Terahertz Emission in Asymmetric Quantum Wells by Frequency Mixing of Midinfrared Waves

Generation of terahertz (THz), and sub-THz, coherent waves has been demonstrated at room temperature in GaAs–AlGaAs asymmetric quantum wells by mixing two <formula formulatype="inline"><tex>${\hbox {CO}}_{2}$</tex> </formula> laser beams. Two overlapping regimes were studied: the double resonance regime where three states are involved and the optical rectification which relies on two states only. The measured conversion efficiency of <formula formulatype="inline"><tex>$\sim {\hbox {10}}^{-7}\ {\hbox {W}}^{-1}$</tex> </formula> is in reasonable agreement with theoretical predictions. In addition, this technique of THz generation provides a model system to study the emission profile from an ensemble of radiating dipoles. For instance, backward emission and strong diffraction effects have been observed.