Ultrafast carrier dynamics in semiconductor superlattices: Terahertz gain and dephasing mechanism

We have directly determined the spectral shape of the complex conductivities of Bloch oscillating electrons by using time-domain terahertz (THz)-electrooptic sampling technique and presented an experimental evidence for a dispersive Bloch gain in superlattices. This unique dispersive gain without population inversion arises from a non-classical nature of Bloch oscillations; that is, the phase of the Bloch oscillation (BO) is shifted by pi/2 from that of the semiclassical charged harmonic oscillation when driven by the same ac field. By increasing the bias electric field, the gain bandwidth reached similar to 3 THz in our particular sample. It was also found that the dominant dephasing mechanism of the BOs is identified to be the interface roughness scattering (alloy disorder scattering) below (above) the critical bias electric field.