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Time-dependent electron current response of GaAs-based miniband superlattice under dual ac electric fields and a magnetic field is studied using balance equation approach. The space charge-induced self-consistent electric field is taken into account in the model. The miniband superlattice operates in the diffusive regime without electric field domain formation. Electron current displays very complicated oscillating behavior with the influence of external fields. The effect of dissipation on nonlinear electron transport is carefully studied based on Poincaré bifurcation diagram and power spectrum. The exhibition of complicate nonlinear oscillation in superlattice is attributed to the nonlinearity induced by self-consistent field and interaction between external radiation and internal cooperative oscillating mode relative to Bloch oscillation and cyclotron oscillation.

Optical feedback exists in most laser configurations and strongly affects laser performances depending on the feedback strength, length, and phase. In this paper, we investigate the frequency comb behaviour of a semiconductor quantum cascade laser emitting around 4.2 THz with external optical feedback. A periodic evolution of the laser inter-mode beatnote from single-line to multiple-line structures is experimentally observed with a minor change of optical feedback length (phase) on the wavelength scale. The comb stability of the laser with feedback is also measured and compared with the same laser without feedback. Furthermore, our simulations reveal that the dynamical oscillations invoked by optical feedback are responsible for the measured multiple-line beatnotes. It is found that the characteristic feedback period is determined by the half wavelength of the laser, while the comb operation is maintained at most feedback length positions. Therefore, terahertz quantum cascade laser combs are robust against the minor position vibration of the feedback mirror in practice, owing to the much smaller feedback phase change than that of common near-infrared laser diodes.

The resistance hysteresis of vanadium dioxide ([Formula: see text]) is a key feature in revealing mechanisms of a phase transition as well as emerging applications. In this study, a dynamical model based on random-resistor networks is developed to simulate the transport properties of [Formula: see text] thin films. The reversible metal–insulator phase transition of each microscopic domain is captured by a modified Landau-type functional. The proposed model enables analysis of not only the formation of conducting filaments driven by an electric field, but also the thermal-driving reversal curves of resistance hysteresis. It is shown that the appearance of a hysteresis loop as well as the aggregation of metallic domains can be tuned via the interactions of each domain with its neighbors and the substrate. The interaction effects are vital for the persistence of metallic domains, which can re-trigger the insulating-to-metallic transition by a subthreshold voltage bias with the delay time much longer than the transition switching time. These results are in agreement with experimental observations and can be helpful in developing [Formula: see text]-based key components ranging from infrared bolometers to the volatile resistive switches for neuromorphic computing.

Chromatic optical lenses have promising applications in three-dimensional imaging, which allows fast spectral tomography without mechanical moving parts. The scanning range of current chromatic optical lenses is limited by their dispersion ability. The recent development in metasurfaces provides ideal blocks for optical wavefront manipulation and dispersion engineering of artificial materials at sub-wavelength scales. Hyper-dispersive metalenses can be realized by utilizing dispersive meta-atoms, which have enhanced dispersion compared to regular diffractive lenses. This is critical for increasing the imaging depth of fast spectral tomography. In this work, a hyper-dispersive metalens is realized with a chromatic dispersion 1.76 times greater than that of a regular diffractive metalens in the THz frequency range of 2.40–2.61 THz by simultaneously controlling the frequency-dependent phase, group delay (GD), and GD dispersion of the metalens. This approach can also be extended to other optical spectra and improve the performance of spectral tomography.

