Photonic metamaterials are artificial structures with inter-“atomic” distances (or “lattice constants”) that are smaller than the wavelength of light. Thus, the light field “sees” an effective homogeneous material for any given propagated direction. The building blocks (“atoms”), however, are not real atoms but are rather made of many actual atoms, often metallic. It is this design aspect that allows tailoring the electromagnetic material properties, in particular the corresponding dispersion relation of light, to a previously unprecedented degree. For example, magnetic dipoles reacting to optical frequencies become possible. This is in sharp contrast to conventional “optics textbook wisdom” which states that the magnetic permeability µ is unified at optical frequencies for all materials. A negative µ together with a negative electric permittivity e can lead to a negative index of refraction n, offering new options for applications (e.g. “perfect” lenses) and science (e.g. different nonlinear optical selection rules).

A team of the CFN (S. Linden and M. Wegener) realized in cooperation with C.M. Soukoulis (Iowa State University) the first metamaterials worldwide with a magnetic response at near-optical frequencies, i.e. at 3 µm wavelength (Science 306, 1351 (2004)).
Later, using improved nanofabrication of split-ring resonators, the same team shifted the magnetic resonance to 1.5 µm wavelength (Phys. Rev. Lett. 95, 203901 (2005)) for the first time. More recently, this value was further improved to 900 nm wavelength (Opt. Lett., in press (2006)).
Furthermore, alternative “magnetic atoms” designs, aiming at easing the nanofabrication, have also been realized (Opt. Lett. 30, 3198 (2005)) by the same team. These results have directly led to low-loss negative-index metamaterials around 1.5 µm wavelength. In direct femtosecond pulse-propagation experiments on these samples it has been shown that both phase and group velocity of light can be negative (Science, in press).
This means that both the carrier wave and the pulse envelope peak appear at the rear side of the sample before their counterparts have entered the front side of the sample – the negative-index analogue of the Garrett and McCumber effect.

This research has led to three recent international and national awards/honors: The European Union René Descartes Award for Cooperative Research 2005 (E. Ozbay, J. Pendry, C.M. Soukoulis, D. Smith, and M. Wegener), a “Helmholtz-Hochschul-Nachwuchsgruppe” starting in 2006 (S. Linden), and the International Carl Zeiss Research Award 2006 (K. Busch and M. Wegener).

Another emerging class of optical materials comprises three-dimensional photonic crystals and photonic band gap materials, independently introduced by S. John and E. Yablonovitch in 1987.
Such three-dimensionally periodic dielectric structures with lattice constants in the order of half the relevant wavelength of light (few hundreds of nanometers) can have a complete energy gap in the dispersion relation of light in analogy to the band gap of electrons in usual semiconductors. Thus, these materials are often referred to as “semiconductors for light”. They offer new options for applications and science. Applications are envisioned, e.g. in telecommunications via “molding the flow of light”, as novel lasers, as efficient thermal light emitters, or as novel efficient materials for nonlinear optics.
New scientific avenues are foreseen, e.g. in quantum optics using photonic crystals as a “designer vacuum” in which spontaneous emission can be totally different from ordinary vacuum.

The nanofabrication of such structures for telecommunication wavelengths poses severe challenges. For the visible regime it has not yet been achieved by any means. The best three-dimensional photonic crystal templates worldwide using the technique of direct laser writing have been fabricated in the CFN (K. Busch, G. von Freymann, S. Linden, and M. Wegener). These structures first approached telecommunication wavelengths (Nature Materials 3, 444 (2004) and Appl. Phys. Lett. 85, 1895 (2004)) – a cooperation with C.M. Soukoulis (Iowa State) and S. John (Toronto).
A recent direct experiment-theory comparison has revealed outstanding sample quality (Appl. Phys. Lett. 87, 221104 (2005)). 3D-2D-3D photonic crystal heterostructures have been realized for the first time (Opt. Lett. 31, 805 (2006)), paving the road for complex three-dimensional photonic circuitry – a cooperation with S. John (Toronto). High-index three-dimensional photonic band gap materials have been realized by replication of photoresist templates in silicon (Adv. Mater. 18, 457 (2006)) or by direct laser writing into chalcogenide glasses (Adv. Mater. 18, 265 (2006)), both for the first time – in cooperation with G. Ozin (Toronto) and S. John (Toronto).

This research has led to two recent international/national awards/honors: A “DFG-Emmy Noether Nachwuchsgruppe” starting in 2005 (G. von Freymann) and the International Carl Zeiss Research Award 2006 (K. Busch and M. Wegener).