Photorefractive ac-enhanced nonlinear response of sillenites: Low- and high-contrast effects O. Filippov, K. H. Ringhofer and B. I. Sturman Eur. Phys. J. D, 23 2 (2003) 285-290 Published online: 11 February 2003 DOI: 10.1140/epjd/e2003-00042-8