Doubling of gyrotron radiation frequency due to nonlinear susceptibility in A3B5 semiconductorsVladimir V. Rumyantsev, A.A. Dubinov, A.P. Fokin, V.V. Utochkin, M.Yu. Glyavin and S.V. MorozovEPJ Web Conf., 195 (2018) 02010DOI: https://doi.org/10.1051/epjconf/201819502010