Terahertz Conductivity and Possible Bloch Gain in Semiconductor Superlattices

We have investigated terahertz emission due to dynamical electron transport in wide-miniband $\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/{\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{G}\mathrm{a}}_{0.7}\mathrm{A}\mathrm{s}$ superlattices. By noting that the time-domain THz emission spectroscopy inherently measures the step-response of the electron system to the bias electric field, the obtained THz spectra were compared with the high-frequency conductivities predicted for miniband transport. Excellent agreement between theory and experiment strongly supports that the THz gain due to Bloch oscillating electrons persists at least up to 1.7 THz. It was also found that Zener tunneling into the second miniband sets the high-frequency limit to the THz gain for the samples studied here.