Double-resonance Semiconductor Superlattice Parametric Oscillator for Generation of Subterahertz Radiation

We report on a subterahertz superlattice parametric oscillator that operated simultaneously at two different harmonic frequencies of a microwave pump field. A pump field (frequency near 100 GHz) was coupled to a GaAs/AlAs superlattice in a resonator for the third and the fifth harmonic. The pump field produced a third harmonic field and this together with the pump field created a fifth harmonic field. A theoretical analysis indicates that the nonlinearity, which is based on the dynamics of miniband electrons, should allow for the upconversion of pump radiation of higher frequency into the terahertz frequency range.      

Subterahertz superlattice parametric oscillator

We report a superlattice parametric oscillator (SPO), with a GaAs/AlAs superlattice as the active element. The SPO was pumped by a microwave field (power 4 mW) and produced third harmonic radiation at subterahertz frequencies (near 300 GHz; 0.1 mW). We attribute the parametric gain to the nonlinearity of the miniband transport.     

Nonlinear properties of two-dimensional semiconductor superlattices in biharmonic fields

Conditions for the dynamic localization of electrons and electromagnetic transparency in a two-dimensional semiconductor superlattice in harmonic and biharmonic fields are obtained. The effect of static current excitation by a biharmonic field with orthogonally directed components is predicted.     

Response of a Bloch oscillator to a THz-field

In this paper we report on the observation of response of a Bloch oscillator at room temperature to a THz-field of a frequency larger than the Bloch frequency. The oscillator consisted of a semiconductor superlattice structure, with an applied dc voltage giving rise to a dc electron drift current. Submitting the oscillator to a field at a frequency of 3.3 THz caused a sizeable reduction of the current; the THz-field was generated by use of intense THz-radiation pulses focused on an antenna coupled to the superlattice. We attribute the THz-field induced reduction of the current to a frequency modulation of the Bloch oscillations of electrons at the frequency of the THz-field, leading to reduction of the electron drift velocity and, consequently, of the current.     

Frequency multiplication of microwave radiation in a semiconductor superlattice by electrons capable to perform Bloch oscillations

We report the observation of frequency multiplication of microwave radiation in a GaAs/AlAs semiconductor superlattice at room temperature. We observed, for a fundamental frequency of 9 GHz, second and third harmonic generation. We associate the harmonic generation with a nonlinear current-voltage characteristic that is determined by Bloch oscillations of electrons propagating along the superlattice axis. Our results suggest for the frequency multiplication an upper limit in the tetrahertz frequency range.     

Gain recovery dynamics and photon-driven transport in quantum cascade lasers

Quantum cascade lasers are semiconductor devices based on the interplay of perpendicular transport through the heterostructure and the intracavity lasing field. We employ femtosecond time-resolved pump-probe measurements to investigate the nature of the transport through the laser structure via the dynamics of the gain. The gain recovery is determined by the time-dependent transport of electrons through both the active regions and the superlattice regions connecting them. As the laser approaches and exceeds threshold, the component of the gain recovery due to the nonzero lifetime of the upper lasing state in the active region shows a dramatic reduction due to the onset of quantum stimulated emission; the drift of the electrons is thus driven by the cavity photon density. The gain recovery is qualitatively different from that in conventional lasers due to the superlattice transport in the cascade.     

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.     

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.     

Terahertz radiation of Bloch oscillators excited by an electromagnetic field in lateral semiconductor superlattices

The action of a strong high-frequency electromagnetic field on a lateral semiconductor superlattice is considered based on the quasi-classical electron transport theory in the self-consistent wave formulation. The theory predicts that a lateral superlattice can emit terahertz radiation wave trains, which are associated with periodic excitation of Bloch oscillations in the superlattice arising because of the development of transient processes in it in a variable self-consistent electric field. The conditions necessary for observing Bloch oscillator radiation were found. The spectral composition of radiation transmitted through the superlattice and the energy efficiency of frequency multiplication related to Bloch oscillator excitation were calculated.     

