Ultrafast carrier dynamics in the 10fs-time range probed by time-domain terahertz emission spectroscopy

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In ultrafast transistors with cutoff frequencies higher than 500 GHz, carriers are expected to travel in the channel in a very nonstationary manner. It is, therefore, essential to understand nonequilibrium transport of carriers subjected to high electric fields. The aim of this study was to make a quantitative comparison between the nonequilibrium transport data obtained by the time-domain THz spectroscopy and ensemble Monte Carlo (EMC) results and extract intrinsic carrier transport properties. It is found that the experimental results are strongly affected by finite sample thickness. Furthermore, the contribution of holes to the THz signal is found to become significant when the acceleration field is greater than 20 kV/cm. When the sample geometries and the photoexcitation conditions were property taken into account, the agreement between the THz data and the EMC calculations was good.Furthermore, we have also investigated the quantum mechanical transport in quantum heterostructures. … More Since the first proposal of Bloch oscillators by Esaki and Tsu, considerable effort from both experiments and theories has been made to search for Bloch oscillations and obtain terahertz (THz) emission. Even after three decades from the proposal, however, THz Bloch oscillators have not been realized yet It is, therefore, of prime importance to establish an understanding of the dynamical electron motion in the THz regime.In this work, we obtained a strong experimental support for the THz gain due to Bloch oscillating electrons in wide miniband GaAs/Al_<0.3>Ga_<0.7>/As SLs. For determining the high-frequency electron conductivity in SL minibands, we proposed a totally new approach, by noting that the time-domain THz spectroscopy inherently measures the step-response of the electron system to the applied bias electric field and that the Fourier spectra of the THz emission is closely related with the high-frequency electron conductivity. The Fourier spectra of the emitted THz electromagnetic wave were compared with the predicted conductivity spectra. The excellent agreement between theory and experiment strongly suggests that the THz gain due to Bloch oscillating electrons persists at least up to 1.7THz. Less