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Electrons performing Bloch oscillations in an energy band of a dc-biased superlattice in the presence of weak dissipation can potentially generate THz fields at room temperature [1]. The realization of such Bloch oscillator is a long-standing problem due to the instability of a spatially homogeneous field profile in conditions of negative dc differential conductivity. The electric instability can be circumvented if the sample exhibits positive dynamical conductivity at low frequencies, including dc, while the high-frequency conductivity is still negative giving rise to an amplification [2, 3]. Our theoretical analysis demonstrates that such kind of gain profile can be achieved in superlattices under an action of an auxiliary ac field [4, 5]. In the case of a monochromatic auxiliary field, the frequency of the ac field has to belong to the THz frequency range limiting the applied aspect of this suggestion [4]. However, the unstable Bloch gain profile can be made stable also by a means of a polychromatic low-frequency field with proper phase differences between the harmonics [5]. In both cases the sign of the dynamical conductivity at low frequencies is sensitive to the ac fields, but the gain profile in the vicinity of the Bloch gain maximum is robust. The degree to which the large magnitude of THz gain near the Bloch resonance may be preserved here depends on the modulation waveform.
[1] S.A. Ktitorov et al., Sov. Phys. Solid State 13, 1872 (1972). [2] J. C. McGroddy and P. Gueret, Solid-State Electron. 14, 1219 (1971). [3] A. Wacker et al., Phys. Stat. Sol. B 204, 95 (1997). [4] T. Hyart et al., Phys. Rev. B 77, 165330 (2008). [5] T. Hyart et al., arXiv:0812.4046v1 (2008). |
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