1. A pedagogical discussion of self-focusing and self-trapping is presented in Chapter 6 of R. W. Boyd, Nonlinear Optics ( Academic, San Diego, 1992 ).
2. M. Hercher, J. Opt. Soc. Am. 54, 563 (1964).
3. R. Y. Chiao, E. Garmire, and C. H. Townes, Phys. Rev. Lett. 13,479 (1964); Erratum, ibid14, 1056 (1965).
4. G. A. Askar’yan, Zh. Eksp. Teor. Fiz. 42,1567 (1962); English translation: Sov. Phys. JETP 15, 1088 (1962).
5. The correction factor of 1/2 introduced by H. A. Haus, Appl. Phys. Lett. 8,128 (1966) has been included.