Photonic Structures

A periodic modulation of the material's refractive index leads to dramatic change of its optical properties, when the modulation is in the length scale of the light wavelength. So-called photonic crystals are prominent examples of such synthetically produced materials. Typically, the periodic modulation of the optical properties is obtained by a alternating deposition of different materials. Already in the one-dimensional case a dielectric layer structure can exhibit a photonic bandgap. In analogy to an electronic bandgap in solid state physics where the existence of electrons with a specific energy is forbidden, a photonic bandgap inhibits the propagation of photons. The waves that are able to propagate in such periodic structures are described by the so-called Bloch modes.

The investigation of nonlinear effects in photonic crystals is one of our fields of research. For example, we have found an enhancement of the efficiency by two orders of magnitude in four-wave-mixing and phase conjugation experiments in one-dimensional synthetically produced photonic crystals.

To employ the technological potential of photonic crystals, it is crucially important to achieve dynamic tunability of their properties. Such tunability can be achieved in nonlinear photonic crystals where the actual band structure is influenced by an incident beam through self-induced refractive index changes. Such dynamically reconfigurable photonic structures can be induced optically in photorefractive crystals, where the refractive index can be modulated by the linear electro-optic effect, the Pockels-effect. This effect is not only responsible for the excitation of optical spatial solitons but allows for the investigation of light-induced photonic structures. An example for photonic structures with a full photonic bandgap in multiple dimensions is shown in Figure 1. When the photorefractive nonlinearity is exploited a modulated probe beam is able to excite fundamental band gap solitons from the second band that are mobile on the lattice. For a deeper understanding of these structures it is furthermore possible to excite individual Bloch waves.

If one does not address the weak electro-optic coefficient to produce a periodic refractive index modulation, but uses the strong nonlinearity in the direction of the crystallographic c-axis of the photorefractive crystal the single lattice sites undergo self-focussing. Nonlinear photonic structures created like this differ from their linear counterparts that are used as light-induced photonic crystals. In general, the modulation of the light field and and the resulting modulation of the refractive index are significantly different. The investigation of Bloch modes that are able to propagate in such a structure and their properties is a current topic of research.