Usual optical materials are composed of atoms that are often periodically arranged into a crystal structure. The corresponding period or lattice constant is less than one nanometer. Thus, the light field with wavelengths of several hundreds of nanometers effectively averages over this fine periodicity, it doesn’t “see” the granularity.
Artificial Optical Materials for the 21st Century
In periodic nanostructures, the atoms are replaced by nanostructured dielectric and/or metallic building blocks. If the period is roughly equal to half the wavelength of light, Bragg reflection occurs. This leads to a band structure for light, the “photonic band structure”. In analogy to the forbidden energy range - the band gap - for electrons in a semiconductor, such materials can show a complete band gap for the energy spectrum of photons, the “photonic band gap” (PBG). Thus, the surface of such PBG materials can act like a perfect dielectric mirror for light impinging from all directions. This allows for capturing, i.e., localizing light in nano-resonators and for guiding light in two and three dimensions with radii of curvature which were previously inaccessible. Furthermore, photonic crystals, PBG materials and wave guides therein allow tailoring the dispersion relation of light. For example, the group velocity of light can be made very small or even zero.
Theoretical and Experimental Challenges
The quantum optical and the nonlinear optical properties of atoms, molecules and semiconductor quantum dots in such a dielectric environment are distinctly different from usual free space. The inhibition or strong modification of spontaneous emission in a controlled fashion is just one fascinating example of basic research - the PBG material can act as a tailored quantum electrodynamic vacuum (see project A1.2: Light-Matter Interaction in Nano-Photonic Systems). At the same time, this new physics allows to design novel photonic devices such as ultralow threshold lasers or ultracompact and ultrafast nonlinear optical switching elements (see subproject A1.1: Theory of Photonic Crystal Structures and Concepts for Photonic Crystal-Based Devices). The controlled incorporation of "defects" can lead to allowed energies within the forbidden gap which then act like extremely narrow optical filters.
In addition to considerable design challenges, the actual fabrication of these generally three-dimensional nanostructures represents a major task as well. Using direct laser writing, which can be viewed as the three-dimensional counterpart of planar electron-beam lithography, complex photonic crystal architectures become accessible experimentally (see A1.4: Three-Dimensional Photonic Crystals).
If the wavelength of light is much larger than the period of the nanostructure, the light again averages over the fine details and “sees” an effectively homogeneous material, a “photonic metamaterials”. By suitably tailoring the metamaterial’s building blocks, such structures can effectively not only control the electric but also the magnetic component of the light field. This magnetism at optical frequencies is another route towards increased control of light and enables, e.g., negative phase velocities of light, giant circular dichroism, invisibility cloaking devices, or unusual optical nonlinearities (see subproject A1.5: Photonic Metamaterials). Ideally, the metamaterial’s optical properties can reversibly be tuned by external parameters, for example by simply applying an electric voltage (see subproject A1.6: Tunable Photonic Metamaterials).