  We demonstrate that due to strong modal interactions through cross-gain
saturation, the onset of a new lasing mode can switch off an existing mode via
a negative power slope. In this process of interaction-induced mode switching
(IMS) the two involved modes maintain their identities, i.e. they do not change
their spatial field patterns or lasing frequencies. For a fixed pump profile, a
simple analytic criterion for the occurrence of IMS is given in terms of their
self- and cross-interaction coefficients and non-interacting thresholds, which
is verified for the example of a two-dimensional microdisk laser. When the
spatial pump profile is varied as the pump power is increased, IMS can be
induced even when it would not occur with a fixed pump profile, as we show for
two coupled laser cavities. Our findings apply to steady-state lasing and are
hence different from dynamical mode switching or hopping. IMS may have
potential applications in robust and flexible all-optical switching.
