.1, p.179, 2002.
9. D. Sievenpiper, J. Schaffner, J. J. Lee, and S. Livingston, "A Tunable Impedance Surface Performing as a Reconfigurable Beam Steering Reflector," IEEE Trans. Antennas Propag., vol.50, no.3, p.384, 2002.
10. D. Sievenpiper, J. Schaffner, H. J. Song, R. Y. Loo, and G. Tangonan, "Two-dimensional beam steering using an electrically tunable impedance surface," IEEE Trans. Antennas Propag., vol.51, no.10, p.2713, 2003.
11. G. Eleftheriades, K.G Balmain, Negative-Refraction Metamaterials: Fundamental Principles and Applications, Wiley: NY, 2006.
12. S. Simms and V. Fusco "Thin radar absorber using artificial magnetic ground plane," Electron. Lett., 41(24):1, 2005.
13. D. Chicherin, S. Dudorov, D. Lioubtchenko, V. Ovchinnikov, S. Tretyakov, and A. Räisanen, "MEMS-based high-impedance surfaces for millimeter and submillimeter wave applications," Microw. Opt. Technol. Lett., vol.48, no.12, p.2570, 2006.
14. D. Chicherin, S. Dudorov, D. Lioubtchenko, V. Ovchinnikov, and A. Räaisanen, "Millimetre Wave Phase Shifters Based on a Metal Waveguide with a MEMS-Based High-Impedance Surface," Proc. 36th Eur. Microw. Conf., p.372, 2006.