We systematically investigate the laser ridge width effect on comb operation of single plasmon waveguide quantum cascade lasers emitting around 4.2 THz. The total group velocity dispersion (GVD), including the gain, waveguide, and material dispersions, is numerically evaluated for 6-mm long lasers with ridge widths varying from 100 to 200 μm. The simulation reveals that although calculated waveguide GVDs of lasers with different ridge widths are almost identical, the clamped gain dispersion partially determined by the frequency-dependent waveguide loss strongly contributes to the total GVD. From the simulation, we find that the laser with a 150-μm-wide ridge shows the flattest total GVD in the lasing range between 4.05 and 4.35 THz. The optimal ridge width of 150 μm for comb operation is also experimentally verified by intermode beat note and on-chip dual-comb measurements.We systematically investigate the laser ridge width effect on comb operation of single plasmon waveguide quantum cascade lasers emitting around 4.2 THz. The total group velocity dispersion (GVD), including the gain, waveguide, and material dispersions, is numerically evaluated for 6-mm long lasers with ridge widths varying from 100 to 200 μm. The simulation reveals that although calculated waveguide GVDs of lasers with different ridge widths are almost identical, the clamped gain dispersion partially determined by the frequency-dependent waveguide loss strongly contributes to the total GVD. From the simulation, we find that the laser with a 150-μm-wide ridge shows the flattest total GVD in the lasing range between 4.05 and 4.35 THz. The optimal ridge width of 150 μm for comb operation is also experimentally verified by intermode beat note and on-chip dual-comb measurements.

High-performance terahertz (THz) detectors are in great need in the applications of security, medicine, as well as in astronomy. A high responsivity p-GaAs homojunction interfacial workfunction internal photoemission (HIWIP) detector was demonstrated for a specific frequency (5 THz) below the frequency of the Reststrahlen band. The experimental results indicate that the optimized detector shows significant enhancement of the response below the Reststrahlen band in contrast to the conventional detectors. With the bottom gold layer serving as a perfect reflector, nearly 50% increment of responsivity and quantum efficiency was obtained further due to the cavity effect. Though very simple, such reflector design shows a satisfactory effect and is easy to be realized in practical applications. The resultant peak responsivity of the detector with a bottom reflector could be as high as 6.8 A/W at 1 V bias. The noise equivalent power is 2.3 × 10 − 12 W / Hz 1 / 2. Due to the absorption ability to normal incident light and high responsivity, the p-GaAs HIWIP detector is promising for the focal plane array and large-scale pixelless imaging applications.

In this letter, a broadband bias-tunable terahertz (THz) quantum well infrared photodetector (QWIP) with a GaAs/AlGaAs asymmetric step multi-quantum well using the intersubband transition was proposed, designed, fabricated, and characterized. It is found that the THz QWIP has two photon response peaks at 4.5 THz and 6.5 THz and the bias-tunable photon response in the 4.0–6.5 THz frequency range of the device is attributed to the field-assisted tunneling of electrons in the second excitation subbands to continuum states.

We study terahertz (THz) generation from anisotropic streaming distribution of a two-dimensional electron gas (2DEG) in a GaN/AlGaN quantum well using the ensemble Monte Carlo method. Under proper electric fields, electrons do not have enough energy for intersubband and intervalley transitions. They make nearly cyclic motion in momentum space within the first subband, which gives rise to a spindle-shaped electron distribution. This effect is improved due to large polar optical phonon energy, strong electron–phonon interaction and much sharper increase of the polar optical phonon emission rate in GaN 2DEG. We demonstrate that electrons show dynamic negative differential mobility at the transit-time frequency and its harmonics at low temperatures due to this cyclic motion. The resonant frequency can be tuned by simply varying the applied electric field. We may thus use this effect to develop voltage-tunable THz masers.

The optical absorption spectra in a quantum well driven both by an intense terahertz (THz) and by an optical pulse are theoretically investigated within the theory of density matrix. We found that the optical absorption spectra and the splitting of the excitonic peaks splitting can be controlled by changing the THz field intensity and/or frequency. The Autler–Towns splitting is a result of the THz nonlinear dynamics of confined excitons, which is in agreement with the experiments. In addition, the dependence of the optical absorption on the quantum well width and the carrier density is also discussed.

