Dispersive terahertz Bloch gain in semiconductor superlattices

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 is shifted by pi/2 from that of the semi-classical 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.