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.     

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.     

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.     

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.     

High-field transport in semiconductor superlattices for interacting Wannier–Stark levels

We developed a microscopic theory of electron transport in superlattices within the Wannier–Stark approach by including the interaction associated with Zener tunneling between the energy levels pertaining to adjacent quantum wells. By using a Monte Carlo technique we have simulated the hopping motion associated with absorption and emission of polar optical phonons and determined the main transport parameters for the case of a GaAs/GaAlAs structure at room temperature. Interaction between the levels is found to be responsible for a systematic increase of the level energy with respect to the bottom of the quantum well at electric fields above about 20 kV/cm. When compared with the non-interacting case, at the highest fields the average carrier energy evidences a consistent increase, which leads to a significant softening of the negative slope of both the drift velocity and diffusivity versus electric field behavior.     

Electron spectrum and infrared transitions in semiconductor superlattices with a unit cell allowing for quasi-localised carrier states

We studied theoretically the electron spectrum and infrared transitions in a superlattice with a unit cell allowing for quasi-localised carrier states. The dispersion relation and the band structure of such a system have been found. We calculated the dipole matrix element for inter-subband carrier infrared transitions. The wave functions and the electron spectrum in this superlattice show a peculiarity when the energy of a band state approaches the energy of the quasi-localised state in the single cell. The absorption strength peaks up at the respective frequencies.     

Photoexcited charge carrier dynamics in silicon nanocrystal/SiO2 superlattices

We report in detail on the dynamics of photoexcited charge carriers in size-controlled silicon nanocrystals in silicon nanocrystal/SiO₂ superlattices. The samples were prepared using plasma enhanced chemical vapor deposition and subsequent thermally induced phase separation. This unique approach allows preparation of well-defined Si nanocrystals. Experimental techniques of time-resolved absorption and photoluminescence were used to monitor the carrier dynamics on a wide time scale from picoseconds to milliseconds for a set of samples with different parameters (nanocrystal size, hydrogen annealing). The initial fast decay (tens of picoseconds) dependent on pump intensity for excitation levels exceeding one electron–hole pair per nanocrystal can be interpreted in terms of the bimolecular recombination with constant B=(2–3)×10⁻¹⁰ cm³ s⁻¹. The slow pump intensity independent decay (microseconds) can be reproduced well by a stretched-exponential function. The dependence of stretched-exponential parameters on photoluminescence photon energy and sample properties agrees well with the picture of trapped carriers.     

Interaction of electrons with polar optical phonons in semiconductor superlattices

On the basis of the microscopic theory of lattice dynamics, simulation of the electric potentials created by optical phonons in semiconductor superlattices is performed. It is shown that the spatial distribution of the amplitudes of electric potentials differs from that in a dielectric continuum predicted by the conventional macroscopic model without dispersion. A modified macroscopic continuum theory is proposed that takes into account the dispersion of short-range interatomic forces and allows one to obtain analytical expressions for the potentials of electron-phonon interaction.     

低温环境中位移测量的实现

针对EAST装置极向场中心螺管线圈测试实验中线圈位移量的测量,对低温环境中的位移测量方法进行了探讨,并提出了一种有效消除温度影响的位移测量方法:恒流源输入式位移测量。     

Magneto-oscillation of mid-gap photoluminescence in AlAs:Yb/GaAs superlattices

On AlAs:Yb/GaAs superlattice samples, we measured photoluminescence (PL) spectra including their temperature dependence, magnetic field dependence and resistance up to 25 T. In case of selective excitation of well layers, two broad band PLs were observed in additional to the exception of intra-4f PL from Yb. These peaks show oscillatory behavior similar to that of two-dimensional electron system. From the periods of the oscillation, the electron densities are estimated of the order of which are cannot be archived by usual photoexcitation. It was found that the electron density shows a linear dependence on the excitation energy. To explain such distinctive phenomena, we proposed a new model where Yb ions form hole traps in AlAs.     

Analytical evaluation of Askerov functions arising from anisotropy of the thermoelectric power in superlattices

In this work, we propose an analytical approach which enables more accurate calculation of Askerov functions arising from components of the thermoelectric power in superlattices. Using the binomial expansion theorem, these functions are expressed through the binomial coefficients and familiar basic functions. This work is the first attempt in the literature to analytically evaluate Askerov functions. The convergence of the series is tested by concrete cases of parameters. The exact analytical and numerical solutions are compared with each other and good agreements are obtained.     

