Semiconductor Oscillator Research Articles

Mutual phase-locking of microwave spin torque nano-oscillators

The spin torque effect that occurs in nanometre-scale magnetic multilayer devices can be used to generate steady-state microwave signals in response to a d.c. electrical current. This establishes a new functionality for magneto-electronic structures that are more commonly used as magnetic field sensors and magnetic memory elements. The microwave power emitted from a single spin torque nano-oscillator (STNO) is at present typically less than 1 nW. To achieve a more useful power level (on the order of microwatts), a device could consist of an array of phase coherent STNOs, in a manner analogous to arrays of Josephson junctions and larger semiconductor oscillators. Here we show that two STNOs in close proximity mutually phase-lock-that is, they synchronize, which is a general tendency of interacting nonlinear oscillator systems. The phase-locked state is distinct, characterized by a sudden narrowing of signal linewidth and an increase in power due to the coherence of the individual oscillators. Arrays of phase-locked STNOs could be used as nanometre-scale reference oscillators. Furthermore, phase control of array elements (phased array) could lead to nanometre-scale directional transmitters and receivers for wireless communications.

Theory of space-charge waves in semiconductors with negative differential conductivity

Phenomena that accompany optical excitation of eigenmodes of electron gas oscillation in semiconductors with an N-shaped current-voltage (I-U) characteristic in a strong electric field are investigated theoretically. The dependence of the current flowing through a sample on the oscillation frequency of the interference pattern of light illuminating the sample is analyzed. Nonsteady-state and nonuniform illumination produces an internal electric field, which interacts resonantly with the eigenmodes when the oscillation frequency of the interference pattern coincides with an eigenfrequency of electronic gas oscillation. As the maximum of the I-U curve is approached, the interaction becomes nonlinear in character.

Commemorating the 100-th anniversary of O.V. Losev, the first constructor of semiconductor oscillators and amplifiers

Commemorating the 100-th anniversary of O.V. Losev, the first constructor of semiconductor oscillators and amplifiers

Magnetic modes in a resonator built around a circular evanescent waveguide and coaxial line

The eigenfrequencies of magnetic modes in a resonator made up of two segments of a circular evanescent waveguide and a coaxial line are found by solving an electrodynamic problem. For the fundamental \(H_{11\delta } \) mode, the performance of the resonator is studied analytically and experimentally over a wide range of its parameters. The feasibility of a high-power microwave semiconductor oscillator based on the resonator under study is demonstrated.

Introduction to low frequency local plasmons in bulk extrinsic semiconductors

It is shown that restoring force acts on the electronic cloud of the outer electrons of a neutral or charged impurity atom when it is shifted relative to the inner charged core (or expanded). Because of this the dipole oscillations arise which influence considerably the dispersion law of the plasma oscillations in bulk extrinsic semiconductors. Assuming that only one transition of electron from the ground state to the first excited state is essential, the dispersion law is calculated. It is shown that the calculated dispersion law consists of two separate branches, one of them originates from the regular plasma oscillations of the free electrons of a conductivity band, and the other one stems from the local oscillations of the outer electrons bounded to the impurity atoms.

Nonlinear dynamics оf microwave and optical semiconductor oscillators

The results оf experimental аnd theoretical investigations of the nonlinear phenomena in microwave and optical semiconductor oscillators are represented. It is shown that one оf the reasons of occurrence оf various nonlinear phenomena in semiconductor devices оf the microwave range is qualitative change оf their voltage-current characteristics, in particular occurrence оr disappearance of sections оf negative differential resistance under the influence оf high-power microwave radiation оn semiconductor structures. The external optical feedback forming autodyne operating mode of semiconductor lasers acts as the priority mechanism оf realization of their nonlinear operating mode in semiconductor laser structures.

Detection of terahertz radiation from longitudinal optical phonon–plasmon coupling modes in InSb film using an ultrabroadband photoconductive antenna

Terahertz radiation from longitudinal optical (LO) phonon–plasmon coupling modes in InSb films is observed using an ultrafast photoconductive antenna detector. We demonstrate a response frequency of up to 7 THz for a low-temperature-grown GaAs-based photoconductive antenna gated with 25 fs laser pulses. It is found that the emission frequencies of the coupling modes are dependent only on the residual carrier density, not on the excitation carrier density. It is also found that the LO phonon–plasmon oscillations in semiconductors can serve as an efficient THz source.

