20 08 A metamaterial analog of electromagnetically induced transparency

We present a new type of electromagnetic planar metamaterial that exhibit strong dispersion at a local minimum of losses and is believed to be the first metamaterial analog of electromagnetically induced transparency. We demonstrate that pulses propagating through such metamaterials experience considerable delay, whereas the thickness of the structure along the direction of wave propagation is much smaller than the wavelength, which allows successive stacking of multiple metamaterial slabs. This leads to a significant increase in the band of normal dispersion, as well as in transmission levels. Changes in the velocity of light propagating through dispersive media have been the subject of extensive investigations in the past [1]. In fact, it was predicted [2] and later observed [3, 4] that light pulses can propagate with apparent velocities greater or smaller than in vacuum without strong distortion of shape and width due to pulse reshaping phenomena. Today, control over these effects is essential for the development of optical communication technologies leading to a number of different the quantum phenomenon of electromagnetically induced transparency (EIT) [17, 18]. In that case, an otherwise opaque atomic medium is rendered transparent in a narrow spectral region within the absorption line through quantum interference of a pump and a probe laser beam [17]. This sharp dispersion has important consequences such as dramatically reducing the group velocity [5] and enhancing non-linear interactions [19]. Most of the proposed techniques, however, involve special experimental conditions, e.g. cryogenic temperatures, coherent pumping and high intensities. Recently, the implementation of EIT-like behavior in classical systems has attracted a lot of attention, since in this case the operating frequency can be tuned simply by varying the system geometry, while no pumping is necessary. In particular, interference between coupled classical res-onators can lead to the same effects as EIT, i.e. narrow transmission resonances within the single-resonator stop-band. Nevertheless, existing approaches involve spatial dispersion at the wavelength scale, introducing, therefore , fundamental constraints on the minimum thickness of the medium. In this letter, we demonstrate a classical analogue of EIT in planar metamaterials, namely metal-dielectric slabs of vanishing thickness along the propagation direction, periodically patterned on a subwavelength scale. The phenomenon arises as a result of engaging " trapped-mode " resonances that are weekly coupled to the free space leading to low transmission losses and exceptionally high quality factors. Such modes are normally forbidden, but can be excited in planar metamate-rials with special patterning [29]. …