Time-domain measurement of intersubband oscillations in a quantum well

We report time-domain measurements of electron intersubband oscillations in quantum wells. We use an interferometric technique to measure the change in the profile of a few-cycle THz pulse due to propagation through a modulation-doped ${\mathrm{Al}}_{\mathrm{0.3}}{\mathrm{Ga}}_{\mathrm{0.7}}\mathrm{As/GaAs}$ multiple quantum well structure $\text{(10×510\hspace{0.17em}Å}$ wells). From this data we obtain the absorption and the index of refraction due to electrons in the quantum well, and due to the GaAs substrate. Unlike existing studies of coherent charge oscillations of electrons and holes in heterostructures excited by ultrashort pulses of band-gap light, our all-THz measurements quantitatively determine the linear optical properties of the quantum well electrons.