Excitation of Bloch oscillations in a lateral semiconductor superlattice under the influence of electromagnetic pulses

The conversion of the carrier frequency of electromagnetic pulses in lateral semiconductor superlattices, associated with the excitation of Bloch oscillations in the superlattice, is studied theoretically. Conditions are found that are necessary for the observation of the radiation of a Bloch oscillator. The energy characteristics of the efficiency of frequency multiplication and the spectral distribution of the radiation transmitted through the superlattice are calculated. It is shown that low-frequency collisions of electrons do not suppress the excitation of Bloch oscillations, which can be observed under the interaction of the superlattice not only with a pulsed, but also with a continuous-wave signal.     

Electric field distribution in biased GaAs microstructures with field-pinning layers

Field-pinning layers are an approach to improve the homogeneity of the electric field in a biased semiconductor structure of length above the Kroemer criterion. Building a THz Bloch oscillator with such a structure requires superlattice regions. Nevertheless, GaAs layers are investigated here. We compare different periodic structures (alternating transit and field-pinning layers) via simulating the field distribution. It is shown that the development of propagating Gunn domains is suppressed when field-pinning layers are included, but the homogeneity of the field is still not satisfying for the purpose of building a Bloch gain THz source. Depending on the temperature, intra- and inter-period inhomogeneities occur.     

Frequency multiplication using induced dipole domains in a semiconductor superlattice

We theoretically studied the possibility of frequency multiplication using propagating dipole domains which are induced in a semiconductor superlattice by microwave radiation. We have investigated the dynamics of electrons in a superlattice submitted to both a static voltage and a microwave field by performing a simulation based on a drift-diffusion model and incorporating current-limiting boundary conditions. The motion of electrons in the superlattice was governed by an Esaki–Tsu drift velocity field characteristic with a negative differential mobility above a critical electrical field. The simulation delivered, for a static voltage larger than a critical voltage, the periodic formation and annihilation of propagating dipole domains and, as a consequence, a reduction of the direct current through the superlattice. Our simulation showed that an additional microwave field can periodically induce and subsequently quench domains giving rise to a strongly anharmonic current. The anharmonicity of the current is the origin for the generation of higher harmonics of the microwave field. Both the formation and annihilation of a domain can take place within a time of about 1 ps suggesting that the mechanism of domain induction and quenching can be used for generation of radiation up to almost 1 THz.     

Generation of Acoustic Phonons in Semiconductor Superlattice in the Case of an Intraband Absorption of Electromagnetic Wave

We obtain the damping coefficient of an acoustic wave for the case of intraband multiphoton absorption of an electromagnetic wave in a superlattice. The ranges in which the acoustic damping coefficient reverses sign are determined for the sound-propagation directions which are transverse and parallel to the superlattice axis. Numerical summation of the series for the acoustic-wave gain is performed for typical parameters of the superlattice. The gain is estimated numerically. It is noted that multiphoton absorption affects the acoustic-wave gain in the superlattice if the field value is much smaller than that in a standard semiconductor. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 48, No. 5, pp. 436–440, May 2005.     

THz-field induced nonlinear transport and dc voltage generation in a semiconductor superlattice due to Bloch oscillations

We report on a theoretical analysis of terahertz (THz-) field induced nonlinear dynamics of electrons in a semiconductor superlattice that are capable to perform Bloch oscillations. Our results suggest that for a strong THz-field a dc voltage should be generated. We have analyzed the real-time dynamics using a balance equation approach to describe the electron transport in a superlattice miniband. Taking account of both Bloch oscillations of electrons in a superlattice miniband and dissipation, we studied the influence of a strong THz-field on currently available superlattices at room temperature. We found that a THz-field can lead to a negative conductance resulting in turn in a THz-field induced dc voltage, and that the voltage per superlattice period should show, for varying amplitue of the THz-field, a form of wisted plateaus with the middle points being with high precision equal to the photon energy divided by the electron charge. We show voltage to the finite voltage state, and that in the finite voltage state dynamic localization of the electrons in a miniband occurs.     

