Laser pulse frequency up-shifts by relativistic ionization fronts N. C. Lopes, G. Figueira, J. M. Dias, L. O. Silva, J. T. Mendonça, Ph. Balcou, G. Rey and C. Stenz Europhys. Lett., 66 3 (2004) 371-377 Published online: 01 April 2004 DOI: 10.1209/epl/i2003-10225-9