Barmentlo, M., Thooft, G. W., Eliel, E. R., van der Ham, E. W. M., Vrehen, Q. H. F., van der Meer, A. F. G., & van Amersfoort, P. W. (1994). Sum-Frequency Generation with a Free-Electron Laser - a Study of Gallium-Phosphide. Physical Review A, 50, R14-R17. https://doi.org/10.1103/PhysRevA.50.R14 (Original work published 1994)
DIFFER