Terahertz conductivity and possible BLOCH gain in semiconductor superlattices

We have investigated terahertz emission due to dynamical electron transport in wide-miniband GaAs/Al(0.3)Ga0.7As superlattices. By noting that the time-domain THz emission spectroscopy inherently measures the step-response of the electron system to the bias electric field, the obtained THz spectra were compared with the high-frequency conductivities predicted for miniband transport. Excellent agreement between theory and experiment strongly supports that the THz gain due to Bloch oscillating electrons persists at least up to 1.7 THz. It was also found that Zener tunneling into the second miniband sets the high-frequency limit to the THz gain for the samples studied here.     

Carrier dynamics and 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; i.e., the phase of the Bloch oscillation is shifted by π/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 in our particular sample.     

Rectification of terahertz radiation in semiconductor superlattices in the absence of domains

We study theoretically the dynamical rectification of a terahertz AC?electric field, i.e.?the DC?current and voltage response to the incident radiation, in strongly coupled semiconductor superlattices. We address the problem of stability against electric field domains: a spontaneous DC?voltage is known to appear exactly for parameters for which a spatially homogeneous electron distribution is unstable. We show that by applying a weak direct current bias the rectifier can be switched from a state with zero DC?voltage to one with a finite voltage in full absence of domains. The switching occurs near the conditions of dynamical symmetry breaking of an unbiased semiconductor superlattice. Therefore our scheme allows for the generation of DC?voltages that would otherwise be unreachable due to domain instabilities. Furthermore, for realistic, highly doped wide miniband superlattices at room temperature, the generated DC?field can be nearly quantized, that is, be approximately proportional to an integer multiple of ?ω/ea where a is the superlattice period and ω is the AC?field frequency.     

Photon BLOCH oscillations in porous silicon optical superlattices

We report the first observation of oscillations of the electromagnetic field in an optical superlattice based on porous silicon. These oscillations are an optical equivalent of well-known electronic Bloch oscillations in crystals. Elementary cells of our structure are composed by microcavities whose coupling gives rise to the extended collective modes forming optical minigaps and minibands. By varying thicknesses of the cavities along the structure axis, we have created an effective electric field for photons. A very high quality factor of the confined optical state of the Wannier-Stark ladder may allow lasing in porous silicon-based superlattices.     

Intraband dynamics and terahertz emission in biased semiconductor superlattices coupled to double far-infrared pulses

This paper studies both the intraband polarization and terahertz emission of a semiconductor superlattice in combined dc and ac electric fields by using the superposition of two identical time delayed and phase shifted optical pulses. By adjusting the delay between these two optical pulses, our results show that the intraband polarization is sensitive to the time delay. The peak values appear again for the terahertz emission intensity due to the superposition of two optical pulses. The emission lines of terahertz blueshift and redshift in different ac electric fields and dynamic localization appears. The emission lines of THz only appear to blueshift when the biased superlattice is driven by a single optical pulse. Due to excitonic dynamic localization, the terahertz emission intensity decays with time in different dc and ac electric fields. These are features of this superlattice which distinguish it from a superlattice generated by a single optical pulse to drive it.     

Self-oscillations in semiconductor superlattices

An investigation is made of nonlinear oscillations of the field and current in semiconductor superlattices driven by strong terahertz radiation. Regimes of periodic, quasi-periodic, and stochastic self-oscillations are determined and mechanisms for their formation are discussed. It is shown that the self-oscillation spectra are many-valued functions of the external field amplitude and the static field in them is either absent, weak, or fractionally quantized. Previously predicted states of self-induced superlattice transparency and dynamic electron localization are destroyed as a result of the evolution of dissipative and parametric instabilities and can only be observed in transient processes whose duration decreases with increasing electron concentration.     

Helicon waves in semiconductor superlattices

The dispersion relation for the helicon frequency of a system consisting of a periodic array of two-dimensional gas layers is studied as a function of both q and q_z, the components of the wavevector parallel and perpendicular to the layers. The result is compared with that for a homogeneous three-dimensional electron gas.     

Hopping mobility in semiconductor superlattices

We present results on the electrical transport perpendicular to interfaces in GaAs/Al_xGa_1?xAs superlattices. We have measured the current-voltage characteristics on a series of superlattices. This has been simulated numerically, the superlattice being replaced by an effective medium. Using this model we obtain the values of the effective mobility as a function of the superlattice period. Our data are in good agreement with a theory of phonon-assisted hopping transport between localized states, rather than the theory of phonon-limited band transport of Bloch waves.     