We have theoretically investigated electron transport characteristics of two-dimensional electron gas in the modulation-doped AlGaN/GaN heterostructure by using the balance-equation approach. Temperature-dependent ohmic mobility and hot-electron transport property are calculated with an accurate treatment of the realistic scatterings by remote and background impurities, acoustic phonons (interacting with electrons through deformation and piezoelectric potentials), and polar-optic phonons. It is found that, for the optimized AlGaN/GaN heterostructure, its ohmic mobility could be as high as 4×105 cm2/Vs, as predicted by the Monte-Carlo simulation. The dependence of the ohmic mobility on the remote doping concentration and the spacer width is also discussed.

Inflection of carriers in the ? valley, prior to the usual intervalley scattering when the electric field is approximately less than 300 kV cm?1, can cause a negative differential velocity in zinc-blende (Zb) GaN. GaN n+nn+ oscillators based on this mechanism have a self-oscillating frequency in the terahertz (THz) range. The situation is completely different from the case of traditional Gunn-effect devices. We have carefully studied the electric domain dynamics and the self-oscillation frequency dependence on the applied dc field. When the diode is driven by a dc and an ac bias, a typical nonlinear dynamic system is constructed with the dc bias, the ac amplitude and the ac frequency as the control parameters. Different mode locking and chaotic modes show up when tuning the control parameters.

In order to fabricate good quality ferroelectric thin films, PbZrxTi(1-x)O3 (PZT) and SrBi2Ta2O9 (SBT) films were fabricated on SiO2/Si(100) substrates and on Pt/Ti/SiO2/Si(100) substrates by pulsed laser excimer deposition (PLD). X-ray diffraction, Rutherford backscattering analysis, and atomic force microscopy were used to characterize the structural properties of the samples, which were post-annealed at different temperatures. The results showed that the PZT and SBT films fabricated on Pt/Ti/SiO2/Si(100) substrates and annealed at 700 degreesC exhibited optimum properties. (C) 2002 Elsevier Science B.V. All rights reserved.

The paper presents new hydrodynamic balance equations for semiconductor device simulation, which stem from the Lei–Ting balance equations for spatially inhomogeneous device structures. Their main characteristics, which differ from those of the conventional hydrodynamic equations, is the inclusion of accurate momentum and energy collision terms at the microscopic level. With the help of the nonlinear frictional force and energy transfer functions, the collision terms as well as the relaxation times can be self-consistently calculated from the primary physical parameters, taking full account of dynamical screening and other quantum-mechanical effects. A solution method for this set of differential–integral equations is proposed and applied to a one-dimensional simulation of a submicron Si n+nn+n structure diode. The numerical results for various applied voltages are presented.

Balance equations for high‐field electron transport in nonparabolic multiband (multivalley) semiconductors are proposed based on the Heisenberg equations of motion for the total physical momentum, the total energy and the population of carriers in each energy band (each valley), and the statistical average with respect to an initial density matrix having a lattice wave‐vector shift, an electron temperature, and a chemical potential for each energy band (each valley) as parameters. As an example, these equations are applied to the discussion of hot‐electron transport in bulk Si, assuming Kane‐type energy dispersion for six elliptical valleys. The theoretical results are in good agreement with experiments and with Monte Carlo simulations, over the entire range of the electric field up to 140 kV/cm.

Terahertz quantum-well photodetectors (QWPs) represent a new and emerging photon-type detector in terahertz region. We first discuss the many-particle effects on the accurate design of terahertz QWPs. Grating and metal-cavity light couplers for terahertz QWPs are introduced. At resonant coupling frequencies, the polarization of light field is effectively changed by the light couplers to fulfill the selection rule of intersubband transition. Meanwhile, the electric field intensities in the active multi-quantum-well region of terahertz QWPs are enhanced. The performance of terahertz QWPs with these light couplers is improved significantly.

