ABSTRACT
Since the proposal to high frequency oscillators by using a oscillatory electron motion called Bloch oscillations (BOs) in superiattices [1], considerable effort from both theory [2] and experiment [3] has been made to obtain terahertz (THz) emission from superiattices. BO in the semiconductor superiattices is attractive as a source of coherent THz electromagnetic wave because the frequency of the radiation can be tuned in the THz range by an electric dc bias field. Even after three decades from the proposal, however, THz emitters using BO have not been achieved yet. This is mainly because of the difficulty in understanding electron motion in the THz regime only from quasi-dc measurements. In recent years, a new technique that measures the THz electromagnetic wave emitted by ultrafast transient electron motions has been established (time-resolved THz emission spectroscopy) [4,5]. This new technique has made it possible to observe the ultrafast motion of electrons in superiattices and discuss coherence and scattering of electron transport in minibands. By using the time-resolved THz emission spectroscopy, transient THz emission from BOs has been observed at T - 15 K [6 ]. Emission from coherent plasma oscillations in doped superiattices has also been observed at room temperature [7].
