Giving light yet another new twist

To completely control the flow of an electromagnetic wave inside of matter, one needs the ability to manipulate both the electric and magnetic components of light with microscopic electric and magnetic dipoles, respectively. While the atoms in natural solids generally do not provide this level of control, artificial atoms in metamaterials can. In a paper in Physical Review B, Eric Plum and coworkers at the University of Southampton in the UK and collaborators in the US, China, and Greece have fabricated such a structure, in which both the electric and magnetic components of light excite strong electric and magnetic dipoles [1]. At microwave frequencies this structure has an effective negative index of refraction. This means that the phase velocity of the wave is opposite to the electromagnetic energy flow. In a separate paper appearing in Physical Review Letters[2], Shuang Zhang and colleagues at the University of California at Berkeley and Oklahoma State University, both in the US, describe a closely related structure that can operate at THz frequencies. If these structures could be further engineered to work in optical and visible frequency ranges, strong optical activity might enable novel types of optical devices. In circularly polarized light, the tip of the electric field sweeps out a spiral in real space. The pitch of this spiral is simply the wavelength of light, while the handedness (or chirality) depends on whether the spiral sweeps clockwise or counterclockwise as the light wave moves along its path. It is intuitively clear that the interaction of circularly polarized light with matter that also has a spiral structure—a chiral molecule or an artificial nanostructure—will depend on the relative handedness between the two. This is the basis of the well-known optical activity in milk or sugar solutions, both of which contain chiral molecules that cause the polarization of an incident, linearly polarized wave to gradually rotate as it passes through the medium. It is perhaps less well known that the microscopic origin of this effect is magnetic dipoles that are excited by the electric component of the light and vice versa [3–7]. In a plane wave, the incident electric and magnetic vector components are perpendicular to each other. If, as the wave passes through a medium, the magnetic component induces an electric dipole parallel to the magnetic field vector, the resulting net local electric field vector will be rotated a bit. It can be shown that reciprocity demands that, likewise, magnetic dipoles are excited by the electric vector component. Hence the magnetic field vector rotates as well. Mathematically, the relations between the relevant components of the electromagnetic fields in the macroscopic Maxwell equations become D = e0eE + iκ/c0H and B = −iκ/c0E + μ0μH[3, 5]. Here, κ is the dimensionless chirality parameter and the other symbols have their usual meaning. The refractive index, n, for right(+) and left-handed (-) circular polarization becomes n± = (eμ)1/2 ± κ. For a sugar solution, however, the magnitude of the induced magnetic dipoles is extremely small and both refractive indices, n± , are positive, while the difference between them, (n+ − n−) = 2κ, is tiny. To observe a significant polarization rotation effect in sugar water, the light would have to pass through more than several centimeters (tens of thousands of visible wavelengths) of liquid. Metamaterials [8, 9] can boost this effect by several orders of magnitude. In place of molecules, one can engineer tiny, subwavelength resonant electromagnets that act as magnetic dipoles [10, 11]. Figure 1 shows an array of split-ring resonators that could make up a metamaterial: by Faraday’s law, a time-varying magnetic field perpendicular to the plane of these rings induces an os-