Frequency conversion by advanced phase matching
Only for a perfect phase-matched frequency conversion process, an accumulation of intensity of the generated light filed can be achieved. The absence of phase-matching leads to an oscillating energy transfer of the contributed light fields. The origin of the phase mismatch lies in the inherent presence of dispersion. However, to ensure an efficient conversion process commonly the birefringence of the crystals is exploited. This means that the refractive index depends on the polarization as well as propagation direction. Depending on the specific degree of birefringence the bandwidth of the accessible wavelengths of the conversion process is limited.
To increase the bandwidth tremendously by more than one order of magnitude, we developed the concept of the nonlinear beam splitter. By structuring a nonlinear material by means of tightly focused ultrashort laser pulse (Link: DLW), we create a refractive index structure, which acts as an integrated volume phase grating. An incoming fundamental light field is diffracted by the volume phase grating and the phase matching condition is fulfilled between the diffracted light fields in a noncollinear geometry. Using a computer-generated hologram instead of a basic grating, we can in addition modulate the amplitude of the generated higher harmonic light field and thus create complex light fields.
Beside this noncollinear phase matching configuration with integrated volume phase grating, femtosecond laser-induced structure also show a reduced Χ(2) nonlinearity In the modified area. Similar to periodically poled crystals this can be exploited for all-optical induced quasi phase matching.