Dispersive terahertz gain of a nonclassical oscillator: BLOCH oscillation in semiconductor superlattices

We have directly determined the spectral shape of the complex conductivities of Bloch oscillating electrons by using the time-domain terahertz (THz) electro-optic sampling technique, and presented experimental evidence for a dispersive Bloch gain in superlattices. This unique dispersive gain without population inversion arises from a nonclassical nature of Bloch oscillations; that is, the phase of the Bloch oscillation 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 approximately 3 THz in our particular sample.