Direct electromegnetic generation of high frequency acoustic waves in semiconductor superlattices

The periodic space charge layers formed in semiconducting superlattice structures can be used for direct electromagnetic generation of high frequency acoustic waves. The mechanism for and resonant nature of the acoustic generation are discussed.     

Intersubband absorption in highly photoexcited semiconductor quantum wells

Transient mid infrared (MIR) absorption spectroscopy is used to investigate transitions between higher electronic subbands in semiconductor quantum well (QW) structures after interband photoexcitation with intense picosecond pulses in the visible spectral range. Our investigation focuses on the e₂–e₃ intersubband transition in an asymmetric undoped GaAs/AlGaAs QW structure. At an injected nonequilibrium carrier density of 1×10¹³ cm⁻²/QW, an e₂–e₃ absorption band at 99 meV with a spectral width of 5 meV is found. For a higher density studied, 3×10¹³ cm⁻²/QW, the band is broadened and blueshifted by 30 meV. Intersubband absorption signals are distinguished from free-carrier absorption signals in the MIR by their characteristic time behavior.     

Strong impact of impurity bands on domain formation in superlattices

The formation of electric field domains in doped semiconductor superlattices is described within a microscopic model. Due to the presence of impurity bands in low-doped samples the current–voltage characteristic is essentially different compared to medium-doped samples.     

The study of fractal correlation method in the displacement measurement and its application

The classical digital speckle, or digital image, correlation method of deformation measurement is based on gray level correlation between unformed and deformed digital images. The pattern of artificial random speckles and the natural textures on some object's surfaces have fractal characteristics, and their fractal dimensions represent both gray and morph information. Furthermore, the fractal dimensions are stable feature parameters of the patterns. The digitized images of the patterns are confirmed to be also fractals. By this fact, a new method of displacement measurement is developed in the paper, based on the fractal dimensions correlation. The in-plane displacement fields of a body can be acquired. In order to verify the validity of the new method, an experiment has been designed and the results have been compared with those obtained from the classical digital image correlation method. The validity of the new method is not less than that of classical method. Further discussions about the traits and the developing vista of the method are given at the end.     

Real-time precision displacement measurement interferometer using the robust discrete time Kalman filter

This paper proposes a robust discrete time Kalman filter (RDKF) for the dynamic compensation of nonlinearity in a homodyne laser interferometer for high-precision displacement measurement and in real time. The interferometer system is modelled to reduce the calculation of the estimator. A regulator is applied to improve the robustness of the system. An estimator based on dynamic modelling and a zero regulator of the system was designed by the authors of this study. For real measurement, the experimental results show that the proposed interferometer system can be applied to high-precision displacement measurement in real time.     

Multiple modes of surface plasmonic polaritons in transversely-truncated metal/dielectric superlattices

The surface plasmonic polariton (SPP) of a transversely-truncated metal/dielectric superlattice (SL) structure has been solved with an approximate method. The effect of inter-layer interfaces in the SL is taken into consideration efficiently in comparison with the effective-medium method. The silver/air and silver/SiO₂ SLs with a shorter period are regarded as two specific examples in numerical calculation. A series of separated SPP modes are found and highly localized at the surface, and the highest-frequency mode is the only one also predicated by the effective-medium method. These results obviously show the effect of inter-layer interfaces in the case of short period, whilst the reliability and limitation of the effective-medium method is presented as well. Because the skin depths of the modes are extremely small, the SLs can be used as ideal surface-wave waveguides.     

Super Bloch oscillations in the Peyrard-Bishop-Holstein model

Recently, polarons in the Peyrard-Bishop-Holstein model under DC electric fields were established to perform Bloch oscillations, provided the charge-lattice coupling is not large. In this work, we study this model when the charge is subjected to an applied field with both DC and AC components. Similarly to what happens in the rigid lattice, we find that the carrier undergoes a directed motion or coherent oscillations when the AC field is resonant or detuned with respect to the Bloch frequency, respectively. The electric density current and its Fourier spectrum are also studied to reveal the frequencies involved in the polaron dynamics.