Associate Professor Martijn Wubs and his colleagues at DTU Fotonik have discovered a way to look beyond the refractive index of metamaterials with quantum states of light. This may apply in the development of metamaterial lenses. Their discovery links the metamaterial and the quantum optics communities. Their work is featured in Physical Review Letters.
Metamaterials are designed to have effective refractive indices that fully describe their unusual optical properties. However, in this Letter we show that by probing metamaterials with quantum states of light, we can 'open the effective-index black-box' and tell apart different metamaterials that have the same refractive index. Metamaterials are man-made periodic structures composed of unit cells much smaller than the wavelength of light. They are engineered to behave effectively as a homogeneous medium, with effective dielectric parameters often not occurring in nature, such as a negative refractive index. Our results go against the common belief that optical experiments do not reveal information about the unit cell beyond the usual effective refractive index. Metamaterials often contain metals, which are lossy. A popular strategy is to compensate for the losses by adding gain material. Especially these loss-compensated metamaterials affect quantum states of light differently than the usual effective-medium theories predict. We identify an additional effective parameter of metamaterials that is needed to describe well the propagation of quantum states of light.