The effects of heparin-superoxide dismutase (SOD) conjugate (heparin-SOD) on γ-radiation induced DNA damage in vivo and in vitro were evaluated. Plasmid pcDNA3.0 solution was mixed with heparin-SOD, SOD, and a mixture of heparin and SOD (heparin + SOD), respectively, and irradiated with (60)Co at a dosage of 120 Gy. DNA injury was analyzed using agarose gel electrophoresis. The results showed that the degree of injury of pcDNA3.0 mixed with heparin-SOD, SOD, or heparin + SOD was less than that of untreated pcDNA3.0, and among them the degree of injury of pcDNA3.0 mixed with heparin-SOD was the least. It also showed that the protective effect increased with an increase of heparin-SOD concentration. The effects of SOD and heparin-SOD on the DNA damage and tumor inhibition rate of (60)Co γ-radiation exposure on tumor-bearing mice were also studied. Agarose gel electrophoresis showed that, when different SOD samples were administered before irradiation, the thymus DNA injuries of heparin-SOD, SOD, or heparin + SOD groups were more serious than that of the control group, and the DNA injuries of heparin-SOD or heparin + SOD groups were the most serious, which contradicted the above in vitro experiments. However, when heparin-SOD was administered post irradiation, it showed a repairing effect on the injured DNA.

We study the combined effects of quantum electronic interference and Coulomb interaction on electron transport through near-degenerate molecular states with strong electron-vibration interaction. It is found that quantum electronic interference strongly affects the current and its noise properties. In particular, destructive interference induces pronounced negative differential conductances (NDCs) accompanying the vibrational excited states, and such NDC characters are not related to asymmetric tunnel coupling and are robust to the damping of a thermal bath. In a certain transport regime, the non-equilibrium vibration distribution even shows a peculiar sub-Poissonian behavior, which is enhanced by quantum electronic interference.

We study the correlations of tunneling currents through an interacting quantum dots (QDs) system composed of a top single QD and a bottom qubit with purely capacitive coupling within a quantum master approach. We find that the super-Poissonian current noise of the qubit near resonance, which is a signature of coherent tunneling within the transport qubit for asymmetrical contact couplings, is strongly dependent on non-equilibrium transport through the top QD with different coupling configurations. For pure-dephasing coupling, such a super-Poissonian feature is asymmetrically washed out by increasing coupling strength showing obvious qubit level position dependence with finite bias and temperature, while for orthogonal coupling we can almost symmetrically lower the double peak to a double minimum by increasing coupling strength or adjusting the ratio of the top QD contact couplings in the large bias limit, indicating the transition from coherent tunneling to sequential tunneling.

Multilayer ferroelectric and ferromagnetic thin films were fabricated with the following composition and structure: La0.67Sr0.33MnOx/(Pb, La)(Ca, Ti)O3 (LSMO/PLCT) and (Pb, La)(Ca, Ti)O3/La0.67Sr0.33MnOx (PLCT/LSMO), with an intention of creating a device of new concept. The films were fabricated using metalorganic deposition. The LSMO/PLCT structured films exhibit the ferroelectric properties while the PLCT/LSMO structured films exhibit the ferromagnetic properties. Compared with the PLCT/Pt structure, the capacitance-frequency and tan δ-frequency properties of the LSMO/PLCT structure are degraded. The magnetoresistance of the PLCT/LSMO structure was measured and the results show that the metal-insulator transition temperature is lower than that of the LSMO/SiO2/Si structure. Furthermore, the magnetoresistance at 77 K and at the transition temperature for the PLCT/LSMO structure is higher than that of the LSMO/SiO2/Si structure. These effects can be explained by the piezoelectricity of the PLCT film depo...

