1. R. Y. Chias, E. Garmire and C.H. Townes, Phys. Rev. Lett. 13 (1964), 479–482.
2. P.L. Kelley, Phys. Rev. Lett. 15 (1965), 1005–1012
3. V.I. Talanov, JETP Lett. 2 (1965), 138–141.
4. J.B. Baillon, T. Cazenave and M. Figueira, C.R. Acad. Sci. 284 (1977), 869–872.
5. W.A. Strauss, Anais Acad. Brasil. Ciencias 42 (1970), 645–651.