Coherent terahertz emission from an original InGaP/InGaAs/GaAs plasmon-resonant high-electron-mobility transistor, having a super-grating dual-gate structure, is investigated. We report total emission from multi-mode of plasmons: Thermally excited incoherent modes and instability-driven coherent modes, using the Fourier Transform Infrared (FTIR) spectroscopic measurement. We discriminate the coherent emission of ultrafast instability-driven plasmon self-oscillation from incoherent hot plasmons using the Electro-Optic Sampling (EOS) method. The resultant emission shows a clear optimal plasmon-resonant mode with sharp peak at 3.2 THz for an optimal bias condition of Vds 2V. It indicates the spectral narrowing effect on the super-grating-gate structure.
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