Terahertz (1 - 10 THz) quantum-well photodetectors and quantum- cascade lasers have been investigated. The design and projected detector performance are presented together with experimental results on several test devices, all working at photon energies below the optical phonons. Background limited infrared performance (BLIP) operations were observed for all samples (three in total) designed for different wavelengths. For lasers, a set of THz quantumcascade lasers with identical device parameters except for the doping concentration has been studied. The δ-doping density for each period was varied from 3.2 × 1010 to 4.8 × 1010 cm-2. We observed that the lasing threshold current increased monotonically with doping. Moreover, the measured results on devices with different cavity lengths provided evidence that the free carrier absorption caused waveguide loss also increased monotonically. Interestingly however, the observed maximum lasing temperature displayed an optimum at a doping density of 3.6 × 1010 cm-2.© (2006) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Intersubband lasing at 12-16 microm based on a CO2 laser pumped stimulated resonant Raman process in GaAs/AlGaAs three-level double-quantum-well structures is reported. The presence, or lack of, lasing action provides evidence for resonantly coupled modes of collective electronic intersubband transitions and longitudinal optical phonons. An anticrossing behavior of these modes is clearly seen when the difference between the pump and lasing energies (i.e., Stokes Raman shift) is compared with the subband separation. This work reveals the significance of the strong coupling between intersubband transitions and phonons and raises a new possibility of realizing a phonon "laser."

‘‘N-shaped’’ velocity-field relation is yielded by using the nonparabolic balance-equation theory with a realistic treatment of carrier scatterings by impurity, acoustic phonon, and optic phonon. The dependence of the self-oscillating mode and its frequency on the dc bias, doping concentration, and lattice temperature is examined in detail. The THz-driven p 1 pp 1 NEM diodes can produce a cooperative nonlinear oscillatory mode which leads to very complicated chaotic dynamics with the dc bias, ac amplitude, and ac frequency as the controlling parameters. The transitions between the periodic and chaotic states are carefully studied by differ

We investigate the electronic and transport properties of a normal insulator in form of a zigzag ribbon based on the Kane-Mele model subjected to circularly polarized radiation. Our results show that chiral edge-states can be induced in the band gap of the quasi-energy spectra under periodic driving. More interestingly, for appropriate parameters, there exists a single chiral edge state at each boundary of the sample. As a result, the conductance shows plateau structure with the step height e2/h as we increase the ac field intensity. These observations may find their potential applications for high-efficiency non-magnetic spin injection which can be readily tuned by modulating an external ac field.

We investigate the carrier transport properties of a three-well terahertz quantum cascade laser which is considered as an open system by using self-consistent Bloch-Poisson equations. The simulation results show that a dynamic equilibrium is achieved, and the electric potential of each period slightly changes with time. Compared to other simulation methods with the cyclic boundary condition approximation, our open system modeling gives more reliable results on the current density-applied electric field and population inversion-applied electric field characteristics. Our modeling method can give more realistic results of working terahertz quantum cascade lasers without increasing the simulation complexity.

The authors investigate the effect of injection coupling strength on the performance of a set of three-well diagonal transition terahertz quantum-cascade lasers using a Monte Carlo method. Device structures with different injection coupling strengths, i.e. from 1.6 to 10 meV, are considered. For an accurate comparison, the level separation between the lasing states is kept the same (around 3.7 THz) across all designs. The simulation results show that the current density corresponding to the negative differential resistance region (JNDR) can be largely increased by increasing the injection coupling. In the range of the injection coupling strength investigated here, an optimal injection coupling of 7.5 meV is found.

We carry out a theoretical study of nonlinear dynamics in terahertz-driven n+nn+ GaAs1−xNx diodes. A cooperative nonlinear oscillatory mode shows up due to the negative differential velocity (NDV) effect in the highly nonparabolic conduction band of the diodes. We have made detailed investigations to the complicated chaotic dynamics by changing the dc bias, ac amplitude, and ac frequency of terahertz fields with chaos-detecting methods, such as Poincare bifurcation diagram, power spectrum, and first return map. The influence of NDV curves on the nonlinear dynamics is also studied. The coupling of dc bias and ac driving voltage causes possible types of transport states (periodic or chaotic) and transitions between them. The resulting power spectrum pattern displays a very complex mosaic scenario with a self-similar emergence of high-order mixing frequencies.

