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This research involves a novel axial strain measurement of helical long-period grating by dual-wavelength ratio. The single-mode fiber is spirally processed via the commercial welding machine. There are two obvious resonance dips near 1473 and 1517 nm, with the pitch length around 782 μ m. The experimental results show that both the transmission of dip1 and dip2 rise along with increasing axial strain. The relationships between axial strain versus transmission of resonance dip and axial strain versus transmission of a fixed wavelength are studied separately. They are all linear change with axial strain. This indicates that the axial strain can be measured by the linear relationship between the transmission and axial strain. Furthermore, to eliminate the influence of light source, the dual-wavelength ratio is studied. The experimental results show that the ratio also changes linearly with axial strain. The fixed wavelength can be selected by a filter or a fiber Bragg grating, and the transmission intensity can be measured by a light intensity detector. This exploits a new way to devise high precision, low cost, and gains great potential application in the axial strain sensor.

In this work, we investigate the effect of cavity size on the change of the physical fields of diamond synthetic cavity and the mechanism of large cavity to improve the growth rate of the large single crystal diamond under high pressure and high temperature (HPHT) conditions. Finite element method (FEM) is used to simulate the temperature and convection field of the Φ15 and Φ10 cavity assemblies. Temperature simulation results indicate that the increase of cavity size leads to the increase of axial and radial temperature difference of catalyst. Convection simulation results show that the convection velocity and strength of the catalyst increases significantly with the increase of the cavity size. The increase of temperature difference and convection velocity and strength lead to the acceleration of crystal growth. The simulation results were perfectly validated by the synthetic experiments. This work not only elucidates the mechanism of large cavity to improve the growth rate, but also provides a theoretical reference for the application of the large-size cavity in the diamond synthetic industry.

The thermoelectric transport properties of a parallel-coupled double quantum dot (PCDQD) system with side-coupled quantum dots (QDs) is investigated by using the Keldysh non-equilibrium Green's function technique. The thermoelectric quantities, including the thermal conductance, thermopower, and thermoelectric figure of merit denoted by ZT, are sensitive to the inter-dot coupling strength. With the help of side-coupled QD, unusual double Fano resonances are created in the conductance spectra to largely enhance the thermoelectric effect at low-temperature. Benefited from the coexistence of local bipolar effect and Fano resonance, the ZT can be improved by one-fold higher than that of original PCDQD system. Moreover, when the asymmetry parameter α, which indicates the geometric arrangement of coupled QDs with a given lead, takes appropriate value, the optimization of ZT can be achieved at high temperature. Our work suggests that the side-coupled QDs scheme holds promise for the designing of high-efficiency thermoelectric conversion devices.

We present theoretically the quantum electronic transport through an interferometer asymmetrically coupled with triple quantum dots. Using the Keldysh non-equilibrium Green’s function method, the photon-assisted transport properties through the asymmetric quantum system are numerically analyzed. The sidebands of the photon-assisted tunneling process appear when driven by the time-modulated field. The average current spectra are simulated as a function of quantum dot energy to understand the roles of side-coupling strength and time-modulated field in sideband effect and electron tunneling. This is helpful in future design of the basic structures required for quantum computation applications.

We theoretically study the average current through a ring embedded with multiple quantum dots in each arm subjected to a time-dependent external field. A current resonance band can be observed in a six-quantum-dot system. In the presence of a time-dependent external field, mutual transformation occurs between the resonance band and antiresonance band, indicating an effective optically-controlled quantum switch can be realized in a wider quantum dot’s energy regime. As the Zeeman effect is introduced, the conversion between 100 and − 100% for spin polarization $$p$$ p can be realized by adjusting the frequency of time-dependent external field, suggesting a physical scheme of an optically-controlled spin filter. The present work sheds lights onto the design and quantum computation of future nano-devices.

The influence of interface structure on the photoelectric properties of InGaN light-emitting diodes grown on (0001) polar, (11–22) semi-polar, and (11–20) nonpolar planes is studied. Nonpolar and semi-polar interface structures are characterized by reduced polarization-related electric fields and suppressed QCSE. The energy bands of the structures grown on these orientations are undistorted, and QWs have more rectangular shapes. Consequently, these structures do not undergo separation of the electron and hole wavefunctions typical of c-plane structures. The simulation results suggest that potoelectric properties and efficiency drop is markedly improved when the polar interface structure is replaced by nonpolar or semi-polar interface structures.

The electrical and optical properties of InGaN LEDs with different threading dislocation densities (TDDs) were investigated. LEDs with low TDDs exhibited low forward voltage and high injection efficiency compared with LEDs with high TDDs. The effect of TDDs on the electrical properties of InGaN LEDs was attributed to efficient carrier injection into the QWs brought about by increased transverse carrier mobility in the InGaN layer resulting from reductions in carrier scattering around dislocation cores. In terms of optical properties, LEDs with low TDDs exhibited high peak efficiency and substantial efficiency droops under increased current densities, whereas LEDs with high TDDs showed low peak efficiency and minimal droops under the same condition. These trends can be explained by the correlations of high TDDs with increased dominance of nonradiative recombination and pronounced suppression of peak efficiency under low current densities.

