Yet another example of a nonlinear system is the p-n junction diode, the simplest semiconductor device. Its current voltage characteristic defined through
(1.3)
is essentially nonlinear as shown in figure 2(a). Even more interesting is the Esaki(tunnel) diode (characteristic shown in figure 2(b)) which exhibits bistable behavior. Naturally all other more complex semiconductor devices such as transistors, thyristors and varistors which are made of multiple junctions are expected to be and indeed are nonlinear.
In fact, nonlinear systems are abundant in nature, be it the chemical reactor involving activator - inhibitor reaction kinetics or morphogenesis in biology.
It thus is not an exaggeration to say that almost every system in nature is nonlinear and optics is no exception
