  We report on tunable terahertz resonant detection of two 1.55 &micro;m
cw-lasers beating by plasma waves in AlGaAs/InGaAs/InP high-electron-mobility
transistor. We show that the fundamental plasma resonant frequency and its odd
harmonics can be tuned with the applied gate-voltage in the range 75-490 GHz.
The observed frequency dependence on gate-bias is found to be in good agreement
with the theoretical plasma waves dispersion law.
