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.
Two-dimensional (2-D) electron plasma in a submicron channel of a high-electron mobility transistor (HEMT) can make resonant oscillation in the terahertz range. The gate bias potential Vgs can control the resonant frequency fr, which offers the possibility of tunable coherent terahertz oscillators. The terahertz plasma-wave excitation can be performed by means of interband photoexcitation in a manner of laser-photomixed difference-frequency (Δf) generation. The 2-D electron plasma in the electron channel is excited by the terahertz Δf component of the photoexcited carriers. Since the photoelectrons perturb the surface density of 2-D electrons, strong photoexcitation dynamically modulates the fr, resulting in considerable resonant-spectral broadening. This effect was modeled analytically in the 2-D plasma hydrodynamic equation. The modulation depth of the density of 2-D electrons by the photoelectrons deeply relates to the resonant intensity and fr. In order to validate the analytical calculation, the plasma-wave resonance was experimentally observed for a 0.15-μm gate-length InGaP/InGaAs/GaAs pseudomorphic HEMT in the terahertz range. At the modulation depth of 30%, the resonance was clearly observed with a double peak (the peak at 1.9/5.8 THz corresponding to the fundamental /third harmonic resonance). On the contrary, under a low modulation depth condition, the plasma resonant intensity decreased. Observed resonant frequencies support the analytical calculation.