5 research outputs found
Bloch gain in dc-ac-driven semiconductor superlattices in the absence of electric domains
We theoretically study the feasibility of amplification and generation of terahertz radiation in dc-ac-driven semiconductor superlattices in the absence of electric domains. We find that if in addition to a dc bias a strong terahertz pump field is applied, a Bloch gain profile for a small terahertz signal can be achieved under the conditions of a positive static differential conductivity. Here, the positive differential conductivity arises, similarly to the case of a large-signal amplification scheme [H. Kroemer, arXiv:cond-mat/0009311 (unpublished)], due to modifications in dc current density caused by the application of a high-frequency ac field [ K. Unterrainer et al. Phys. Rev. Lett. 76 2973 (1996)]. Whereas the sign of absorption at low and zero frequencies is sensitive to the ac fields, the gain profile in the vicinity of the gain maximum is robust. We suggest to use this ac-induced effect in a starter for a terahertz Bloch oscillator. Our analysis demonstrates that the application of a short terahertz pulse to a superlattice allows the suppression of the undesirable formation of electric domains and the achievement of a sustained large-amplitude operation of the dc-biased Bloch oscillator
Terahertz Bloch oscillator with suppressed electric domains: Effect of elastic scattering
We theoretically consider the amplification of THz radiation in a
superlattice Bloch oscillator. The main dilemma in the realization of THz Bloch
oscillator is finding operational conditions which allow simultaneously to
achieve gain at THz frequencies and to avoid destructive space-charge
instabilities. A possible solution to this dilemma is the extended Limited
Space-Charge Accumulation scheme of Kroemer (H. Kroemer, cond-mat/0009311).
Within the semiclassical miniband transport approach we extend its range of
applicability by considering a difference in the relaxation times for electron
velocity and electron energy. The kinetics of electrons and fields establishing
a stationary signal in the oscillator is also discussed.Comment: Submitted to proceedings of the summer school-conference of AQDJJ
programme of ESF, Kiten, Bulgaria, 9-24 June 200
Vibronic spectroscopy of an artificial molecule
With advanced fabrication techniques it is possible to make nanoscale
electronic structures that have discrete energy levels. Such structures are
called artificial atoms because of analogy with true atoms. Examples of such
atoms are quantum dots in semiconductor heterostructures and Josephson-junction
qubits. It is also possible to have artificial atoms interacting with each
other. This is an artificial molecule in the sense that the electronic states
are analogous to the ones in a molecule. In this letter we present a different
type of artificial molecule that, in addition to electronic states, also
includes the analog of nuclear vibrations in a diatomic molecule. Some of the
earlier experiments could be interpreted using this analogy, including qubits
coupled to oscillators and qubits driven by an intense field. In our case the
electronic states of the molecule are represented by a Josephson-junction
qubit, and the nuclear separation corresponds to the magnetic flux in a loop
containing the qubit and an LC oscillator. We probe the vibronic transitions,
where both the electronic and vibrational states change simultaneously, and
find that they are analogous to true molecules. The vibronic transitions could
be used for sideband cooling of the oscillator, and we see damping up to
sidebands of order 10.Comment: 5 pages, 4 figure
Bloch gain in dc-ac-driven semiconductor superlattices in the absence of electric domains
We study theoretically the feasibility of amplification and generation of
terahertz radiation in dc-ac-driven semiconductor superlattices in the absence
of electric domains. We find that if in addition to dc bias a strong THz pump
field is applied, Bloch gain profile for a small THz signal can be achieved
under conditions of positive static differential conductivity. Here the
positive differential conductivity arises, similarly to the case of
large-signal amplification scheme [H. Kroemer, cond-mat/0009311)], due to
modifications of dc current density caused by the application of high-frequency
ac field [K. Unterrainer \textit{et al.}, Phys. Rev. Lett. \textbf{76}, 2973
(1996)]. Whereas the sign of absorption at low and zero frequencies is
sensitive to the ac fields, the gain profile in the vicinity of gain maximum is
robust. We suggest to use this ac-induced effect in a starter for THz Bloch
oscillator. Our analysis demonstrates that the application of a short THz pulse
to a superlattice allows to suppress the undesirable formation of electric
domains and reach a sustained large-amplitude operation of the dc-biased Bloch
scillator.Comment: 13 pages, 12 figure