We show that the polarization states of electromagnetic waves can be manipulated through reflections by an anisotropic metamaterial plate, and all possible polarizations (circular, elliptic, and linear) are realizable via adjusting material parameters. In particular, a linearly polarized light converts its polarization completely to the cross direction after reflection under certain conditions. Microwave experiments were performed to successfully realize these ideas and results are in excellent agreement with numerical simulations. DOI: 10.1103/PhysRevLett.99.063908 PACS numbers: 42.25.Ja, 42.25.Bs, 78.20.Bh, 78.20.Fm Polarization is an important characteristics of electromagnetic (EM) waves. It is always desirable to have full control of the polarization states of EM waves. Conventional methods to manipulate polarization include using optical gratings, dichroic crystals, or employing the Brewster and birefringence effects, etc. [1,2]. Here we propose an alternative approach based on metamaterials [3][4][5][6]. Metamaterials have drawn much attention recently due to many fascinating properties discovered, such as the negative refraction [4], the in-phase reflection [5], and the axially frozen modes [6], etc. Here, we show that a specific metamaterial reflector can be employed to manipulate the polarization state of an incident EM wave. In particular, a complete conversion between two independent linear polarizations is realizable under certain conditions. We show the physics to be governed by the unique reflection properties of the metamaterial, and we perform experiments and finite-difference-time-domain (FDTD) simulations to demonstrate these ideas in the microwave regime.We start from studying a model system as shown in Fig. 1(a), which consists of an anisotropic homogeneous metamaterial layer (of a thickness d) with a dispersive relative permeability tensor $ 2 (with diagonal elements xx , yy , zz ) and a relative permittivity " 2 , put on top of a perfect metal substrate (with " 3 ! ÿ1, 3 1). We consider the reflection and refraction properties of the structure, when a monochromatic EM wave with a wave vectork in !=csincosx sinsinŷ cosẑ and a given polarization strikes on the surface. According to the Maxwell equations, EM waves should satisfyẼ2 k Ẽ inside the metamaterial layer withk the wave vector. Given k x and k y , the dispersion relation between ! and k z is determined by !=c 4 "ii ÿ1 jj k 2 l 0, where i; j; l x; y; z. The above equation has four roots corresponding to two refracted waves propagating forwardly and backwardly. The solution inside the second layer must be a linear combination of these four waves, manifesting the birefringence effect [1,7]. To match the boundary conditions, we must also expand the waves in other regions to linear combinations of four solutions, namely, the forward (backward) waves with s and p polarizations. The reflected beam thus generally consists of both s and p modes, even if the incident wave possesses one polarization. To solve these problems, we have extende...

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The theory of microwave thermal emission from a nonscattering half-space medium is developed for application to regions with nonuniform subsurface soil moisture and temperature variations. A coherent stratified model is presented which is valid for nonuniform temperature profiles and rapidly varying moisture profiles, under which conditions the commonly used emissivity and radiative transfer approaches become inaccurate. For naturally occurring profiles the stratified model gives more accurate results than the other approaches at frequencies below about 4 GHz. Experimental results from groundbased radiometric observations of a controlled target area compare systematically with brightness temperatures predicted from the theoretical model to within approximately 10øK. Results of dielectric constant measurements of the sand are given at seven frequencies in the microwave range and for moisture contents in the range 0-30% by volume. By using this model the thermal microwave emission spectrum is computed for a number of representative moisture and temperature profiles in the frequency range 0.25-25 GHz. A regression technique is used to show that multifrequency data can be used to obtain moisture and temperature gradients in the soil when an estimate of the surface temperature is available.

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