The steady-state field gratings recorded in photovoltaic photorefractive materials have been investigated theoretically for high modulation's depths of the light intensity pattern (up to modulation depth m/spl ap/1). Several numerical methods have been used to deal with this strongly nonlinear regime and their capability critically evaluated. The number of harmonics contributing to the field gratings increases very strongly with m above m/spl gsim/0.8, leading to a strong localization of the fringes. The dependence of grating amplitude has been found to be superlinear on m. The role of the k vector, impurity concentration, and oxidation/reduction state on the holographic recording efficiency has also been analyzed.< >