Self-sustained acoustoelectric and photoelectric oscillations in semiconductor–piezoelectric structures

Existence and possibility of self-sustained oscillations of acoustoelectric and photoelectric voltages in layered piezoelectric–semiconductor structures without any negative differential conductivity and acoustoelectric domains are demonstrated experimentally and theoretically. A joint experimental study of acoustoelectric and photoelectric phenomena in the same structure permits to establish similar electronic processes. Experimental and theoretical results show that nonlinear acoustoelectric and photoelectric voltages with electronic relaxation properties and external feedback are necessary for observing the phenomena. The theory presented explains the experimental results obtained. A criterion for excitation in a soft mode and dynamic characteristics of the self-sustained oscillations under various external conditions, as temperature, illumination and humidity are discussed. Periodic and stochastic modes of oscillation are noted. The quasi-harmonic mode of oscillations described by the Van der Pol equation is very useful and attractive for practical application because of the fact that the oscillation frequency is proportional to the square root of the relaxation time of electronic processes in semiconductors and that it can be precisely measured and digitized. Due to the precise method presented for measuring the relaxation time, new fundamental possible applications to methods of investigation of physical and chemical sensors with a frequency output are noted.

Three-band excitonic Rabi oscillations in semiconductor quantum wells

Optical Rabi oscillations in atomic two-level systems and excitonic Rabi oscillations in two-band semiconductors are already well established. In contrast to two-level (or two-band) Rabi oscillations, three-level (or three-band) Rabi oscillations offer a rich variety of oscillation dynamics involving both radiative and nonradiative (Raman) transitions. We analyze excitonic three-band Rabi oscillations in semiconductor quantum wells and study, in particular, the role of band-coupling effects and Coulomb effects.

Interband absorption cross-section and Rabi oscillations in semiconductors

A numerical method is described that allows to find differential interband absorption cross-section of infrared (IR) radiation in solids in the case of complex energy bands. The method is based on observation of carrier population oscillations between two selected energy bands. Absorption due to hole intervalence transitions in p-InP, when both the nonparabolicity and the warping of heavy, light and spin–orbit split-off valence bands is taken into account, is presented as an illustration of the method. The method allows us to calculate the resonance shift and change of the cross-section at high IR light intensities.

Excitonic Rabi Oscillations in Semiconductors

Excitonic Rabi Oscillations in Semiconductors

Excitonic Rabi Oscillations in Semiconductors

Excitonic Rabi Oscillations in Semiconductors

Direct Observation of Excitonic Rabi Oscillations in Semiconductors

We observe multiple excitonic optical Rabi oscillations in a semiconductor quantum well. Up to eight oscillation periods of the heavy-hole exciton density on a subpicosecond time scale are observed. An approximate linear dependence of the oscillation frequency on the light field amplitude is established. The experiment is based on a two-color detection scheme which allows for the observation of the heavy-hole exciton density via transmission changes at the light-hole exciton. The observations are in good agreement with theoretical computations based on multiband semiconductor Bloch equations.

Electrical current oscillations in vapor-adsorbed ferrocene derivatives having two substituted groups

Oscillations in electrical current as a function of time have been observed in a specific temperature range in the samples of 1,1′-diacetylferrocene and 1,1′-ferrocenedicarboxylic acid with adsorbed ethanol vapor in a sandwich-type cell. The frequency of current oscillations has been found to decrease with bias voltage and sample temperature and to increase with vapor pressure. The frequency of current oscillations for the mono-group substituted derivatives is higher than the corresponding frequency for the derivatives having two substituted groups. Again, the frequency of oscillations for –COCH 3 group substituted derivatives is higher than the corresponding value for –COOH group substituted derivatives. The observation of current oscillations is possibly associated with some kind of time dependent phase changes, arising from the structural nonrigidity of the molecules, in the solid-ethanol vapor system at the sample surface layer. The “ball-bearing” motion of the cyclopentadienyl rings of ferrocene unit, nature and number of substituted groups, cooperative interaction of the neighboring molecules influence the structural nonrigidity and hence the oscillatory behavior of current. Reasons are given for ruling out other models of current oscillations in semiconductors.

