How can we create a nonlinear material


In non-linear optics, new effects such as self-focusing and frequency conversion occur in that light changes a material in a non-linear manner. The prerequisites for non-linear optical effects are generally high light intensities such as can be generated with short laser pulses and a sufficiently large non-linearity of the medium. The non-linear response of a material in the second order (so-called Χ(2)-Non-linearity) leads to the generation of new light frequencies such as twice the frequency (SHG: Second Harmonic Generation). So that the frequency conversion is as efficient as possible, an effective phase adjustment must be fulfilled.

We structure the linear and non-linear properties of crystals in order to tailor them specifically for non-linear photonic applications. The structuring is preferably carried out with light or with electric fields. We modulate the refractive index of crystals permanently in three dimensions by means of direct laser beam writing with fs laser pulses or reversibly with cw lasers using the photorefractive effect. The Χ(2) Nonlinearity can be spatially changed in ferroelectric crystals by applying an electric field above the coercive field. In this way, we produce nonlinear photonic structures which, depending on their dimension (1D or 2D) and order (from periodic to disordered), have different useful phase-matching conditions (.modulated Χ(2) materials). We examine the structure of non-linear photonic crystals using a special type of SHG laser scanning microscopy (.SHG microscopy). We develop advanced phase matching methods using light-induced gratings that either function as diffraction gratings (noncollinear phase matching) or in which the Χ(2) Non-linearity is modulated (light-induced quasi phase adjustment).