High-temperature recording of strong permanent diffraction gratings in copper-doped lithium niobate crystals based on a zero-electric-field photorefractive effect

摘要

Diffraction gratings, which do not rely on the electro-optic effect, are recorded in copper-doped lithium niobate crystals: At elevated temperatures (approx. 180℃), the crystals are illuminated by an interference pattern. The light pattern excites electrons from filled Cu~+ traps into the conduction band. They are captured by empty Cu~(2+) traps elsewhere. The resulting electric fields are compensated by ions, such as H~+, that are thermally mobilized. Therefore the redistribution of electrons can continue which finally results in a high modulation of Cu~+ and Cu~(2+) traps. This process is called "thermal fixing" and is known since decades. However, here we report about a novel and relevant effect resulting from the electron concentration grating. The Cu~+ traps absorb mainly in the near UV and visible spectral range, Cu~(2+) traps have their absorption peak in the near infrared. A modulation of Cu~+ and Cu~(2+) trap concentrations therefore results in a pronounced absorption modulation. This immediately implies an absorption grating, but another consequence is even more dramatic: The Kramers-Kronig relations link changes of the absorption to changes of the refractive index. Thus the narrow absorption bands of copper in lithium niobate lead to refractive index changes over a vast wavelength range. Maximum index changes of the order of 10~(-4) are achieved. The resulting gratings can be recorded in any crystal orientation and are of interest for applications in non-linear optics.