V.  Giliberti, Di Gaspare, A., Giovine, E., Ortolani, M., Sorba, L., Biasiol, G., Popov, V. V., Fateev, D. V., and Evangelisti, F., ?Downconversion of terahertz radiation due to intrinsic hydrodynamic nonlinearity of a two-dimensional electron plasma?, Physical Review B - Condensed Matter and Materials Physics, vol. 91, 2015.