Optical characterization of weakly absorbing PP,PE, and PP/PE films

The interaction between a solitary electromagnetic wave and a narrow layer with an increased electron concentration in a semiconductor superlattice in a preset uniform alternating electric field directed along the superlattice axis is investigated. The model of the electron energy spectrum in the superlattice is chosen in the strong coupling approximation taking into account the second harmonic. It is shown that, for certain relations between the amplitude and frequency of the alternating electric field, a solitary electromagnetic wave approaching the layer with an increased electron concentration can be trapped by this layer.     

Absolute negative conductivity and spontaneous current generation in semiconductor superlattices with hot electrons

We study transport through a semiconductor superlattice with an electric field parallel to and a magnetic field perpendicular to the growth axis. Using a semiclassical balance equation model with elastic and inelastic scattering, we find that (1) the current-voltage characteristic becomes multistable in a large magnetic field and (2) "hot" electrons display novel features in their current-voltage characteristics, including absolute negative conductivity and a spontaneous dc current at zero bias. We discuss experimental situations providing hot electrons to observe these effects.     

Microwave oscillator based on Bloch oscillations of electrons in a semiconductor superlattice

We report on a microwave oscillator based on Bloch oscillations of electrons in a semiconductor superlattice. Our GaAs/AlAs superlattice, at room temperature, was coupled electromagnetically by an antenna to a rectangular cavity resonator, and was operated at a current-voltage state of negative differential conductance. We observed generation of microwave radiation at frequencies, depending on the resonator length, between 7 and 30 GHz. Electronic tuning by several percent was possible; the ratio of linewidth to frequency was of the order of 10^?4. A radiation power up to 1 μW (at 10 GHz) was obtained, corresponding to a generator efficiency of the order of 10^?3 for the conversion of electrical power to microwave radiation.     

A theoretical study of harmonic generation in a short period AlGaN/GaN superlattice induced by a terahertz field

Based on an improved energy dispersion relation, the terahertz field induced nonlinear transport of miniband electrons in a short period AlGaN/GaN superlattice is theoretically studied in this paper with a semiclassical theory. To a short period superlattice, it is not precise enough to calculate the energy dispersion relation by just using the nearest wells in tight binding method: the next to nearest wells should be considered. The results show that the electron drift velocity is 30% lower under a dc field but 10% higher under an ac field than the traditional simple cosine model obtained from the tight binding method. The influence of the terahertz field strength and frequency on the harmonic amplitude, phase and power efficiency is calculated. The relative power efficiency of the third harmonic reaches the peak value when the dc field strength equals about three times the critical field strength and the ac field strength equals about four times the critical field strength. These results show that the AlGaN/GaN superlattice is a promising candidate to convert radiation of frequency ω to radiation of frequency 3ω or even higher.     

SEMICONDUCTOR-SUPERLATTICE FREQUENCY MIXER FOR DETECTION OF SUBMILLIMETER WAVES

We report on a GaAs/AlAs superlattice frequency mixer for detection of submillimeter waves. The mixer is based on the nonlinear miniband transport giving rise to domains excited under the action of a microwave field. We designed the mixer for broadband operation (300–600 GHz). For studying basic properties, we investigated the mixer as a harmonic mixer in 15^th order to detect radiation at a radio frequency (RF) near 300 GHz using local oscillator (LO) radiation of a frequency near 20 GHz. We reached a noise equivalent power (NEP) of about 10 fW/Hz. We also show that the use of the superlattice mixer together with a superlattice frequency multiplier allows to realize a superlattice-based free-space transmission line for submillimeter waves.     

Coherent control of harmonic generation in superlattices: single-mode response

We study the harmonic generation spectrum in a semiconductor superlattice (a quantum-dot array) at slow relaxation. The effect of a single-mode response in an alternating rectangular (meander) electric field is demonstrated: For certain values of field parameters, the extremely wide discrete output spectrum with slowly decaying tails (multiharmonic generation) shrinks to one single harmonic (single-harmonic generation). Similarly, the effect is manifested in the transient continuous spectrum by diminishing the divergencies (peaks) at all odd harmonics but one. Substantial control over the spectrum is demonstrated.