An optical "Janus" device for integrated photonics.

Adv. Mater. 2010, 22, 2561–2564 2010 WILEY-VCH Verlag G T IO N In Roman mythology, the god Janus was depicted with two faces, looking in opposite directions. This led to the phrase ‘‘Janus faced’’ which is mostly used for a ‘‘two-faced’’ or deceitful character of a person. Within integrated photonics a concept like Janus can provide a new class of multi-functional optical meta-elements which could be a key ingredient in achieving compact and high speed photonic systems.While there have been great strides in the miniaturization of optical elements, such photonic integration largely consists of combining discrete components at the chip level. Here, we present a new approach of designing a single optical element that possesses simultaneously multiple distinct functions. We employ transformation optics to design the optical space and manipulate the light propagation using a metamaterial with spatially varying permittivity. Our experiment demonstrates a single optical ‘‘Janus’’ device that acts as a lens as well as a beam-shifter at the same time. The emerging field of transformation optics has provided a new design methodology allowing an unprecedented manipulation of light propagation, with the optical cloak as the most prominent example. However, transformation optics can also be used to enhance the functionality of conventional optical elements. Traditionally, these conventional elements only involve stretching or compressing the optical space in one direction whereas the remaining dimensions in space are unaltered. For example, an optical lens can be interpreted as a result of a simple wavefront transformation that molds the flow of light in a particular direction. A lens works well in one direction whereas light propagating perpendicular to this direction is strongly perturbed. Since space can be modified in two or three dimensions simultaneously, the additional degrees of freedom provided by transformation optics can be used to spatially imprint elements into a single optical Janus or metadevice. Here, we present a transformation optics design approach together with an experimental demonstration that takes advantage of this dimensionality by integrating multiple, independent optical elements into the same footprint. Electromagnetic waves passing through a physical space can be caused to experience a predetermined virtual space by modifying the underlying material properties in a spatial manner. This is possible because of the invariance of Maxwell’s equations under a spatial transformation. Therefore, transformation optics opens the possibility to precisely control the flow of light in a medium and thus allow to design novel devices for microwave and optical frequencies such as invisibility cloaks, light concentrators, beam manipulators, object transformers, and even electromagnetic wormholes and blackholes. On the other hand, transformation optics can also be used to improve conventional optical elements. For example, it is possible to transform a Luneberg lens into a lens with flat focal plane without aberration. The flexibility to control the flow of light at will can also be used to design a new class of optical metadevices that can provide different optical functionalities for different light propagation directions. We demonstrate this capability of transformation optics by imprinting multiple and dissimilar optical functions into one single optical element. This is done by taking advantage of the fact that transformation optics provides a design methodology by which different directions in space can be manipulated independently by employing a well-defined two dimensional permittivity profile. This is in contrast to conventional optical elements which normally alter the phase front only in one propagation direction, without taking advantage of the fact that the refractive index can be engineered for the entire two dimensional space. As an example of a metadevice, we combine a lens with a beam-shifter, acting perpendicular to it, into the same device. Furthermore, we show the flexibility of the transformation optics approach by replacing the lens of the metadevice with a different functionality, in this case, a second beam-shifter. This level of interchangeable dual-functionality cannot be obtained with conventional optical design. Hence, our scheme paves a new way in designing more compact elements in optical integrated circuits. The twometadevices are designed by reshaping the boundaries (black lines in Fig. 1) of a virtual homogeneous space into a physical inhomogeneous space. The horizontal lens and the vertical shifter (Fig. 1a) are created by the compression of the upper and lower boundaries (which originate from the circumference of the same circle) to flat facets. Due to the fact that rays in the untransformed space must stay perpendicular to the boundaries, the rays have to bend (shown by the dark grey arrows) inside the transformed medium. At the same time, the left and right boundaries are compressed, resulting in a device that can focus a diverging wave into a converging wave, i.e., the element behaves as a lens in the horizontal direction. The same principle