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 ~3 THz in our particular sample.

References

• N. Sekine and K. Hirakawa: “Dispersive terahertz gain of non-classical oscillator: Bloch oscillation in semiconductor superlattices”, Physical Review Letters, vol. 94, No. 5, 057408 (2005).
• Y. Shimada, K. Hirakawa, M. Odnoblioudov, and K. A. Chao: “Terahertz conductivity and possible Bloch gain in semiconductor superlattices”, Physical Review Letters, vol. 90, No. 4, pp. 46806-46809 (2003).