Polariton modes in semiconductor superlattices

Polariton electric fields and dispersion relations of some important complex-basis superlattices have been derived by means of electromagnetic theory and the Bloch's theorem. The spatial distribution of the polariton electric fields shows an interesting physical picture: the polaritons are mainly bulk modes as the wavenumber k is small and become typical interface modes with very strong peak intensities as k increases.     

Fano resonances in semiconductor superlattices

We use semiconductor superlattices as a model system for the investigation of Fano resonances. In absorption the excitonic transitions of the Wannier–Stark ladder show the typical asymmetric line shape due to coupling to the continuum of lower-lying transitions. The unique feature of these Fano resonances is that they allow to continuously tune the key parameter – the coupling strength Γ between the discrete state and the degenerate continuum – by varying the bias voltage. Using this feature, we directly show that the Fano coupling leads to a fast polarization decay. We also investigate the dependence of the Fano parameters on the structure of the superlattice and compare with an extensive theoretical model of the resonances.     

Collective excitations in semiconductor superlattices

Collective excitations in semiconductor superlattices are studied beyond tbe random-phase approximation (RPA). The Singwi, Tosi, Land and Sjölander (STLS) theory, which accounts for exchange and short-range correlations effects through an effective potential depending on the structurc factor, is generalized to the layered electron system described by the model of Visscher and Falicov. The exact numerical solution of the STLS self-consistent equations provides information about intraplane and interplane correlations. The plasmon dispersion curves are evaluated for some typical values of the coupling constant r_s of the electron system and the distance between the planes for GaAs/AlGaAs semiconductor superlattices. For comparison, the RPA and the Hubbard approximation are also considered.     

Quantum derivatives and terahertz gain in a superlattice

Simple formulas describing terahertz absorption and gain in a semiconductor superlattice irradiated by a microwave pump field are derived for the case when the signal frequency is a half harmonic of the pump. A simple qualitative analysis provides a geometric interpretation of the derived formulas, which can be used to determine if gain is feasible.     

Operation of a semiconductor superlattice oscillator

We report an experimental study of a semiconductor superlattice oscillator and present an analysis of the origin of gain. The oscillator generated microwave radiation (at frequencies around 60 GHz). An analysis of the results suggests that the oscillator operated in a pure charge accumulation mode that can occur in a medium with a negative differential mobility. We relate the negative differential mobility to miniband transport. Additionally, we propose a microwave-terahertz double oscillator that may be suitable to realize a terahertz Bloch oscillator.     

Collective modes in semiconductor superlattices

Electronic collective modes in a semiconductor superlattice structure are studied within the self-consistent field approach. Plasmon and magneto-plasmon dispersion relations are obtained for the cases of strong and weak coupling between layers. The interaction of these collective modes with optical phonons is also investigated.     

Forcing of chaos in semiconductor superlattices

The chaotic behavior of high-field transport in weakly-coupled narrow-miniband GaAs/AlAs superlattices (SL) under ac + dc biases has been numerically studied within a self-consistent discrete model. It is shown that the regions of entrainment and quasiperiodicity form the Arnol'd tongues on the driving frequency–driving amplitude parameter plane. Chaos is demonstrated to appear at the boundaries of the tongues and in the regions where they overlap. Numerical simulation shows that ac driving can lead to chaos for different regimes: (i) when the electric-field domains in the SL are stable; and (ii) when they are unstable (periodically oscillating).     

Band mixing effect in semiconductor superlattices

The band mixing effect on the electronic and optical properties of semiconductor superlattices is studied within the framework of the empirical tight-binding model. It is found that the superlattice periodic potentials mix the bulk heavy hole, light hole and spin-orbit-split bands in the valence band states. As a consequence, the optical matrix elements associated with various valence-to-conduction subband transitions are very sensitive to the variation of the wavevector in directions parallel to the interface ( _t). We find that band mixing in conjunction with the exciton effect can account for the Δn≠0 forbidden transitions observed in several recent experiments.     

Phonon polariton modes in semiconductor superlattices

Phonon polariton modes in semiconductor superlattices are studied. Polariton electric fields and the dispersion relation are derived by electromagnetic theory, and due to periodicity in the direction normal to the superlattice layers, Bloch's theorem is applied. Polariton modes are found to exist between the TO and LO phonon frequencies, and approach the surface polariton frequency in the limit of large tangential wave vectors. The frequencies are also strongly dependent on the ratio of the layer thicknesses. Results are illustrated by a GaAsGaP superlattice.