We have designed and fabricated our original terahertz plasmon-resonant emitter incorporating doubly interdigitated grating gates and a vertical cavity into an InGaP/InGaAs/GaAs high-electron mobility transistor (HEMT) structure. The fabricated device is subjected to 1550-nm, 1-mW (a) a single CW-laser, (b) 4-THz photomixed dual CW-laser, and (c) a 70-fs pulsed-laser illumination at room temperature. In case of (a), terahertz emission due to the plasmon modes of self oscillation is detected by a Si bolometer under certain bias conditions. In case of (b), a resonant peak of injection-locked 4-THz oscillation is clearly observed on the device photoresponse. In case of (c), an impulsive radiation followed by relaxation oscillation is observed by electrooptic sampling, whose Fourier spectrum exhibited resonant peaks of plasmons' harmonic modes up to 4 THz. Estimated radiation power exceeds 0.1 μW, resulting in excellent conversion efficiency of the order of 10-4.