Anisotropy of the Electron and Hole Drift Mobility in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>KNbO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>BaTiO</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

We determine the anisotropy of the mobility of photoexcited charge carriers in single crystals of ${\mathrm{KNbO}}_{3}$ and ${\mathrm{BaTiO}}_{3}$ by means of a purely optical method. In orthorhombic ${\mathrm{KNbO}}_{3}$ the mobility anisotropy is measured for both electrons and holes giving ${\ensuremath{\mu}}_{a}/{\ensuremath{\mu}}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.05\ifmmode\pm\else\textpm\fi{}0.06$, ${\ensuremath{\mu}}_{b}/{\ensuremath{\mu}}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.9\ifmmode\pm\else\textpm\fi{}0.3$ for electrons and ${\ensuremath{\mu}}_{a}/{\ensuremath{\mu}}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.15\ifmmode\pm\else\textpm\fi{}0.09$, ${\ensuremath{\mu}}_{b}/{\ensuremath{\mu}}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.9\ifmmode\pm\else\textpm\fi{}0.2$ for holes. In ${\mathrm{BaTiO}}_{3}$ we find for holes ${\ensuremath{\mu}}_{a}/{\ensuremath{\mu}}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}19.6\ifmmode\pm\else\textpm\fi{}0.6$. Our experiment also demonstrates that the acoustic phonon contribution to the dielectric permittivity is substantially different for spatially modulated electric fields as compared to the homogeneous case.