Terahertz emission from two-dimensional plasmons in semiconductor heterostructures

半导体异质结中光栅耦合的二维(2D)等离子体的辐射衰变是可调谐固态太赫兹(THz)辐射源的重要候选之一。我们用连续波和时间分辨太赫兹发射谱研究了AlGaAa/GaAs异质结中2D等离子体的激发弛豫机制，利用宽带THz探测器和磁场调谐回旋共振(CR)滤波器相结合，我们可以确定2D等离子体的谱线形状和辐射强度。光谱测量清楚地分辨了由2D等离子激元引起的窄发射峰。观测到的发射能与理论符合得很好。实验发现，等离子体激元的发射强度随电子系统输入功率的增加而增加，并遵循由热2D电子系统的热力学温度表征的玻色-爱因斯坦分布规律，表明在本实验范围内存在2D等离子体的热激发。此外，我们还用时间分辨THz发射光谱研究了AlGaAs/GaAs异质结中2D等离子体的激发。首次观测到飞秒激光脉冲照射产生的非平衡自由载流子等离子体振荡产生的相干亚毫米波辐射。观察到的谱线宽比用连续太赫兹吸收光谱观察到的谱线宽得多。因此，弛豫比动量弛豫快得多，这表明由于小角度的杂质散射，退相过程是重要的。

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.