There are investigated nonlinear elcctrodynamic phenomena in volume narrow-gap semiconductors n-InSb, n-InAs at frequencies 1-5 THz. The nonlinearity of the electron gas due to the Kane dispersion law is dominating there. The equation for the amplitude, which is slowly varying with respect to time only, has been derived. The saturation of nonlinearity is taken into account. Under a propagation of THz transverse limited electromagnetic pulses through the structure intrinsic InSb-n-InSb-intrinsic InSb both the longitudinal and transverse focusing occurs. This leads to the wave collapse, or the concentration of the wave energy near one point. The optimum conditions for realizing wave collapse are obtained. A comparison is given with the structures that include the graphene sheets.
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