  We develop a theory of the propagation and focusing of the THz Gaussian laser
beam through the layered superconductor slab of finite thickness in the
presence of an external DC magnetic field in a nonlinear regime. We show that,
in this case, focusing of radiation, which results from the specific
nonlinearity of the medium, can be flexibly tuned by the external magnetic
field providing a new way to control THz waves. We analytically study the main
characteristics of the Gaussian beam, its waist, and the focusing distance as
the functions of wave frequency, amplitude, and the external magnetic field.
The results of analytic calculations are supported by the numerical simulation
of the electromagnetic field distribution in the slab.