Bicarbonate (HCO3-) causes a significant and reversible stimulation of anion-inhibited electron flow in photosystem II of higher plants and cyanobacteria. To test if selected arginine (Arg) residues are involved in the binding of HCO3-, we utilized oligonucleotide-directed mutagenesis to construct Synechocystis sp. PCC 6803 mutants carrying mutations in Arg residues in the D2 protein. Measurements of oxygen evolution showed that the D2 mutants R233Q (arginine-233----glutamine) and R251S (arginine-251----serine) were 10-fold more sensitive to formate than the wild type. The formate concentration giving half-maximal inhibition of the steady-state oxygen evolution rate was 48 mM, 4.5 mM and 4 mM for the wild type, R233Q and R251S, respectively. Measurements of oxygen evolution in single-turnover flashes confirm that the mutants are more sensitive to formate than the wild type. Measurements of chlorophyll a fluorescence decay kinetics after the second saturating actinic flash indicated that, after formate treatment, the halftime of QA- oxidation was decreased by approximately a factor of 2, 4 and 6 in the wild type, R251S and R233Q, respectively. The recombination rate between QA- and S2 was approx. 2-fold slower in R251S and R233Q than in the wild type. In the presence of 100 mM sodium formate, reactivation of the Hill reaction by bicarbonate showed that the wild type had an apparent Km for bicarbonate of 0.5 mM, while the Km values for R233Q and R251S were 1.4 and 1.5 mM, respectively. We suggest that Arg-233 and Arg-251 in the D2 polypeptide contribute to stabilization of HCO3- binding in Photosystem II.

The authors present the testing method and electrical and optical characteristics of a 4.1 THz quantum cascade laser with a four-well resonant-phonon design. The mode evolution in pulsed and continuous wave modes as a function of drive current has been investigated. An ensemble Monte Carlo model is used to analyze the carrier transport and output characteristics of the device. The calculated current density, lasing domain, and lasing frequency as a function of electric field are consistent with the measured results.

We have theoretically investigated current self-oscillation in doped n+nn+ GaAs1−xNx diodes driven by direct current (dc) electric field. The current self-oscillation is associated with negative differential velocity effect in the highly nonparabolic conduction band of this unique material system. By solving a time-dependent drift-diffusion model that takes into account the negative differential velocity effect, we provide a detailed analysis of the current oscillations. The frequencies of current oscillations are in the gigahertz to terahertz region, depending on the doping concentration and the applied dc electric field. The calculated average current density is in qualitative agreement with the measured result.

The authors demonstrate that the Rashba spin-orbit interaction in low-dimensional semiconductors can enhance or reduce the electron-phonon scattering rate by as much as 25%. The underlying mechanism is that the electron-phonon scattering phase space for the upper (lower) Rashba band is significantly enhanced (suppressed) by the spin-orbit interaction. While the scattering time decreases for the upper level, the mobility of the level increases due to an additional term in the electron velocity.

The authors compare different Coulomb screening models in the Monte Carlo simulation of terahertz quantum-cascade lasers: the static multisubband screening model and two single subband models. In quantum-cascade structures, electrons are located in many different subbands. Coulomb screening from all these subbands influences the intra- and intersubband scattering processes. The simulation results show that one of the two single subband models overestimates the screening effect, while the other underestimates it. The authors show the reasons for this and propose simple modification to the single subband models.

We calculate the photon absorption coefficient of hot two-dimensional electrons in the presence of a strong magnetic field. The electrons interact strongly with the optical phonons and the acoustic phonons in quantum wells. The dependence of the optical absorption on the magnetic field is obtained by using the quantum mechanical kinetic theory. It is found that the photon absorption spectrum displays a local magnetophonon resonance. The magnetophonon absorption resulting from inelastic scattering between Landau levels is more pronounced at higher temperature. The effect of subband nonparabolicity on the absorption coefficient is also discussed.

The dependence of the intersubband absorption spectra on the Coulomb interaction and quantum well (QW) width is studied. Rather than following the Fermi–Dirac distribution, we have solved the intersubband equations of motion to determine the subband population self-consistently. We have gone beyond the linear absorption theory to show the effect of various many-body interactions on the absorption spectra. It is found that the redistribution of electrons in excited states reduces the absorption. Our results indicate that the line shape and peak position are determined by the interplay of different collective excitations, such as the Fermi edge singularity and the intersubband plasmon. The dependence of the absorption spectrum on the QW width and the subband effective masses is also discussed.