Bifurcation and Chaos of Current Oscillations in Semiconductors with NDC

This paper describes a simulational study on bifurcation and chaos of current oscillations in semiconductor with NDC ( N egative D ifferential C onductivity) induced by the recombination instability. A one-dimensional model of gold-doped n -Ge has been proposed in which the recombination instability leads the system to instability and current oscillations, and the computations have been performed under the boundary conditions with d.c. bias. We found that this system exhibits complicated structure of bifurcations for current oscillations including chaotic behavior and unusual truncated branches on bifurcation diagrams at the transition region between two basic oscillation modes in the parameter space.

Coherent phonon oscillations in photo-excited semiconductors and semimetals

Recent progress in laser technology using femtosecond pulses enables us to generate coherent oscillations of high-frequency optical phonons in various materials. The coherent oscillation measurement belongs to time-domain spectroscopy has recently been extended to collective motions of carriers, i.e., plasmons in semiconductors. In this paper, characteristic features of the coherent phonon oscillations in semimetals and semiconductors are summarized. The time-domain spectra are compared with frequency-domain (Raman) spectra. The relaxation of phonons and plasmons is investigated in InP and GaAs.

A Generalized Monte Carlo Approach for the Analysis of Quantum-Transport Phenomena in Mesoscopic Systems: Interplay Between Coherence and Relaxation

A theoretical investigation of quantum-transport phenomena in mesoscopic systems is presented. In particular, a generalization to “open systems” of the well-known Semiconductor Bloch equations is proposed. Compared to the conventional Bloch theory, the presence of spatial boundary conditions manifest itself through self-energy corrections and additional source terms in the kinetic equations, which are solved by means of a generalized Monte Carlo simulation.The proposed numerical approach is applied to the study of the scattering-induced suppression of Bloch oscillations in semiconductor superlattices as well as to the analysis of quantum-transport phenomena in double-barrier structures.

Observation of low-frequency electrical current oscillations in ferrocene sample with adsorbed 2-propanol vapor

Low-frequency oscillations in electrical current (dark) as a function of time were observed, in the temperature range where unusual/anomalous change in electrical conductivity with temperature was noticed in the samples of ferrocene with physically adsorbed 2-propanol vapor (at 40 torr pressure) molecules as impurities in a sandwich type of cell configuration. With the increasing bias voltage across the electrodes, at a constant sample cell temperature, increase in both the time interval between the two consecutive current peaks (i.e. decrease in frequency of current oscillations) and current peak height was noticed. The current-voltage characteristics behavior of ferrocene with adsorbed 2-propanol vapor has indicated the ohmic nature of conductivity. Existing theories of current oscillations in semiconductors have failed to explain the observed low-frequency current oscillations in this organometallic semiconductor. Such current oscillations are possibly associated with some kind of time dependent phase changes in the ferrocene- (2-propanol) vapor system/surface layer.

Few-Cycle THz Emission from Cold Plasma Oscillations

We report on THz emission from plasma oscillations in semiconductors excited by femtosecond optical pulses. Time-resolved correlation measurements are performed on $p\ensuremath{-}i\ensuremath{-}n$ and $n$-doped GaAs structures. In $p\ensuremath{-}i\ensuremath{-}n$ structures coherent oscillations of the hot photogenerated carrier plasma emit THz radiation. A fundamentally new emission process is proposed in $n$-doped GaAs structures. Here, the screening of the surface field starts plasma oscillations of the cold electrons in the GaAs bulk leading to an efficient emission of few-cycle THz radiation.

Effects of some physical parameters on the electrical current oscillations in vapor adsorbed ferrocene

This work deals with the observation of low-frequency electrical current oscillations in a ferrocene sample in the presence of adsorbed ethanol vapor in a sandwich-type cell configuration. The effects of some physical parameters such as (i) vapor pressure, (ii) temperature when adsorption of vapors is completed, (iii) cell temperature during current measurement and (iv) bias voltages on the oscillatory behavior of current have been studied. The current measurements were performed with different bias voltages (27–300 V) at some fixed temperatures in the range from 272 to 300 K for adsorption of vapors at 20, 25, 30, 35 and 40 Torr vapor pressure. This experiment has shown that the frequency of current oscillations increases exponentially with the increase of vapor pressure, when other experimental conditions remained the same. The frequency of oscillations and the current peak height depend on the cell temperature during adsorption as well as measurement of current. With increasing bias, a decrease in frequency of oscillations along with an increase in peak height has been observed for an experiment with a fixed vapor pressure. Existing theories of current oscillations in semiconductors have failed to explain the observed low-frequency current oscillations in this organometallic semiconductor. The results have been discussed.