The nonlinearity of the refractive index of optical media in the terahertz spectral range

Recently, great progress has been made in the generation of high-intensity terahertz (THz) radiation, which has led to the opportunity to explore non-linear processes in the THz frequency range [1]. A key parameter characterizing the nonlinear response of a material is the non-linear refractive index, denoted as n2. It has been predicted theoretically [2] that the THz nonlinear response of crystals is extremely large as a consequence of a strong vibrational contribution. An indirect assessment of n2 for lithium niobate at THz frequencies was reported in [3], leading to a value of approximately 10cm/W. We find that n2 = 4.0 × 10сm/W, which is approximately 3 orders of magnitude larger that the value 2.4×10 cm/W in the NIR spectral range [4]. The present paper is devoted to theoretical analysis and a direct experimental measurement of n2 for crystalline ZnSe in the THz spectral range. In this paper, we also consider the special aspects of self-action of single-cycle waves in media with given nonlinearities. It is shown that self-focusing efficiency in transparent dispersive medium decreases when the number of oscillations in input radiation is decreased for few-cycle paraxial waves with the same maximum electric field and transverse size, i.e., with the same ratio of the radiation power to the critical self-focusing power.