The nonlinearity of the refractive index of optical media in the terahertz spectral range A.N. Tcypkin, S.E. Putilin, M.S. Kulya, A.A. Drozdov, M. Siddiqui, S. Choudhary, J. Zhao, V.G. Bespalov, R.W. Boyd, X.C. Zhang and S.A. Kozlov EPJ Web Conf., 149 (2017) 05005 Published online: 04 August 2017 DOI: 10.1051/epjconf/201714905005