Doubling of gyrotron radiation frequency due to nonlinear susceptibility in A3B5 semiconductors Vladimir V. Rumyantsev, A.A. Dubinov, A.P. Fokin, V.V. Utochkin, M.Yu. Glyavin and S.V. Morozov EPJ Web Conf., 195 (2018) 02010 Published online: 23 November 2018 DOI: 10.1051/epjconf/201819502010