Dynamics of Modulation Instability in Terahertz Range in Different Nonlinear Materials under Focusing

It is investigated the formation of regular sequences of short envelope pulses in terahertz range in different nonlinear crystals due to the modulation instability. The cases of the nonlinear paraelectrics and narrow-gap semiconductors are considered. The initial focusing of the input long pulses is applied. In paraelectrics the modulation instability occurs in the longitudinal direction only whereas in the narrow-gap semiconductors it takes place in all directions. It is shown that in the nonlinear paraelectrics the approximation of the pure cubic nonlinearity is satisfactory for a description of the modulation instability. In the nonlinear semiconductor plasma it is necessary to take into account the saturation of nonlinearity. There exists some interval of amplitudes of the input pulses where the formation of the regular sequences of short pulses is possible.