Theoretical calculations of the interband optical-absorption spectra of a finite quantum dot superlattice in a wire (QDSLW) in the presence of a homogeneous dc or terahertz field are presented. The QDSLW has miniband structures of the electron states as general quantum well superlattices do and shows the two features common to one-dimensional structures, viz., strong excitonic absorption below and weak absorption above the band edge. The applied dc field causes a redshift of the location and a decrease in height of the main excitonic peak and leads to an increase in height of the minor peak next to the main peak. In the presence of a terahertz field, the Autler-Townes splitting of the main excitonic peak and the emergence of one-photon and two-photon gain peaks on the spectra are demonstrated. (c) 2005 American Institute of Physics.

We have extended the balance equations to account for conduction–valence interband impact ionization (II) induced by an intense terahertz (THz) electromagnetic irradiation in two-dimensional semiconductors. We have studied the effect of II on electron transport and electron–hole pair generation–recombination rate in THz-driven InAs/AlSb heterojunctions (HJs). As many as needed multiphoton channels are self-consistently taken into account. Usually II acts as a cooling mechanism in semiconductors. In the present THz-radiation-driven case with a multiphoton process, the electron temperature with II, however, is higher than that without this process. We propose to explain the counterintuitive behavior of electron temperature in THz-radiation-driven HJs.

We have theoretically studied current oscillation and chaotic dynamics in doped GaAs∕AlAs superlattices driven by crossed electric and magnetic fields. When the superlattice system is driven by a dc voltage, a stationary or dynamic electric-field domain can be obtained. We carefully studied the electric-field-domain dynamics and current self-oscillation which both display different modes with the change of magnetic field. When an ac electric field is also applied to the superlattice, a typical nonlinear dynamic system is constructed with the ac amplitude, ac frequency, and magnetic field as the control parameters. Different nonlinear behaviors show up when we tune the control parameters.

Interface and confined optical-phonon modes in wurtzite multi-interface heterostructures are studied numerically based on the dielectric continuum model and Loudon’s uniaxial crystal model. The phonon dispersion relation and electrostatic potential are calculated using the transfer matrix method. The anisotropic characteristics of wurtzite phonons are demonstrated. The electron-optical-phonon Frohlich interaction is studied in a fully numerical manner. The dispersion relation is taken into account in the derivation of Fermi’s golden rule. The method presented here is general and can be easily applied to the design and simulation of optoelectronic devices based on wurtzite multi-interface heterostructures.

The optical absorption of GaAs nanorings under dc and ac electric fields with frequency at the terahertz range applied in the plane containing the nanoring is investigated. The excitonic effects are essential to describe even qualitatively the absorption spectra. The applied lateral electric fields change the optical absorption spectra prominently. In the presence of terahertz electric fields, the optical absorption spectra may be very different for different frequencies. In certain frequencies, there arise other absorption peaks in the below-band-gap spectrum.

In this paper, mouse transplantable hepatocellular carcinomas of both solid and ascitic types were effectively treated by urea. Survival time of tumour-bearing animals was obviously prolonged. Tumour cells in the treated group were significant swelling, degeneration, necrosis, smoothing of cellular surface and loss of infiltrating ability to surrounding soft tissues observed by light, electron and scanning electron microscope. As assayed by cellular kinetics and autoradiography, proliferation was inhibited, appearing as decrease of mitotic index and prolongation of cell cycle time, and so on. The proliferating ability of tumour stem cells in the treated group was severely inhibited as tested by colony formation frequency in soft-agar medium. There was no obvious alteration in relative DNA content in tumour cells or in the ratio of number of cells in various phases to cell population according to the assay of fluorescence activated cell sorter. This means that urea is a cell cycle non-specific antitumour agent and is able to kill tumour cells in various phases.