  The ability to exploit the on-chip nonlinear generation of new frequencies
has opened the door to a plethora of applications in fundamental and applied
physics. Excitation of dispersive waves is a particularly interesting process
that allows efficient nonlinear wavelength conversion by phase-matching of a
soliton and waves in the normal dispersion regime. However, controlling the
wavelength of dispersive waves in integrated waveguides remains an open
challenge, hampering versatile shaping of the nonlinear spectral response.
Here, we show that metamaterial silicon waveguides release new degrees of
freedom to select the wavelength of dispersive waves. Based on this concept, we
experimentally demonstrate excitation of two dispersive waves near 1.55 {\mu}m
and 7.5 {\mu}m allowing ultra-wideband supercontinuum generation, covering most
of the silicon transparency window. These results stand as an important
milestone for versatile nonlinear frequency generation in silicon chips, with a
great potential for applications in sensing, metrology and communications.