Using the non-equilibrium Keldysh Green's function technique, electron transport properties through a two-terminal linear tri-quantum-dot molecule Aharonov-Bohm (A-B) interference are investigated. The conductance as a function of electron energy is numerically calculated. The influence of magnetic flux and interdot coupling strength on the conductance is researched. Fano resonances emerge in the conductance spectrum, and two bound states in the continuum form simultaneously when the interdot couplings take appropriate values. A conductance dip is observed and evolves into an antiresonance band with increasing magnetic flux. The system can be designed as a quantum switch by adjusting the intramolecular couplings.

Electron transport characteristics through a system consisting of a series of triangular quantum dot rings are studied utilizing the non-equilibrium Green’s function. In the presence of a magnetic field, an additional anti-resonance point occurs in the conductance spectrum. As the number of triangular quantum dot rings increases, two anti-resonance points evolve into two well-defined insulating bands. The insulating band disappears as the magnetic flux takes an appropriate value, and therefore, an effective magnetically-controlled quantum switch can be achieved. If a Zeeman magnetic field is introduced, a spin-polarized window (SPW) can be formed, suggesting a physical scheme of a spin filtering. As the intradot Coulomb interactions are taken into consideration, more SPWs can be obtained. This work sheds lights onto the design of future quantum devices.

We design a four-quantum-dot system subjected to a time-dependent external field. The average current through the quantum dot system is obtained by using the method of non-equilibrium Green's function. By adjusting either the amplitude or frequency of the time-dependent external field, the average current can be converted between zero and non-zero. Based on this property, the system can be designed as an optically-controlled quantum switch device. When the asymmetric time-dependent external field is irradiated to the left and right electrodes, negative current can be observed in the current spectrum, i.e., the photon-electron pumping effect. The present work sheds lights onto design and quantum computation of future nano-devices.

Using first-principles density functional theory, we predict a hexagonal structure of boron carbide with two shells, which consists of the sp2 hybridized boron and carbon in (001) plane and the pz–pz (σ) bonding carbon along [001] direction. The calculated results show that the structure is thermodynamically stable and possesses lower formation energy than other candidates. In addition, the quasiparticle calculations within the GW approximation reveal that the boron carbide, which is a two dimensional insulator, exhibits the indirect band gap of 2.4 eV and large exciton bonding energy of 1.35 eV. In optical absorption spectra, a bright Frenkel class bound exciton has been discovered at about 2.98 eV, which is desirable for light emitting applications. Besides, the piezoelectric coefficient (e22) of −2.38×10−10 Cm−1 is predicted for monolayer boron carbide, which indicates that the monolayer boron carbide is a potential candidate for piezoelectric applications in the nanoelectromechanical systems.

A four quantum dots structure is designed. The influence of the interdot coupling strength, energy levels of quantum dots, magnetic flux and intradot Coulomb interactions on the conductance is discussed. An antiresonance point exactly emerges at the location of the energy level of side-coupled quantum dots (SCQD). The conversion between resonance band and antiresonance band can be achieved as parallel-coupled di-quantum dots with or without SCQD, which suggests a physical scheme of an effective quantum switch. The Breit–Wigner resonant and Fano antiresonant peaks may merge into one resonant band. By means of the mathematical formula, we analyze how the resonance band is formed. By varying intradot Coulomb interaction, the resonance band may evolve into a Fano antiresonance with an ultra-narrow width. This study provides theoretical basis for the design of quantum computing devices.

Reversing the direction of polarization current is essential for reversible solid oxide cells technologies, but its effect on cobaltite based perovskite oxygen electrodes is largely unknown. Herein, we report the operating stability and microstructure at the electrode/electrolyte interface of La0.57Sr0.38Co0.18Fe0.72Nb0.1O3-δ (LSCFN) oxygen electrodes assembled on barrier-layer-free Y2O3 ZrO2 electrolyte under cyclic anodic/cathodic polarization mode at 0.5 A cm−2 and 750 °C. During the cyclic polarization, the electrocatalytic activity of LSCFN electrode is drastically deteriorated in cathodic mode, but the performance loss is largely recoverable in anodic mode. This is due to the fact that the surface segregation of Sr and accumulation at the electrode/electrolyte interface by cathodic polarization can be remarkably mitigated by anodic polarization. The time period in each cycle plays a key role in determining the accumulation of Sr species at the electrode/electrolyte interface. A full cell operating in a time period of 12 h fuel-cell/12 h electrolysis is reversible for a duration of 240 h, in contrast to the performance degradation in a shorter time period of 4 h fuel cell/4 h electrolysis. The present study sheds lights on applying cobaltite based perovskite oxygen electrodes on barrier-layer-free YSZ electrolyte for reliable solid oxide cells.

Employing the non-equilibrium Green’s function technique, we have obtained the formula for dc current of two one-dimensional multi-quantum dot arrays, which couple to each other via tunneling coupling between two quantum dots connected to four leads, respectively. The retarded Green’s function is a staircase type, terminating at the four leads. Furthermore, the four quantum dots case is demonstrated. The influence of inter-dot coupling strength and quantum dot energy level on the transmission probability for TL, TM and TN branches is investigated. A non-resonant band is observed. By adjusting energy levels of quantum dots, a resonance emerges in the region of the non-resonance band. The system can be used as a quantum switch.