Downconversion of terahertz radiation due to intrinsic hydrodynamic nonlinearity of a two-dimensional electron plasma

We have measured the electric signal downconverted from a terahertz frequency by an unbiased high mobilitytwo-dimensional electron-gas (2DEG) device. The 2DEG was confined in an asymmetric plasmonic microcavity,and the radiation frequency was continuously tuned in the 0.2–0.4 THz range. The presence of resonant peaksat three frequencies corresponding to three plasma oscillation modes of the ungated 2DEG clearly points to theintrinsic nature of the hydrodynamic nonlinearity responsible for the downconversion as opposed to previouslyproposed plasmonic cavity configurations where the 2DEG oscillates under the metal gate that also acts as thesource of the nonlinearity