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The advance of terahertz science and technology yet lays wait for thebreakthrough in high-efficiency and high-power solid-state terahertz sourcesapplicable at room temperature. Plasmon in two-dimensional electron gas (2DEG)has long been pursued as a type of promising active medium for terahertzemitters. However, a high wall-plug efficiency sufficient for high-poweroperation has not been achieved. Here we report spontaneous terahertz emissiontunable in a frequency range from 0.3 to 2.5 THz through electrical excitationof plasmon or plasmon-polariton modes in a grating-gate coupled AlGaN/GaN 2DEGat 7 K. A wall-plug efficiency ranging from $10^{-6}$ to $10^{-1}$ is found bytuning the energy (4.5 eV to 0.5 eV) of tunneling electrons between the gratinggates and the 2DEG. The total emission power, albeit below 15 nW, is alreadydetectable by using a direct terahertz field-effect transistor detector basedon the same type of 2DEG. The findings indicate that both high-efficiency andhigh-power terahertz plasmon-polariton emission could be obtained by a properinjection of tunneling electrons with low energy and high current density.

High-efficiency spontaneous terahertz emission from electrically-excited plasmons in AlGaN/GaN two-dimensional electron gas