Terahertz emission from two-dimensional plasmons in semiconductor heterostructures

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The radiative decay of grating-coupled two-dimensional (2D) plasmons in semiconductor heterostructures is one of promising candidates for tunable solid-state terahertz (THz) radiation sources. We have investigated the excitation relaxation mechanisms of 2D plasmons in AlGaAa/GaAs heterostructures by cw and time-resolved THz emission spectroscopy.By using a combination of a broadband THz detector and a magnetic-field tuned cyclotron resonance (CR) filter, we can determine the spectral line shape as well as the intensity of the radiation due to 2D plasmons. Spectroscopic measurement clearly resolved narrow emission peaks due to 2D plasmons. The observed emission energies are in good agreement with theory. The plasmon emission intensity is found to increase with increasing electrical input power to the electron system and follow the Bose-Einstein distribution law characterized by the thermodynamic temperature of the hot 2D electron system, indicating the thermal excitation of 2D plasmons inthe present experimental regime.Furthermore, the excitation of 2D plasmons in AlGaAs/GaAs heterostructures has been investigated by timeresolved THz emission spectroscopy. Coherent submillimeter-wave emission from nonequilibrium free carrier plasma oscillations created by femtosecond laser pulse illumination has been observed for the first time. The observed spectral linewidths are found to be much broader than those observed in a cw THz absorption spectroscopy. Thus, the relaxation is much faster than the momentum relaxation, suggesting the importance of the dephasing process due to the smallangle impurity scattering.